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Duct Design and InstallationDuct Design and Installation
John Proctor, P.E.
Proctor Engineering Group, Ltd.
San Rafael, CA
1-888-455-5742 [email protected] www.proctoreng.com
2004 Proctor Engineering Group, Ltd.
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Duct DesignDuct Design
ACCA Manuals J, T, and D or Equivalent
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Proper Design & Selection Estimate LoadsProper Design & Selection Estimate Loads
1. Determine Design Heating Loads
2. Determine Design Cooling Loads Sensible and Latent
3. Determine the airflow required to eachroom
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A Good Duct Design ProvidesA Good Duct Design Provides
Quiet
Efficient
Comfort
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Recommended Velocity (in fpm)Recommended Velocity (in fpm)
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EfficientEfficient FAN WATT DRAW REDUCTION
SUFFICIENT AIRFLOW
LOW DUCT LEAKAGE
SEAL WITH MASTIC OR SNAP-DUCT
PUT INSIDE THE CONDITIONED SPACE
LOW CONDUCTION LOSS
SHORT RUNS
FEWER LARGER DIAMETER RUNS
PUT INSIDE THE CONDITIONED SPACE
USE METAL DUCT AND FITTINGS
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ComfortComfort
TEMPERATURE DIFFERENCE
BETWEEN ROOMS
< 2 F IDEAL
4 F MAXIMUM
SO USE A CONTINUOUS FAN? NO
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Why not just add anElectrically Commutated Motor (ECM)
and use a continuous fan?
Why not just add anElectrically Commutated Motor (ECM)
and use a continuous fan?
1. An ECM uses as
much powerrunningcontinuously asa PSC motor
does running onAuto(Scott Pigg,Wisconsin Study)
0
1000
2000
3000
4000
5000
PSC Motor ECM
Annu
alkWh
Continuous Fan Auto Fan
2. Continuous duct leakage losses and conduction losses
3. ECM motors are not magic When pressures are too high they canuse more electricity and burnout
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ECM Motors Maintain Airflow (up to apoint) but can use more watts than a PSCECM Motors Maintain Airflow (up to apoint) but can use more watts than a PSC
0
200
400
600
800
1,000
1,200
1,400
1,600
1,800
0.0 0.2 0.4 0.6 0.8 1.0
Static Pressure (IW)
Airflow
(CFM)
ECM
PSC
0
100
200
300
400
500
600
700
800
0.0 0.2 0.4 0.6 0.8 1.0
Static Pressure (IW)
Wa
tts
ECM
PSC
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Comfort comes from a designedsystemComfort comes from a designedsystem
Duct System Must Be Designed to Deliver CorrectAmount of Air to Each Supply Terminal
AND
Each Supply Terminal Must Be Carefully Sized andLocated
AND
Each Supply Terminal Must Be Chosen With AdequateMixing and Throw
AND
There Must Be an Adequate Return Path for EachSupply
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SUPPLY LOCATION (old recommendation)
PERIMETERUP OUTSIDE WALL (HEATING)
CEILINGPARALLEL TO CEILING OUT TO
WALLS(COOLING)
HIGH INSIDE WALLPARALLEL TO CEILING TO OUTSIDE
WALL (COOLING)
TerminalsTerminalsTerminals
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Returns Adequate Air PathwaysReturns Adequate Air Pathways
High Cost
Excessive Duct Leakage
Unnecessary in OpenFloor Plan
Adequate Return PathsEvery Room
Potential for InadequateReturn Pathways(Pressure)
Potential for Noise
Low Cost
Provides Accessible FilterLocation
Low Surface Area
Single
DisadvantagesAdvantagesReturn Type
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Duct Design ConsiderationsDuct Design Considerations
WHAT DETERMINES FLOW THROUGH UNIT?
EXTERNAL SYSTEM = EQUIPMENT
PRESSURE DROP PRESSURE GAIN
AT DESIRED FLOW
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External System Pressure Drop =Equipment Pressure Gain
External System Pressure Drop =Equipment Pressure Gain
External System
Pressure Drop
Fan Pressure
Gain
Operating
Point
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Generic External SystemPressure Drops
Generic External SystemPressure Drops
DEVICE
Standard FilterHigh Efficiency Filter
Humidifiers/Electric Heaters
Supply Outlet
Return Grille
Balancing Damper
Coil
Duct System
PRESSURE DROP
.10 Clean
.20 Clean
.10 to .20
.03
.03
.03 Open
.15 to .45 wet coil
What is left
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Coil Pressure DropCoil Pressure Drop
0.451400
0.331200
0.231000
Static Pressure(Wet Coil)
CFM
G2FD036(S,H)21(T)
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Filter pressure drop is .05 0.24 in H2OMedian is 0.15Filter pressure drop is .05 0.24 in H2OMedian is 0.15
Cooling Filter Pressure Drop
0.05
0.10
0.15
0.20
0.25
1 6 11 16 21 26 31 36 41 46
Number
in
ofwater
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20" X 20" MERV 8 Air Filter
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 200 400 600 800 1000 1200
Air Flow (CFM)
StaticPre
ssureDrop
New Filter
Filter Loading
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Example Pressure DropExample Pressure Drop
DEVICE
Supply RegisterReturn Grille
Balancing Damper
Coil
Total EXCLUDING DUCTS
PRESSURE DROP
.03
.03
.03
.33 wet coil
.42
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Equipment Pressure GainEquipment Pressure Gain
.45 IWC at 1200 cfm
C k l 03 IWC
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Cannot work only .03 IWCavailable for duct systemCannot work only .03 IWCavailable for duct system
WHAT DETERMINES FLOW THROUGH UNIT?
