00 customer service - noraweb.org

00 Customer Service

CombustionChapter 7

Chapter 7—Combustion 7-3

Combustion theoryAs a technician, you have an obligation

to assure the equipment you are working onis operating at peak performance levels.Understanding combustion theory is thebasis for adjusting oilburners for safe,clean, reliable, and economical operation.

Combustion is a controlledchemical reaction

Three things are needed to make a fire:oxygen, ignition, and fuel. When heatingoil is burned, the chemical energy in thefuel is converted to heat.

The oxygen required for combustioncomes from the air that is delivered by theoilburner fan. The spark delivered by theelectrodes provides the heat needed to startthe combustion process.

The heat from the spark vaporizes the oildroplets delivered by the nozzle then lightsthe vapor on fire. If the conditions forcombustion are right, this process contin-ues until all the droplets vaporize and burncompletely and cleanly within the combus-tion zone.

Combustion is the rapid oxidation of anymaterial that will combine readily withoxygen.

The resulting flame contains the hotgases produced when the hydrogen andcarbon in the fuel react and combine withthe oxygen in the air. This reaction createslight, and releases large quantities of heat.This heat from the combustion gases is

extracted by the heat exchanger in thefurnace, boiler, or water heater, and heatsthe air, water, or steam we use for spaceand domestic water heating.

Every gallon of oil contains about140,000 Btus per gallon. A Btu is theenergy required to raise one pound of waterone degree Fahrenheit—about the amountof energy contained in a birthday candleflame. In a typical oil-fired appliance,every gallon of oil burned puts about119,000 Btus into the building and about21,000 Btus go up the chimney. Figure 7-1.

Heating oil is 85% carbon and 15%hydrogen. These fixed ratios of hydrogenand carbon in the fuel combine with aspecific quantity of oxygen to formcombustion gases. Therefore a precise Figure 7-1: Heat

loads, heat loss

Chapter 7Combustion

Heat to Load = Heat Energy Input - Stack Loss

Heat Energy to Load119,000 Btuh

Heat Energy LossUp Chimney(Stack Loss)21,000 Btuh

Heat EnergyInput

140,000 Btuh

7-4 Combustion

quantity of combustion air is needed so allthe fuel will burn completely. Too much airwill lower efficiency, and too little aircauses incomplete combustion and smoke.

During combustion, new chemicalsubstances are created from the fuel andthe air. These substances are called com-bustion gases. Most come from chemicalcombinations of the fuel and oxygen, butthe gases can also include chemicalcombinations from the air alone.

When a hydrocarbon-based fuel (oil)burns, the exhaust gases include water(hydrogen + oxygen) and carbon dioxide(carbon + oxygen). Combustion gases canalso include carbon monoxide (CO), oxidesof nitrogen (NOx, nitrogen + oxygen) andsince sulfur is present in the fuel, sulfuricoxide (sulfur + oxygen).

As the fuel and air are turned intocombustion gases, heat is generated. Heat isrequired to start combustion and is itself a

product of combustion. Once combustiongets started, we don’t have to continue toprovide the heat source, because the heatproduced by the combustion process willkeep things going.

What is air?Air is 20.9% oxygen, 78% nitrogen, and1.1% other gases. For every one part ofoxygen, we get four parts nitrogen, seeFigure 7-2. Nitrogen is an inert gas andmost of it goes through the combustionprocess unchanged. It cools the chemicalreaction (burning temperature) and lowersthe maximum heat content deliverable bythe fuel. Therefore, it is impossible toachieve combustion efficiencies above 95%for most fuels, including natural gas, whenair is used as the source of oxygen for thecombustion process.

With flame retention burners, some ofthe nitrogen combines with the oxygen toform nitrogen oxide or NOx.

Figure 7-2:Combustionprocess in a burner

Combustion

Air1,400 Cubic Feet Per Gallon21% Oxygen, 79% Nitrogen

Spark

Atomization

CombustionFuel Oil85% Carbon, 15% HydrogenTrace of Sulfur

Vaporization

Chapter 7Combustion

Heat138,690

Hot NitrogenCarbon DioxideCO2

Water VaporH2O

Nitrogen OxidesNOx

Excess OxygenO2

Sulfur OxidesSOx

Free CarbonSmoke

Carbon MonoxideCO


Heating oil flamesCombustion is a series of exact chemical

reactions that create exact quantities ofcombustion gases. It takes 14.4 pounds ofair to burn each pound of heating oil andwe produce 15.4 pounds of combustiongases, Figure 7-3.

If we could achieve stoichiometric, orperfect combustion, each gallon of oilconsumed would need 1,360 cubic feet ofcombustion air. The actual amount of airrequired will vary by the heat value of thefuel and the design of the burner. Heatingoil contains about 19,500 Btus per pound.Non-flame retention burners need at least1,700 cubic feet of air to burn clean.Current burners need about 1,500 cubicfeet.

Buildings today are so well insulatedand weather-stripped that getting adequatecombustion air to the burner is becoming aproblem. When troubleshooting combus-tion problems, get into the habit of askingyourself, “Where is my combustion aircoming from?” and “Do I have enough toburn all of my fuel?”

Oilburner flames produce variouscombustion gases in fixed quantities. With

perfect combustion, every pound of oilburned will produce 3.2 pounds of carbondioxide, 1.1 pound of water vapor, and11.1 pounds of hot nitrogen. This constantratio of combustion gases allows us to testthe quality of a flame against this perfectstandard to determine optimum adjustment.

Heating oil atomizationand vaporization

Heating oil will not burn as a liquid. Itmust be converted to a vapor before therapid reaction between the fuel and the aircan produce a flame. The oilburner’s job isto convert the liquid fuel into a vapor so itcan be burned.

The oil is pumped to the nozzle at highpressure (100 psi or more) where it isbroken up into a mist of small droplets(atomized). The droplets evaporate quicklywhen exposed to the heat of the spark orflame, producing vapors that burn easilywith the air supplied by the burner fan.

Combustion air supplyand air-oil mixing

The better the air and the oil vapor aremixed, the better our combustion. Burnerair parts, (including turbulators, spinners,end cones, and flame retention heads), aredesigned to give good mixing of theatomized fuel droplets and the combustionair. Good fuel and air mixing assures thatall the fuel vapors contact enough oxygenfor complete burning.

In high pressure atomizing burners,several factors control the quality of air-oilmixing. The spray pattern of the oildroplets must be similar to the air patterncreated by the burner.

Flame retention burners have muchbetter air/oil mixing capabilities than olderburners. They use high speed burner

Figure 7-3: Fueloil combustion in air

Chapter 7Combustion

7-6 Combustion

motors (3,450 RPM) and air patternshaping to create the high static air pressureneeded to make the high velocity air swirland the internal recirculation needed forclean, efficient combustion.

This recirculation is created by the dropin pressure in the center of the air swirl,like the eye of a tornado. This pulls some ofthe hot flame gases toward the burner head,the way the spray from a showerhead pullsin the shower curtain.

These hot gases add heat to the fueldroplets coming out of the nozzle speedingup their vaporization and burning rates,which gives us a nice clean, stable fireclose to the burner head.

New oilburners should notproduce smoke

Smoke and soot, which are nothingmore than unburned carbon, are created byoutdated burner designs and incorrectburner service and adjustment. Smokeproduction is unnecessary and must be

Figure 7-4:Soot affects fuelconsumption

Effect of Soot on Fuel Consumption

Soot Layer on Heating Surfaces

Chapter 7Combustion

eliminated, because it reduces efficiency,increases service calls, and is a nuisance tohomeowners. It can be prevented usingmodern burners and by careful adjustmentof burners using combustion test equip-ment.

Excessive smoke wastes fuel because itdeposits soot on the heat exchangersurfaces, Figure 7-4. This insulates the heatexchanger, limiting its ability to extract theheat from the combustion gases. A layer ofsoot only 1/8" thick can reduce heatabsorption by over 8%.