EXTERNAL SYSTEM = EQUIPMENT
PRESSURE DROP PRESSURE GAIN
AT DESIRED FLOW
1200 cfm
0.42 IWC +
Ducts
0.45 IWC
i k i h ll i dlSti k ith S ll Ai H dl
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Stick with Smaller Air Handler Smaller Fan MotorStick with Smaller Air Handler Smaller Fan Motor
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SolutionSolution
Keep smaller air handler
Use a less restrictive coil (0.21 IWC)Drop the dampers
External system pressure drop becomes:
Return Grille .03 Supply Register .03
Coil .21
TOTAL .27 + Ducts
Equipment Pressure at Med. Hi and 1200 cfm 0.45
Available for ducts 0.45 0.27 = 0.18
N d k hN d t k th
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Now we need to know thefriction rate (IWC/100 ft.)Now we need to know thefriction rate (IWC/100 ft.)
We have an available static pressure of
0.18 Inches of Water ColumnWhat do we want to divide that by to get
the friction rate?
The Total Effective Length
H L i th D t S t ?H L i th D t S t ?
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How Long is the Duct System?(from an air molecule's view)
How Long is the Duct System?(from an air molecule's view)
A 200 ft. straight pipe is 200 feet long
This 4 ft. section of pipe is how long?
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30 ft.4-J80 ft.4-G
EffectiveLength
FittingNumber
Supply BootsSupply Boots
4-J
4-G
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40 ft.1-P withVanes
60 ft.1-O
EffectiveLength
FittingNumber
1-O1-P
2-A 2-B
5045403530202-B8070655545352-A
5 or+
43210DownstreamBranches
Supply Plenums and TakeoffsSupply Plenums and Takeoffs
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Furnace
Plenum
Trunk
Run 1
Run 2
4-G
4-J
2-A
2-B
5 ft. 15 ft.
10ft.
10ft.
1-O
A Simple Duct DesignA Simple Duct Design
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Trunk
Run 2
4-J
2-B
20ft.
Run 6
4-J
2-B
12ft.
1-P
A Less Simple Duct DesignA Less Simple Duct Design
Furnace
Plenum
160 ft EL Return
Run 1
4-J 4-J 4-J
12ft.
10ft.
5 ft.5 ft.
2-B 2-B2-B5 ft. 5 ft. 5 ft.5 ft.
Run 3
4-J
2-B
12ft.
18ft.
550 CFM
90 CFM
C l l t th T t l Eff ti L thC l l t th T t l Eff ti L th
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Calculate the Total Effective Lengthfrom the return grille to the supply
register
Calculate the Total Effective Lengthfrom the return grille to the supply
register
A simple takeoff can be as little as 10 feet
Or as much as 115 feet
Through good design we got the overallequivalent length to 300 feet
So we have 0.18 IWC available staticpressure over 300 ft.
The friction factor is .06 IWC per 100 ft(0.18/300 = 0.06)
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Recommended Velocity (in fpm)Recommended Velocity (in fpm)
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Why not just use 0.10 IWC /100 ft.Friction Rate on a Ductalator?Why not just use 0.10 IWC /100 ft.Friction Rate on a Ductalator?