Efficiency loss caused by a smoky burneroccurs as the soot slowly builds up. Sootalso affects the reliable operation of theburner. If it builds up on the cad cell or thebimetal of the stack relay, it can act like aflame failure and cause the control to lockout on safety, creating an unnecessaryservice call.

Overfiring can cause smoke: If a unitis overfired, the burner will create heatfaster than the heating system can distributeto the building. When this happens, theburner short cycles (goes on and offfrequently for short periods of time). Theproblem is that older oilburners createsmoke when they start and stop.

Up to two thirds of all the smokeproduced by burners made before the year2000 is produced on start up and shutdown. Therefore, properly sized nozzleswill produce less frequent burner cyclingand less smoke.

SulfurSulfur exists in varying degrees in all

fossil fuels. The sulfur content of heatingoil ranges from 0.5% to 0.05% by weight.

When burned, the sulfur mixes withoxygen to form sulfuric oxide (SOx). Itreacts with the water vapor in the combus-tion gases to create sulfuric acid aerosol.


Figure 7-5:Carbonmonoxide levelsof concern

Chapter 7Combustion

Death Within 1-3 Minutes

Nausea Within 20 Minutes,Death Within 1 Hour

Nausea and Convulsions,Death Within 2 Hours

Frontal Headaches 1 to 2 Hours,Life Threatening After 3 Hours

Maximum Concentration forContinuous Exposure in Any8 Hour Period

Maximum Indoor Air Quality Level

Desirable Level

Carbon Monoxide (CO) Levels

When the acid condenses (at about 150 to200°F), it adheres to the flue pipe and heatexchanger surfaces in a film and reacts withthe iron in the pipe and heat exchangerwall. This creates iron sulfates, the lightyellow to rust colored crusty scale you findclinging to the heat exchanger.

Scale buildup downgrades efficiency by1% to 4% over the year. It also blocks fluepassages, restricting air flow and increasingsmoke and soot. Sulfur levels in heating oilare gradually being reduced, so this will beless of a problem in the future.

Carbon monoxideCarbon monoxide, or CO, is a toxic gas

that can occur in homes and buildingswhere combustion by-products are gener-ated, not properly vented and allowed toaccumulate. CO is a colorless, odorless,tasteless poison. Carbon monoxide isreadily absorbed in the body and canimpair the oxygen-carrying capacity of theblood (hemoglobin).

Impairment of the body’s hemoglobinresults in less oxygen to the brain, heart,and tissues. Even short-term over exposureto carbon monoxide can be critical or fatalto people with heart and lung diseases, andto the young or the elderly. It may alsocause headaches and dizziness and othersignificant medical problems in healthypeople. At low concentrations, CO can goundetected and contribute to naggingillnesses, and can compound pre-existinghealth problems. Figure 7-5.

Carbon Monoxide is a result of incom-plete combustion due to unburned fuel.During combustion, carbon in the fueloxidizes through a series of reactions to formcarbon dioxide (CO

2). However, 100%

conversion of carbon to CO2 is rarely

achieved under field conditions and somecarbon only oxidizes to the intermediate step,carbon monoxide or CO. Carbon Monoxide

is usually produced by insufficient combus-tion air. However, excess air and mismatchedoil to air patterns and ratios can also reduceflame temperature to a point where CO isproduced. So, adding too much air to cleanup a smoky fire can create CO. When anypart of the flame is reduced below 1,128°F,CO will be produced. Flame impingementalso results in lower flame temperature andCO production.

Ambient CO limits(Recommended)

0 ppm. This level is most desirable, butcannot always be achieved due to ciga-rettes, candles, and appliances such as gasstoves.

1-9 ppm.Normal levelswithinthe home.

10-35 ppm.Advise occupants,check for symptoms(slight headache,tiredness, dizziness,and nausea or flulike symptoms),check all appli-ances, including thefurnace, waterheater and boiler,check for othersources includinginternal combustionengine operation inattached garages.

36-99 ppm.Recommend freshair, check forsymptoms, ventilatethe space, recom-mend medicalattention.

7-8 Combustion

100+ ppm. Evacuate the home(including yourself!) and contact emer-gency medical services (911). Do notattempt to ventilate the space. Short-termexposure to these levels can cause perma-nent physical damage.

Carbon monoxide is released into homesby vent blockage, flue pipe damage, heatexchanger cracks, and restricted air supplyinto the house. This last problem is pro-gressively getting worse as new homesbecome tighter in their construction, andmany homeowners are weather strippingand insulating their older homes.

Most homes have a number of devicessuch as exhaust fans, clothes dryers, andfireplaces, that remove air from the home.This suction is often stronger than thesuction of the heating system’s chimney orpower vent. This back drafting causes theemissions from the heating system, thewater heater, gas ovens, gas stoves, gasdryers, and wood stoves or fireplaces toenter the living area and elevate CO levels.

Oilheat’s CO warning signsIf you see smoke near the burner, dark

smoke coming from the chimney, or smella sharp raw oil smell, the burner is prob-ably producing unacceptable levels ofcarbon monoxide. With insufficientcombustion air, oilburners usually produceelevated smoke levels before high COlevels are reached. This smoke is a warningsignal.

The result is that the danger from highCO levels is much lower from oilburnersthan any other hydrocarbon burner.However, if oilburners are operated withtoo much combustion air, it chills the flameand creates CO with no smoke! Impropernozzle to air patterning can also produceCO.

What to watch out forCO is odorless and tasteless, therefore in

order to detect its presence, we performcombustion tests and look for other cluesfor combustion or ventilation problemssuch as:

• Sharp gas or oil smell

• Stale or stuffy air

• Soot, rust, or scale build-up on oraround appliances and vents

• Loose or disconnected chimney orvent connections

• Debris or soot falling from chimney,fireplace, or appliance

• Excessive moisture on the inside ofwindows or walls

• Chalky white powder forming on thechimney or vent

• Visible smoke in the living space

Light off CO levels: High CO levels atlight off may be an indication of rough ordelayed ignition, warranting further investi-gation. The CO readings will peak onstartup, then dramatically drop. CO readingsshould stabilize within 10 minutes ofoperation and should never be rising duringoperation.

Mechanical problems and CO: If theappliance being tested has sufficient combus-tion air and is still producing higher thanacceptable CO air-free levels, it could be amechanical problem.

Inspect the burner for cleanliness, properalignment, fuel pressures, and evidence ofimpingement. Impingement occurs when theflame hits an object that has sufficient massto transfer enough heat from the flame tocause low flame temperatures and incom-plete combustion. This can be as simple as ascrew poking into the heat exchanger or asmajor as a collapsed refractory.

Missing burner covers, improper air bandadjustment or oil pressures can also contrib-ute to higher than normal CO levels.

Chapter 7Combustion

CO ambient air testing(Combustion air zone & living space)

Ambient CO levels should be checked andthe equipment should be run through acomplete cycle if you suspect any combus-tion problems.

If at any time ambient CO levels exceed100 ppm, evacuate occupants and callemergency services.

The most common sources of CO areexhaust from a vehicle in an attachedgarage, and depressurization of the homeresulting in insufficient air for combustion.If CO is detected, all possible sources ofCO should be checked, including—but notlimited to—water heaters, gas ovens andstoves, the furnace, (non-electric) spaceheaters, and vented or unvented appliancessuch as gas logs.

Combustionefficiency testing

Combustion efficiency testing is one ofthe most important things we do whenservicing oilburners, Figure 7-6. The testsdetermine the quality of the combustion

process and tell us if we have set up theburner correctly. In this section, we willexplore how to use instruments to measureand improve efficiency, cleanliness, andsafety of the unit. We will also cover thereasons for high and low efficiency, andhow testing can pinpoint current and futureproblems.

It is imperative to perform a combustionanalysis during routine service, or any timechanges are made that will affect combus-tion. Combustion testing provides numerousbenefits to the customer and service techni-cian including:

• Saving money

• Saving time

• Avoiding callbacks

• Limiting liability

• Maintaining equipment warranty

• Providing confidence

• Providing increased comfort

• Providing increased safety

• Increasing energy efficiency

• Lowering environmental emissions

Modern burners require proper setup,making the use of instruments necessary.Using instruments assures low smoke andsoot, improves your image, and increasescustomer comfort and satisfaction. Today’sburners are superior to the older modelswhen set up correctly—but can be trouble-some when setup incorrectly. You cannot seea #2 smoke, you also cannot feel a 350°stack, smell a 6% CO

2, or 100 ppm of

carbon monoxide, yet if you leave the unitoperating in any of these conditions, you willnot be doing your job.

With the older units, you could observethe flame, see its shape and color, anddetermine to some extent how the burnerwas performing. However, even with the


Figure 7-6: Combustion efficiency test

Chapter 7Combustion

7-10 Combustion

how much improvement you have made inthe combustion efficiency, and how muchmoney you have saved your customer.

Combustiontest equipment

Combustion testing equipment can beseparated into two groups: the manualinstruments (Figure 7-7), and the continu-ous sampling digital electronic instruments,Figure 7-8. The manual equipment has been

older units, there is no way you can accu-rately determine whether the chemicalreaction is complete by just observing thefire.

The most important setting you make onthe burner is excess air. You cannot really dothis without instruments. If you give testinga fair trial, you will find it will reduce thetime required to accurately service, trouble-shoot, and adjust a heating system.

Principles ofcombustion testing

Combustion test instruments measure thecomposition and temperature of flue gasesas they leave the boiler or furnace. We usethis information to calculate the amount ofexcess air and the combustion efficiency.We also measure the amount of smoke anddraft produced in order to properly adjustthe flame and identify problems.

Combustion testing measurements:

• Temperature of the flue gases

• Draft produced by the chimney,power-venter or venting system

• Smoke concentrations in theflue gases

• Composition of the flue gases (excessair, CO

2, O

2, and NOx)

• Carbon Monoxide concentrations inthe flue gases

The three things we adjust that affect thecombustion process are: fuel pressure,combustion air, and draft. Other factors canaffect the combustion process, includingimpingement, excess air leaks into the heatexchanger, insufficient combustion air dueto tight construction or improper ventila-tion, or an improperly installed ventingsystem.

If you perform combustion testing priorto and after a tune up, you can calculate

Figure 7-7: Manual instruments

Figure 7-8: Continuous samplingdigital electronic instruments

Chapter 7Combustion


pipe near the boiler or furnace outlet (thebreech), see Figure 7-9.

With electronic testers, the hole may haveto be larger to accommodate the probe. Thehole should not be in an elbow.

With the older test equipment, you willwant to drill two holes to speed up theprocess. They should both be as close to thebreeching (the place where the flue pipeconnects to the furnace, boiler or waterheater) as possible.

The holes should be at least 6" from thedraft regulator on the furnace or boiler sideof the regulator. There is no need to plug theholes in the stack, but we do suggest thatyou insert self-threading screws or snap capsin the holes.

You will also need a hole in the fire orobservation door over the burner. Some newunits do not have a door over the burner. Ifthis is the case, check to see if the manufac-turer has provided a special port for you todo your over-fire test. See Figure 7-6.

used for many years and can producereliable results if used and maintainedproperly.

The problem is, testing with the oldermanual equipment is time-consuming andonly gives you a fuzzy snapshot of theburner performance. (Each squeeze of awet kit bulb represents a different snapshotof the flue gas. A manual test blends allthose snapshots together into one reading.)

The digital equipment is much quicker,and does efficiency calculations automati-cally. The best feature of the digitalequipment is that they sample continu-ously, like using a video camera, so youcan see the results change as you make theadjustments. It gives you a much betteridea of what is going on.

Holes for testingMeasurements are taken through a three-

eighths (3/8) inch hole drilled in the flue

Figure 7-9:Measuring forcombustionefficiency

Chapter 7Combustion

A. Locate hole at least one flue pipe diameter on the furnace or boiler side of draft control.

B. Ideally, hole should be at least two flue pipe diameters from breeching or elbow.

Horizontal Flue Connection Vertical Flue Connection

Chimney

Note A

Note B

Flue Pipe

Draft RegulatorLocation forSampling Hole

Oilburner

Location forSampling Hole

Note B Note A

Breeching

7-12 Combustion

Stack Temperature: With manualequipment, two holes will speed uptesting and the thermometer should beplaced into the unit on start and the restof the testing done in the other holewhen the temperature reading stabilizes.

Draft: Do draft second because theother tests will be affected by anyincrease or decrease of draft.

Smoke: To adjust for zero smokenumber, you start by opening the burnerair shutter or adjusting the head. Then

Figure 7-12:Orsat tester

Figure 7-13:Stackthermometer

Chapter 7Combustion

Figure 7-10:Draft gauge

fine-tune the air to reach a zero smoke.

CO2 or O2 : This is done last becauseif draft or smoke are wrong, it does notmatter what CO

2 is—you cannot leave the

unit that way.

Calculate Efficiency: Once the air isadjusted properly, take a final tempera-ture reading, then compare the CO

2

reading and the net stack temperatureusing the slide ruler to get the combus-tion efficiency of the unit.

Important: Make a record of theresults of each test on a CombustionSurvey Form, which should show allbefore and after readings. Then you willknow just how much improvement youare achieving.

Figure 7-11:Smoketester

Step-by-step testingprocedureThe traditional order for taking thesetests is:

We also need a vacuum gauge to determinethe condition of the oil delivery system.(See Chapter 4.)

Steady stateFor accurate test results, measurements

should be made after the unit has achievedsteady state. Steady state is the point atwhich the stack temperature stops rising. Atsteady state there are no changes in thecombustion gases, the unit has thoroughlywarmed-up and will maintain constantconditions as long as the burner runs. Thiswill require you to run the unit for 5 to 10minutes, until the stack temperature reachesits highest point and levels off.

Stack temperatureThe stack (flue gas) temperature is the

temperature of the combustion gases leavingthe appliance, and reflects the energy that didnot transfer from the fuel to the heatexchanger. The lower the stack temperature,

Manual combustiontest equipment

To successfully adjust oilburners withthe manual equipment, we need thefollowing:

1. To test for draft, we need a draftgauge. (Figure 7-10).

2. To test for smoke, we need a smoketester and a smoke scale. (Figure 7-11).

3. To test carbon dioxide (CO2) we need

an Orsat tester. (Figure 7-12).

4. A stack thermometer is used tomeasure the temperature of the flue gases.(Figure 7-13).

5. We need the combustion efficiencyslide ruler to calculate efficiency.

While not needed to calculate efficiency,it is important to know if the oil pressure tothe nozzle is correct, and to measure thiswe need a pressure gauge capable ofreading up to at least 300 pounds pressure.


Figure 7-14:Relationship ofexcess air, flametemperature,and volume ofcombustion gases

Figure 7-15: Efficiency vs. net stack temperature

Chapter 7Combustion

Net Stack Temperature °F

Eff

icie

ncy

%

Efficiency vs. Net Stack TemperatureNo.2 Fueloil, 12% CO2

% Excess Combustion Air

Fla

me

Tem

per

atu

re °

F

Volu

me

of

Ho

t C

om

bu

stio

n G

as,

Cu

bic

Fee

t P

er G

allo

n o

f F

uel

Bu

rned

the more effective the heat exchanger designand heat transfer. Stack temperature is ameasurement of the heat exchanger’s abilityto draw the heat from the combustion gases.

As the excess air goes up, so does thestack temperature, Figure 7-14. To under-stand why this happens, we must look atthe heat exchanger. The longer the combus-tion gases are in the heat exchanger, themore the heat the exchanger can pull fromthem, and the lower the stack temperaturewill be. As the excess air increases so doesthe volume of combustion gases.

The volume of gases traveling throughthe heat exchanger determines how fast thegases must travel. The more air we put in,the faster it goes, and the less time theexchanger has to suck the heat out of thegases.

Therefore, as the excess air goes up, thestack temperature does too, even though theflame temperature is reduced. As stacktemperatures go up, efficiency comesdown, and our customers’ heating costsincrease, see Figure 7-15.

The flue gas temperature andall other tests should be measuredin the flue gas hot spot. This isthe point in the center of the fluewhere the stack temperature andthe CO

2 are at the highest level

and the O2 is at its lowest level.

The primary importance ofstack temperature is to provideenough heat in the flue to preventwater formation. If the tempera-ture is not high enough, water inthe combustion gases can con-dense in the flue pipe or chimney.Condensing in non-condensingappliances can cause chimney

7-14 Combustion

Chapter 7Combustion

Draft is the flow of air andcombustion gases through the

burner, heating unit, andventing system. Draft is

required to remove the fluegases from the heat exchanger.

deterioration, liner failure, and rusting ofthe appliance.

Take a gross stack temperature reading,subtract the temperature of air entering theburner (ambient air temperature) from thegross stack temperature to get the net stacktemperature needed for calculating effi-ciency. The relationship between thepercentage of excess air in the flue gases(CO

2 or O

2) and the net stack temperature

is the combustion efficiency.

Causes of high stack temperature• Soot Deposits: The insulating effect of

soot deposits prevents good heat transferthrough the heat exchanger. Inspect the heatexchanger, clean if necessary, and adjust theburner to a zero to trace smoke.

• Excess Air: Excess air cools thecombustion gases and increases theirvolume. This results in lower heat ex-changer efficiency. Excess air can be due topoor air-fuel mixing, poor burner adjust-ment (more air than needed to stop smoke),and air leaks into the heat exchanger.

• Overfiring: Firing a heating system ata higher GPH (gallons per hour) than itwas designed for causes high rates of gasflow through the heat exchanger and resultsin high stack temperature.

Dew point temperatureThe dew point temperature is the

temperature below which the water vaporcontained in the flue gas turnsinto a liquid. This change isoften referred to as condensa-tion. Below the dew pointtemperature, moisture exists;above the dew point tempera-ture, vapor exists. If the

chimney or venting material falls below thedew point temperature, condensation in theflue will occur.

To prevent condensation, the stacktemperature should range from 270°F to370°F above the ambient air temperaturefor non-condensing appliances. (With newhigh efficiency equipment that does nothave a draft regulator, combustion gasescan be on the low end of this range; if thereis a draft regulator, they should be closer tothe high end.)

With the new condensing appliances, thestack temperature will be close to the returnair or water temperature from the heatingsystem and usually below 125°F. The lowerthe heating system return air or watertemperature, the higher the efficiency willbe on a condensing appliance.

Using the dial typestack thermometer

Slide the holding clip out to the end ofthe thermometer stem. Insert the small tabinto the top of sampling hole in the stackand push the thermometer in far enough sothe tip is in the center of the flue pipe.Operate the burner until the thermometerreading is rising no faster than 3 degreesper minute and record the reading. If thestack temperature begins to approach 1,000degrees, remove the thermometer, becausereadings above this temperature willdamage it.

DraftDraft is the flow of air and combustion

gases through the burner, heating unit, andventing system. Draft is required to removethe flue gases from the heat exchanger.

If draft is too low, then the combustion


Chapter 7Combustion

An importantthing torememberwhen using adraft gauge isto “zero it

out” beforeusing it

air flow will be reduced, causing excessivesmoke. If the draft is too high, then toomuch excess air will be drawn into the unitand the combustion gases will be pulledthrough the heat exchanger too fast,lowering efficiency.

During steady state operating conditions,the draft should be stable. Over-fire draftsof -.01" to -.02"wc [water column, seebelow] are generally recommended forresidential units. Measurement and adjust-ment of draft are important because draftaffects all other burner adjustments.

Draft measurementTo measure draft, we use a manometer, a

U-tube or a gauge. The U-tube is a glasstube bent into the shape of a U. The tube isfilled with water up to a zero mark on ascale etched on the tube. A sampling tube isattached to the right leg of the tube, and itis inserted into the unit to be tested. Theleft leg is left open to atmospheric pressure.

The draft on the right leg causes apressure drop on that side and pulls thewater up in the tube, and the atmosphericpressure on the open side pushes the waterdown in the left leg.

The difference between the water levelsin the two legs is the draft in inches ofwater column. The suction from draft is soslight the difference is only a few hun-dredths of an inch. One one-hundredth ofan inch of draft is expressed as -.01"wc.

Since it is a vacuum and not a pressure,we call it negative draft. If the right legmeasured a pressure higher than atmo-spheric the water would go up in the leftleg and we would call it positive draft.

Our modern draft gauges (manometers)are the mechanical equivalent of the U-

tube. An important thing to remember whenusing the draft gauge is you must “zero itout” before using it. Atmospheric pressurechanges all the time, so we take thisvariable pressure out of the equation byadjusting the gauge reading to zero beforewe take each test.

To zero out the draft gauge place it on alevel surface near where you are going totest and slide the lever on the side of theunit until the arrow points to zero. Youshould also be sure the gauge is functioningproperly by spinning the rubber sampletube or blowing gently across the end of itto be sure the needle is not stuck. It shouldmove smoothly and return to zero.

The most important draftreading on residential equipment isthe over-the-fire draft. On mostresidential and light commercialnegative draft units, this readingshould be -.01" to -.02"wc. If uponreaching steady state you cannotobtain this reading, you shouldnext check the draft at the breech,and adjust the draft regulator. Ifyou still cannot achieve sufficientover-fire-draft, then you probablyhave one of the following problems.

Common causes of poor draft

• Chimney is too small for the load ofthe attached appliances

• Chimney is too large in diameter orcross-section

• Chimney too short or improperlyconstructed

• Leakage of air into chimney, thimble,stack, or breeching

7-16 Combustion

Figure 7-16:Effects ofleaving a unitoperating witha 5 smoke

Chapter 7Combustion

Per

cen

t o

f F

uel

Was

ted

Per

cen

t o

fC

om

bu

stio

n E

ffic

ien

cy

Net

Flu

e G

asTe

mp

erat

ure

°F

• Obstruction in chimney or at topof chimney

• Top of chimney is lower than the peakof the house

• Flue pipe diameter is too small. Itshould never be smaller than thebreeching size

• Too many 90 degree elbowsin the stack

• Flue pipe does not have sufficientpitch upward from breeching tothimble; it should be at least ¼" perrunning foot

• Flue pipe extends too far into thechimney flue

• Heat exchanger passages are cloggedwith soot and scale, too restricted, ortoo baffled

• Appliance is overfired; the volume offlame and combustion gases is toogreat for the heat exchanger designand are creating back pressure or apositive draft over the fire

• Unit is underfired, so that the chim-ney gases never get hot enough tocreate normal draft conditions

• Insufficient ventilation or combustionair in the appliance room, starving theflame for air and draft flow

• Improper adjustment of thedraft regulator

• Differences between breeching andover-fire draft (draft drop): More than.05" difference between the two draftreadings usually means trouble. Checkfor heavy soot deposits in the heatexchanger, particularly if the burnerwas found with a heavy smoke

• Little or no difference between thebreeching and over-fire draft: If thestack temperature is high, baffling orheating unit replacement should beconsidered

• Over-fire CO2 higher than breeching

CO2: Check for air leaks between top

of combustion chamber and heatexchanger outlet (Breeching)

• Visible openings for air leakage:Study how the heating unit is put

Total Burner Operating Time (Hours)


Figure 7-17: Smoke vs. percent CO2 curve

Chapter 7Combustion

Figure 7-18:Smoke scale

Percent CO2

Sm

oke

Sp

ot

Nu

mb

er

together and check all joints for possibleleaks. If you suspect air leaks, but cannotfind them, temporarily set the draftregulator to give maximum draft and run acandle or lighted match around the joints tosee where the smoke is drawn in. Repairthe leaks and set the over-fire draft to -.01"to -.02". Also, look for locations wherelight from the flame can be observedthrough cracks.

SmokeAfter you have determined that draft

conditions are correct, the next step is toadjust the burner to create a flame that willnot produce smoke.

Keeping smoke to a minimum is a mustif the heating system is to operate at peakefficiency without further attention duringthe entire heating season.

We measure smoke concentrations bytaking a sample of the flue gases with asmoke tester and smoke scale, whichidentify smoke numbers from zero (no

smoke) to a number 9(heavy smoke). Any smokenumbers above a number 2cause rapid soot accumula-tions on heat exchangersurfaces. This causes fluegas temperatures to rise andefficiency to drop, Figure7-16.

Excess combustion air isneeded to reduce smokeconcentrations to a mini-mum acceptable number(less than a number 1).

As we have discussed,this excess air must be heldto a minimum in order toreach peak efficiency. Thus,correct burner adjustment isthe proper balance betweensmoke and combustion air.

To reach high efficiency,we must adjust burners forZero Smoke and low excess

air whilemaintain-ing a highlevel ofCO

2 or a low level of O

2,

Figure 7-17.

The typical smoketester draws a total of2,200 cubic centimeters(ten full strokes of thesampling pump illus-trated) of flue productsthrough a standard gradefilter paper. The color ofthe resulting smoke stainon the filter paper ismatched to the closestshaded spot on the

7-18 Combustion

When pumping ten full strokes of the smoke testerpump, do not pump too fast. Hold the handle at its mostextended position a second or two between strokes. If

you take morethan one samplefrom a burner, usethe same uniformmethod of pump-ing each sample. Ifyou fail to followthis method, youcould get twodifferent smokenumbers, eventhough you havemade no adjust-ments to theburner.

Chapter 7Combustion

standard graduated smoke scale, Figure 7-18. This scale has ten shaded spots with anequal difference between successive spots.Spot Number 0 is white, and representssmoke-free combustion. Spot Number 9(darkest) represents the maximum smokethat typically will be produced.

An oily or yellow smoke spot on thefilter paper is a sign of unburned fuel,indicating very poor combustion (andlikely high emissions of carbon monoxideand unburned hydrocarbons). If too muchexcess combustion air is supplied, thecombustion process will be chilled so muchthat some of the fuel cannot burn. Theflame is actually being blown out. Furtherevidence is liquid fuel in the heat ex-changer, white smoke (fuel vapor) andstrong odors outside the home.

To adjust for a zero smoke, first adjustthe burner for a trace smoke then open theair gate just a bit farther to go to zero

smoke. By initially adjusting to a zerosmoke you will be introducing an unknownamount of excess air which can lead to:lower CO

2, cooler flame temperatures,

higher stack temperatures, and possibleelevated carbon monoxide production.

Remember that excess air cools thecombustion products and increases theirvolume so that it is difficult for the heatexchanger to absorb the heat before itescapes to the breeching and up the chim-ney. Most new burners are designed tooperate at zero smoke.

Common causes of smoke and soot:Poor fuel atomization: small fuel

droplets vaporize quicker than large ones.Large droplets are caused by:

• Damaged or worn nozzles

• Low fuel pump pressure

• Cold oil

Inadequate combustion air is caused by:• The burner air control is not open

far enough

• Poor chimney draft

• Build up of soot and scale in theheat exchanger

• Accumulation of lint, hair, sawdust,and dirt on the air shutter and theburner fan

• Restrictions of the air flow to theroom the burner is in, and the oper-ation of exhaust fans that de-pressurizethe building

Effects of insufficientcombustion air

For the proper operation and venting ofgas or oilheating appliances, sufficient


Figure 7-19:Calculatingconfined space

Chapter 7Combustion

no doors or with fully louvered doors canbe considered part of the unconfined space.

Note: If the actual free area of thelouvers is not known, wood louvers areassumed to have a 20% to 25% freeopening. Metal louvers or grills areassumed to have 60% to 70% free opening.

Calculatingconfined spaceExample:A room 20' by 30' with an 8' ceiling heightand the heating appliance is 140,000 Btu.See Figure 7-19.

Determine the maximum total inputfiring rate allowable in a room withoutmodification.

Example: Boiler room 20x30x8 =4,800 cu ft.

4800 cu. ft. x 1000 Btu/50 cu. ft. =96,000 Btu

96,000 Btu x 1 gph #2 fuel/140,000 Btu =0.69 GPH

Result: If you fire greater than 0.69 GPHor 96,000 Btus, you will need additionalcombustion air.

outside air must be supplied to the structureto make up for the air lost from ventingheating appliances, fireplaces, clothesdryers, exhaust fans and other building airlosses.

Insufficient combustion air can causemajor problems for proper draft andoperation of both gas and oilheatingsystems.

For years it has been assumed that whena heating appliance was located in anunconfined area, there was sufficient air forboth ventilation and combustion. Today, inmost cases, that is not true! New construc-tion standards, insulation, weather strip-ping, and energy efficient windows anddoors have reduced the amount of airchanges per hour.

The combustion and make up airrequirements in the codes are based on 1/2air changes per hour.

For newer homes and conversion ofelectrically heated homes, the air changescould be reduced to 1/3 or less air changesper hour. Air problems are most notable onthe coldest days when heat loss is thegreatest and there is a chance that windowsor doors are closed for an extended periodof time.

When installing new equipment ortroubleshooting problem equipment, thefirst determination that needs to be made iswhether the equipment is located in aconfined or unconfined space. In accor-dance with NFPA 31 and NFPA 54, anunconfined space is defined as follows:Any space whose volume is equal to orgreater than 50 cubic feet per 1,000 Btu (or20 Btu/Cubic Foot). This is calculated onthe sum of the total input ratings of all fuelburning appliances installed in that space.Only areas connected to the space that have

7-20 Combustion

Figure 7-20:Relationshipbetween excessair and CO2

Chapter 7Combustion

Percent Excess Air

Per

cen

t C

O2

in F

lue

Gas

Measure Percent CO2 to DeterminePercent Excess Air

To add air from an adjacent room, twoopenings between the room could be made12 inches above the floor and 12 inchesbelow the ceiling. The size of theseopenings is based on 1 square inch per1,000 Btu input.

To add air directly from the outside ofthe structure, two openings could be made.The size of these openings is based on 1square inch per 4,000 Btu input. The aboverequirements are based on guidelines inNFPA 31 or NFPA 54.

Alternately, if operating in a confinedspace, additional air may be added by aduct to the outside, sized on 1 square inchper 5,000 Btu input.

Incomplete air-oil mixingImproper mixing creates fuel rich and

fuel lean pockets in the combustion areaand prevents complete burning. Some ofthe causes are:

• Mismatch of the fuel droplet sprayand the burner air pattern

• Inadequate air swirl and turbulencecaused by outdated burner design

• Improper burner head size

• Improper adjustment of air handlingparts of the burner

• Irregular or unbalanced fuel spraycaused by a partially plugged nozzle

• Off center installation of the nozzle

• Dirt or soot accumulation on burnerair forming parts or air fan blades

• Defective or damaged burner partssuch as the end cone, air tube, fan,and motor coupling

• Using a nozzle that is either too smallor too large for the burner head andcombustion area

• Fuel pump pressure set too lowor too high

• Cold oil producing larger fueldroplets, and increasing the firingrate beyond the air setting


Correlation of Percent ofCO2, O2 and Excess Air

Carbon Oxygen Excess AirDioxide (Approx.)

15.4 0.0 0.015.0 0.6 3.014.5 1.2 6.014.0 2.0 10.013.5 2.6 15.013.0 3.3 20.012.5 4.0 25.012.0 4.6 30.011.5 5.3 35.011.0 6.0 40.010.5 6.7 45.010.0 7.4 50.0

The ranges that you will use mostfrequently are bold-faced and incolor.

Figure 7-21:Relationship ofCO2, O2 andexcess air

Chapter 7Combustion

Percent Excess Air Percent Excess Air

PercentCO2

PercentO2

(Oxygen)

Flame impingementThe flame must not touch any solid

surfaces of the burner, the end cone, thecombustion chamber, or the heat exchanger.If this happens, the flame will be cooledand the unburned carbon atoms will turn tosmoke and soot. Possible causes:

• Overfiring, too large a nozzle orexcessive oil pump pressure

• Incorrect oil and/or air pattern

• Burner installed too high, too low, oroff center

• Incorrect combustion chamber size orshape, or base of the chamberfull of soot

• Partially plugged nozzle

• Cold oil

Smoky shut downCheck the cut-off valve by using a

pressure gauge in the fuel pump nozzleport. See the nozzle and fuel unit chaptersfor causes of after drip. (Chapters 4 and 5).

Excess air: CO2 and O2As we add excess combustion air, the

amount of nitrogen and unburned oxygenincrease. The amount of water and carbondioxide remain the same because we havealready burned all the fuel; therefore the

7-22 Combustion

Figure 7-22:0% Excess air

Figure 7-23: 50% Excess air

Chapter 7Combustion

(Amount of water vaporis not considered in % ofCO2 determination

1.18 lb. water11.02 lb. nitrogen3.16 lb. carbon dioxide

15.36 lb. total

1.00 lb. oi l14.36 lb. air15.36 lb. total

1.18 lb. water11.02 lb. nitrogen3.16 lb. carbon dioxide7.18 lb. excess air

22.54 lb. total

1.00 lb. oi l21.54 lb. air22.54 lb. total

percentage of CO2 drops and the percentage

of oxygen starts to increase. (Figure7-20). By measuring either the percentageof CO

2 or the percentage of unused oxygen

(O2) we can determine the quantity of

excess air. They are the opposite of eachother. As the percent of O

2 increases the

CO2% decreases. (Figure 7-21).

Why we must control excess airProperly adjusting the burner air shutter

is a compromise between too little air,which produces smoke, and too much air,which lowers efficiency. See Figures 7-22and 7-23.

Earlier we found that a fixed quantity ofcombustion air is required to burn eachpound of oil. We call this the theoreticalfuel-air ratio.

Oilburners require a controlled amountof excess air above the theoretical value toassure complete combustion and smoke freeoperation. This excess combustion airserves as a safety margin to preventincomplete combustion and smoke.

The air pressure in the building isalways changing, the temperature of thefuel changes, and the draft produced by thechimney is not constant. Excess air gives usthe safety factor we need for all thesevariations.

While excess air is needed for reliableclean operation, it also reduces efficiency.When increasing air for more oxygen,excess nitrogen comes along for the ride.This dramatically increases the amount ofcombustion gases that must be vented. Heatexchangers need time to absorb the heatfrom the combustion gases. The more gasesforced into a heat exchanger, the faster theytravel through the heat exchanger. Thisgives the heat exchanger less time to pullout the heat so the stack temperature goesup, and the efficiency goes down.

Limiting the amount of excess air canincrease efficiency and save our customersmoney.

We measure excess air by determining


Figure 7-24:An analyzer

Chapter 7Combustion

Valve

Scale

Flexible Diaphragm

the percentage of oxygen (O2) in the

combustion gases. For each 1% decrease inoxygen levels introduced into the combus-tion process, efficiency increases by up to1%. While some excess air is needed forcomplete combustion, we must set the airto the manufacturer’s specifications toensure maximum efficiency.

Another problem with excess air is thatit cools the flame. This creates incompleteburning in the combustion area and carbonmonoxide levels can rise. Therefore wemust exactly control the amount of excessair we allow into the burner. The only waywe can achieve this delicate compromisebetween smoke and efficiency is by usingcombustion test equipment. You cannot seeflue gases; the only way you can be sureyou are right is testing.

CO2 measurementThe Orsat CO

2 analyzer, named for its

inventor, uses chemicals that absorb CO2

from a mixture of gases without absorbingany other gas. The chemical usually used ispotassium hydroxide (KOH) because it hasthe capacity to absorb large amounts ofCO

2. When it does so, it expands. By

measuring how much it expands we candetermine the amount of CO

2 in the gases.

There are two main partsto this analyzer:

1. The sampling pump:

A. The sample tube that is inserted intothe stack gases, or replaced by a longertube for over-fire sampling.

B. The yarn filter and water trap, whichstops soot and water from entering theanalyzer.

C. The sample pump, a rubber bulb withrubber flapper suction and discharge checkvalves that allow flow in only one directioninto the analyzer.

D. The rubber connector, which seals the

sampling pump system to the analyzer.

2. The analyzer:

A. A body, which has two cavities or“cups” at the top and the bottom, connectedby a narrow tube with an adjustable scalealongside. (Figure 7-24).

B. A valve system that either seals thegases and liquid inside the analyzer, or elselets a sample be pumped into the top cavitywhile the narrow tube and the lower cavityare sealed off.

C. A diaphragm, or flexible disc in thebottom cavity, which prevents a vacuumfrom forming inside the analyzer and letsthe liquid in the bottom cavity be drawn upinto the narrow tube after CO

2 is absorbed.

Using a CO2 analyzerA. Prime and “wet” the instrument by

tipping it over and then back once, andallow the fluid to drain from the uppercavity while holding the instrument at a 45-degree angle. Next, hold upright anddepress valve on top several times andrelease the valve. Loosen the lock nut onthe sliding scale on the rightside. Slide the scale until “0”lines up with top of fluid inthe center tube. Tighten thelocknut.

B. Insert the sampling tubeinto the stack to draw the gasesto be sampled through thesampling hole in the stack. Thisgives you a stack CO

2 reading.

However, under certainconditions, it is desirable todetermine the over-fire CO

2. To

do this you must replace theshort metal sampling tubewith a ¼" metal tube about30" long. This must extendthrough the sampling hole inthe fire door to a point above

7-24 Combustion

Chapter 7Combustion

Study the instructionmanual and take the

time to learn how youranalyzer works before

you expect it toperform for you. Besure to follow the

manufacturer’srecommended

maintenance andservice procedures.

measure the temperature of the flame dueto dilution of the gases and absorption ofthe heat by radiation to the surroundingareas, we use a combustion equation todetermine the quality of the combustionprocess. The efficiency calculation is basedupon the theoretical heat value of the fuelbeing burned minus the stack losses.

To calculate the combustion efficiencywith older test equipment, you need toknow:

1. The Net Stack Temperature: This isthe measured or gross temperature asindicated by the stack thermometer, minusthe ambient or intake air temperature.

2. The % CO2 in the Stack Gases: Do

not use the % CO2 measured over the fire.

This is for air leak checks only. Test theCO

2 at the breech.

To calculate the steady state efficiency,use combustion efficiency tables or thecombustion efficiency slide ruler. Adjustthe large slide to the right so that the netstack temperature appears in the smallwindow in the upper right corner marked“Net Stack Temperature”; then move thesmall vertical slide until the arrow points tothe CO

2 reading. Through the window in

the arrow on this slide you now read thefigures: combustion efficiency in black andon-cycle heat loss in red.

Electronic combustiontest equipment

One drawback of manual combustiontest instruments is the time required to usethem properly. This is especially true whenthe tests must be repeated during oilburnerfine-tuning. New digital electronic testinginstruments have been developed in recent

the center of the flame inthe combustion chamber.

Place the rubberconnector end of samplepump on top of valve andpush it down to open thevalve. Squeeze and releasethe rubber bulb 18 timesto pump combustion gasesinto the analyzer. Releasethe valve while the bulb isstill collapsed on the 18th

stroke, and remove therubber connector from the instru-ment.

C. Tip the analyzer completelyover and back twice, allowing gas tobubble completely through the

fluid. Hold at 45 degree angle for 5seconds to complete draining of fluid. Thefluid has now absorbed the CO

2 from the

gas sample, and has been pulled up in thecenter tube. Hold the analyzer upright andread the CO

2 on the scale opposite the top

of the fluid.

D. Depress the valve on top of analyzerseveral times, release, and check to see thatthe zero setting has not changed. If it has,repeat the above steps.

E. If the CO2 analyzer is cold, run one

or two tests to warm it up before using thereading.

NOTE: The CO2 test fluid is hazard-ous. Do not invert analyzer while theplunger valve is depressed. If fluid getson your hands, wash them immediately.

Combustion efficiencycalculations

Since it is not possible in the field to


Chapter 7Combustion

years that can reduce the time needed foroilburner testing and adjustment.

Digital instruments are faster, moreaccurate, more reliable, and have a higherrepeatability. Digital instruments stay incalibration, allow trending, allow morecomplex functions and save time.

Digital instruments allow data to berecorded and reported without human error,and provide reliable and accurate results foryou and your customers. Data can berecorded much faster than any techniciancould ever do the calculations.

With the optional printer, the data can berecorded as an un-editable record, so theprint out is what was measured at thejobsite. Permanent records allow the user totrack system changes and determine if thesystem is operating within the designparameters or if changes have taken place.

Only digital analyzers allow you to takereal time tests.

Study the instruction manual and takethe time to learn how your analyzer worksbefore you expect it to perform for you. Besure to follow the manufacturer’s recom-mended maintenance and service proce-dures. Most manufacturers ask that you notexpose the analyzer for extended periods oftime to freezing temperatures, as this has animpact on sensor life. In the winter, do notleave the tester in a cold truck over night.

The electronic combustion analyzers useadvanced methods of measuring flue gascomposition and temperature through asingle probe.

A pump within the electronic analyzerdraws a flue gas sample through a series ofsensors. These electronic analyzers are

capable of measuring oxygen percentage(O

2), carbon monoxide (CO), carbon

dioxide (CO2- calculated), excess air, draft,

stack and ambient temperatures and manycan even determine other gases like oxidesof nitrogen (NO, NO

2, NOx) for advanced

flue gas emissions level readings. The onlything they cannot measure is smoke.

Since the electronic analyzers aresensitive to carbon build-up, to extend thelife of the sensors and instrument, takesmoke tests with a smoke tester, and adjustfor a low smoke reading before using theelectronic analyzers.

The electronic analyzers eliminate thecumbersome work required by the manualtest equipment and more importantly, theyeliminate human error caused by varyinginterpretation of results we have withmanual equipment. With the electronicanalyzers, any two technicians will getidentical results.

The use of these electronic instrumentsalso indicates to customers that you are aprofessional equipped with the most up-to-date testing equipment. It adds to yourimage as an energy expert. This can assistwith customer acceptance of energyconservation options, such as flameretention burners and new boilers andfurnaces.

Some of the electronic instrumentscompute efficiency and display the resultson a printed readout. This can be used todemonstrate to the homeowner the actualefficiency of their boiler or furnace. Thisinformation can serve as a sales tool toidentify the fuel savings that are availableby installing new equipment and can alsoprovide a written record that everythingwas as you say it was when you left the job

7-26 Combustion

Chapter 7Combustion

site.

Measurement procedureConnect the flue gas sampling probe

assembly to analyzer per manufacturer’sinstructions. Verify the condensate trapplug is properly seated in combustionanalyzer, and that water trap and thermo-couple are not touching the side of theprobe assembly. Before inserting the probeinto the unit, start the analyzer and allow itto do its start up procedure. This is calledthe zeroing process. Never allow the

analyzer to zero in the stack unlessmanufacturer’s design allows this.

As with older test equipment, measure-ments must be made in the stack betweenthe barometric damper (if equipped) andthe breech. Allow burner to operate untilstack temperature stabilizes before testing.

Observations to make before testingWith experience, you will be able to

quickly pinpoint the likely problems in theheating unit by looking at the instrumentresults, and making a few quick observa-tions.

• Look at the flame. Is it normallyshaped? Is it lopsided? Is it striking thesides or floor of the combustion chamber?(Be sure the flame observation door isclosed when you do your tests. The airflowing into the open door will lower theCO

2 readings and raise the stack tempera-

ture.)

• Check the burner-operating period.Burning periods of less than 5 minutes donot usually produce enough heat to givegood results. Short burner operatingperiods can be caused by overfiring of unit,or by improper functioning of automaticelectrical controls.

• Watch the flame while the burnershuts down. A flame that lasts more thantwo seconds after the burner shuts off maybe due to a poor cut-off. The primary twocauses are air in the oil or the cut off in thefuel unit is defective. Refer to the FuelUnits Chapter for testing pump cut off.(Chapter 4).

• Test for air leaks around the burnerair tube, the combustion chamber and allaccess doors on the furnace. A commoncause of low CO

2 readings is air leaks into

the unit. To see if the problem is air leaks,take a CO

2 test over the fire. If it is higher


Chapter 7Combustion

than the reading at the breech, you have airleaks into the heat exchanger. Because ofleakage, air has been pulled into the heatexchanger passages by the negative draftwithin the unit, lowering system efficiency.

Keep a recordof efficiency tests

They have a saying in the nursingprofession, “If you didn’t write it, youdidn’t do it.” Why go to all the trouble oftaking these tests if you don’t brag aboutit? Create or buy an efficiency test reportform, and use it to record the results ofyour efficiency tests. These reports areimportant for three reasons:

1. They serve as a starting point fordiagnosis and service.

2. Efficiency test results can be valuablesales tools when trying to convince custom-ers to upgrade their equipment.

3. Having a written record can helpprotect you if there is ever a future prob-lem.

With most of the electronic combustiontest equipment available today, a printeroption is available. This gives you proofthat you performed the test. Additionally, itprovides time, date, and exact informationas to burner performance when you left theunit. This can be invaluable when having toprove the burner operation at the time youmade the adjustments.

Typical Combustion Test Readings

Non-flame retention burners:Oxygen: 5 - 9%

Flame Retention Oxygen:3 - 6%

Carbon Dioxide:10–12.5%

Stack Temp:60–79% Efficiency 400ºF to 600ºF

Stack Temp:80+ Efficiency 330ºF to 450ºF

Stack Temp:90+ Efficiency less than 125ºF

Draft:-0.02"wc Over fire

Draft (Stack):-.04"wc to -.06"wc

Carbon Monoxide:Less than 50 ppm (diluted)

Smoke spot:Zero to #1

Always Follow Manufacturer’s Specifications

Report to the customerYour report to the customer can be

written or oral. A properly written report,including a conservative estimate of savingsyou anticipate from any changes you havemade or suggested, can build customergood will and increase equipment sales.

Combustion air testModern buildings are much tighter than

the old buildings; some do not allow enoughair to leak into the building from theoutdoors to replace the air going up thechimney. Winterization practices on olderhomes have sealed many of the openingsthat formerly provided combustion air.

If the building does not allow enoughinfiltration air in, provisions must be madeto bring in the outdoor air to replace the airused in the combustion process. Thefollowing test will tell you if you haveenough infiltration.

7-28 Combustion

Chapter 7Combustion

1. Visually inspect the venting systemfor proper size and horizontal pitch and besure there is no blockage, restriction,leakage, corrosion, or other deficiencies thatcould cause an unsafe condition.

2. Shut off the unit and any other fuel-gas-burning appliance within the sameroom.

3. Inspect burners for blockage andcorrosion.

4. Furnaces: Inspect the heat exchangerfor cracks, openings, or excessive corrosion.

5. Boilers: Inspect for evidence of wateror combustion product leaks.

6. Insofar as is practical, close allbuilding doors and windows and all doorsbetween the space in which the appliance islocated and other spaces of the building.Turn on the clothes dryer. Turn on anyexhaust fans, such as range hoods andbathroom exhausts, so they will operate atmaximum speed. Close fireplace dampers.

7. Place the appliance being inspected inoperation.

8. Determine that the burner ignitionand operation is satisfactory.

9. Turn on all other fuel and gas-burning appliances within the same room.

10. Determine that all units are burningproperly.

11. Use a monometer to test if thepressure in the room that the unit is in isless than the pressure outdoors.

12. Return doors, windows, exhaust fans,fireplace dampers, and any other fuel-gas-burning appliance to their previous condi-tions of use.

Testing for a leakingfurnace heat exchanger

A combustion analyzer can be used forfinding leaking cracks or holes in a furnaceheat exchanger. The static pressure createdby the system blower can overcome anypositive pressure in the heat exchangercausing air to leak into the fire side of theexchanger rather than out.

Readings that change when the blowercomes on after stabilization has taken placeare indicative of a combustion air, venting,or mechanical problem such as a crackedheat exchanger. Furnaces can have cleanoutdoors leaking that will test positive forleakage. This is not a heat exchangerfailure. Inspection door gaskets should bereplaced and doors should be properlysealed.

Procedure:1. Follow the manufacturer’s instruc-

tions to properly zero the combustionanalyzer. Insert the combustion analyzerprobe in the flue pipe.

2. Start the furnace and observe theoxygen reading for stability (1-3 minutes).

3. When the blower starts, watch for achange in the O

2 reading. If the O

2 reading

goes up, there is a leak.

Corrective action: Attempt to visuallyfind the crack or hole.

A. If you can find the defect, show itto the customer.

B. On the service invoice, write thatyour testing indicates a leak in the heatexchanger. (Do this even if you cannot findthe leak.)

C. Inform the customer, in writing,that the heat exchanger has a defect andthat the furnace should be replaced. Youshould not attempt to repair a heat ex-changer.


Figure 7-25:CO2 analyzer

Chapter 7Combustion

SamplingTube

PumpSuctionValve

PumpDischarge

Valve

Rubber Connector

Filter & Water Trap

Checking and caringfor the instruments1. Smoke tester

A. Clamp a piece of filter paper in thesmoke tester. Place a finger over the end ofsample tube and try to pull the plungerhandle out and then release. The handleshould return by itself to approximately theoriginal position. If it does not, the hose,paper holder, and plunger should bechecked for leaks. If leakage is indicated,the hose in the sampling line may needreplacement. Leakage may also occur if thesurfaces of the filter paper holder aredamaged. These areas (on the nylon inserts)should be inspected. The plunger can beremoved for inspection if leakage isoccurring across the plunger.

B. A clean piece of filter paper shouldbe clamped in the smoke tester and a 10

stroke sample of fresh air taken after verydark smoke readings have been observed. Ifthe air sample does not give a clean spot,repeat until it does. In extreme cases, thetube, and possibly the pump, may have tobe cleaned with soap and water.

C. Sticky action of the plunger can behelped by coating the plunger cup withpetroleum jelly (Vaseline) after wiping thecup and the bore of the body clean.

2. CO2 analyzerA. After about 200 samples have been

measured with the analyzer, the fluid willbegin to lose its strength. This can bechecked by measuring a sample in thenormal manner, after the height of the fluidis measured on the % CO

2 scale, invert the

instrument two more times, let it drain, andreread. If the % CO

2 reading changes more

than ½ a number, the fluid is becomingweak and should be replaced.

B. The sampling pump should bechecked for leaks in the following manner:

1. Place a finger over the end of thesampling tube and squeeze the rubber bulb.The bulb should stay collapsed. If it doesnot, an air leak exists between the bulb andthe sampling tube, or the discharge valve inthe rubber bulb is leaking, Figure 7-25.

2. Place a finger over the rubberconnector at the other end of the pump andseal off the hole tightly. Apply moderatepressure to the rubber bulb. If the bulbcannot hold pressure, a leak exists betweenthe bulb and the rubber connector or thesuction valve in the rubber bulb is leaking.

C. Any evidence of fluid leakage fromthe analyzer should be checked. Suchleakage is an indication of possible air leaksin the analyzer, which will give inaccurateCO

2 readings.

D. If the CO2 scale will not slide low

enough to adjust the zero setting, it may benecessary to add water or more KOH fluidto bring up the fluid level. Add 2 or 3drops of tap water into the space around thevalve in the top of the analyzer. Depress thevalve and release it several times, allow thewater to drain down, and then check fluidlevel. If the level is still too low, add 2 or 3more drops and repeat. This should be usedonly as a stopgap procedure until you canobtain new potassium hydroxide (KOH)fluid.

E. If the fluid must be changed, refillkits are available and a spare should alwaysbe kept on hand. This kit includes fluid,gasket, screws, and filter yarn for thepump. To change the fluid:

(1) Remove four screws in top ofanalyzer.

(2) Remove top and discard fluid andrubber gasket (do not spill fluid, it isharmful to hands, paint, etc.)

(3) Clean unit using soap and water.

(4) Pour in full bottle of new fluid.

(5) Install new rubber gasket cover, andnew screws. Tighten screws evenly. Thefilter yarn in the sampling pump must alsobe changed. This filter yarn may bechanged more often than the fluid toprovide easy sampling pump operation.

F. The analyzer should be stored in anupright position with the valve on top toprevent any possible fluid leakage. Whenplacing the analyzer inthe carrying case, itshould be placed with itsvalve toward the handleside of the case. Exten-sive storage in warmplaces such as car trunks

during summer should be avoided. Thefluid should be checked for strengthfollowing long storage periods.

3. Oxygen(O

2 analyzers)Measuring the oxygen content of the

surrounding air can check the accuracy ofoxygen sensors. When fresh air is drawnthrough the instrument, a reading of 20.9percent O

2 should be observed. The

instrument should be adjusted to this value.If you cannot get a 20.9% reading, replacethe oxygen sensor.

4. Stack temperature gaugeA. Other than checking for obvious

damage, the only problem that might occurwith a temperature gauge is a bent stem.The gauge will not read accurately with abent stem.

B. Many manual and electronic tempera-ture instruments can be checked in boilingwater (212ºF), and readjusted if needed.

5. Draft gaugeA. Check to be sure the indicator needle

moves freely. This can be done by blowinglightly across, and then toward, the end ofthe sampling tube. The needle should movesmoothly and return accurately to the zerosetting position.

B. The draft gauge should be checkedagainst an accurate manometer at least oncea year.

6. Electronic analyzercalibration

All manufacturerinstructions concerningequipment care andmaintenance must be

followed carefully. Oxygen cells, batteries,and calibration of the instrument must bedone as needed to assure accurate readings.Electronic analyzers should be sent back tothe manufacturer for calibration as requiredby the manufacturer. This will insure7-30 Combustion

Chapter 7Combustion

WARNING:The analyzer fluid is hazardous.Always follow the manufacturer’s

cautions for safe handling.

Co

mbu

stion

Trou

blesh

oo

ting

Gu

ide

Courtesy of Bacharach


00 customer service - noraweb.org

Documents