BECAUSE
DESIGN VALUE FOR FRICTION RATE IS
NOT ARBITRARY
FRICTION RATE DESIGN VALUE
DEPENDS ON AVAILABLE STATIC
PRESSURE AND TOTAL EFFECTIVE
LENGTH
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Desirable DesignDesirable Design
SHORT DUCT RUNS
LOW STATIC PRESSURES
INSIDE CONDITIONED SPACE
GOOD THROW ON REGISTERS
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Duct InstallationDuct Installation
Put in what is designed
Straight Leakless
Proper Flow
Proper Distribution
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Duct TestingDuct Testing
Leakage
FlowDistribution
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Duct Leakage in Existing HomesDuct Leakage in Existing Homes
0%
2%
4%
6%
8%
10%
12%
%
ofSample
5% 15% 25% 35% 45% 55% 65% 75% >80%
CFM 25 / Nominal Airflow
Sample size: 1210 (no mobile homes)
Test method: Duct Blaster at 25 pa. (0.10WC)Source: CheckMe! database
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Duct Sealing Saves Energy and PeakDuct Sealing Saves Energy and PeakRequired Sensible Cooling (BTUh)
0
5,000
10,000
15,000
20,000
25,000
80 85 90 95 100 105 110 115 12
Manual J7
16% Supply Leakage, 11% Return Leakage
2%Supply Leakage, 11% Return Leakage
2% Supply Leakage, 3% Return Leakage
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DUCT LEAKAGE TESTINGDUCT LEAKAGE TESTING
Total Leakage
Leakage to Outside
Supply Side Leakage
Return Side LeakageOperating Leakage
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TOTAL LEAKAGE SET-UPTOTAL LEAKAGE SET-UP
REMOVE FILTERS
MOUNT DUCT BLASTER @ AH ORRETURN GRILLE
ALL SUPPLY REGISTERS COVERED
ALL RETURN GRILLES COVEREDDUCT TEST REFERENCE PRESSURE IN
SUPPLY PLENUM (STATIC PRESSURE
PROBE)
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Duct Blaster Test MethodDuct Blaster Test Method
MEASURE CFM TO KEEPDUCTS AT 25 PA
MEASURE CFM TO KEEPDUCTS AT 25 PA
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DUCT TESTING PROCEDUREDUCT TESTING PROCEDURE
KEEP FAN PRESSURE ABOVE 30 PASCALS
(SWITCH FLOW RING IF NEEDED)
FLOW RING SELECTION
OPEN 1,500 TO 500RING 1 800 TO 200RING 2 300 TO 75RING 3 125 TO 30
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Duct Leakage StandardsDuct Leakage Standards
CFM 25 as % of Cooling Nominal
Flow Nominal Flow as 400 CFM per ton
5%, 6%, 8%
CFM 25 as % of Actual FlowCFM 25 as % of Floor Area
L Ai fl
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Low Airflow
All of the audited forced air systems showed low air handler flowDanny Parker Florida Study 1997
Arizona NewConstructionBlasnik et al.1996
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Measuring AirflowMeasuring Airflow Temperature Split Method
Duct Blaster Flow Capture Hood
Flow Grid Coil Pressure Drop
Fan Curve
Pitot Tube Traverse Anemometer
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AC Temperature SplitAC Temperature SplitRun to steady state (15 minutes)
Measure return wet bulb and dry bulbtemperatures
Measure the supply dry bulb temperature
This is the most critical part of theprocedure
System stabilized
The measured temperatures as close to the mixedPLENUM AIR TEMPERATURES
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55 80
67 wb
D t i i T tD t i i T t
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Determining Target
Temperature Split
Determining Target
Temperature Split Target temperature split is not constant
Target temperature split varies with Return dry bulb
Return wet bulb
Determine the target temperature split Based on chart or slide rule
Maximum TemperatureMaximum Temperature
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Maximum TemperatureSplit TableMaximum TemperatureSplit Table
Return Air Wet-Bulb (F)
50 52 54 56 58 60 62 64 66 68 70 72 74
70 23.9 23.6 23.1 22.5 21.7 20.7 19.5 18.2 16.7 14.9 1372 24.9 24.7 24.2 23.6 22.8 21.8 20.6 19.3 17.7 16 14.1 12
74 26 25.8 25.3 24.7 23.9 22.9 21.7 20.4 18.8 17.1 15.2 13.1
76 27.1 26.9 26.4 25.8 25 24 22.8 21.5 19.9 18.2 16.3 14.2 11.9
78 - - 27.5 26.9 26.1 25.1 23.9 22.5 21 19.3 17.4 15.3 13
80 - - - 28 27.2 26.2 25 23.6 22.1 20.4 18.5 16.4 14.1
82 - - - - 28.2 27.2 26.1 24.7 23.2 21.5 19.6 17.5 15.2
Return
AirDryBul
b
(F)
84 - - - - - 28.3 27.2 25.8 24.3 22.5 20.6 18.6 16.3
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TrueFlow Flow GridTrueFlow Flow Grid
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Grid Installs in Filter SlotGrid Installs in Filter Slot
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Duct Blaster Test MethodDuct Blaster Test Method
Take supply static pressurewith air conditioner running
Block blower compartmentfrom return system
Install Duct Blaster on theblower compartment door
Turn on air handler
Adjust Duct Blaster speed toduplicate supply static
Read airflow from DuctBlaster
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Desired Air FlowDesired Air Flow
450Above 0.85
4000.80 to 0.85
3500.74 to 0.79
300Below 0.74
CFM per TonSHR
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Flow to RoomsFlow to Rooms
Within +- 10%
+- 20% Max.
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Write the Specifications
Sign the ContractBreathe Easy --- NOT
Write the Specifications
Sign the ContractBreathe Easy --- NOT
Any equipment installed must perform asintended
These items are accomplished by
human beings and are subject to error.
With Initial Feedback the Crews Learned RapidlyWith Initial Feedback the Crews Learned Rapidly
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With Initial Feedback the Crews Learned Rapidly.With Continued Feedback They Maintained GoodSavings
When Feedback Was Removed the Savings Droppedto 30% of the Maximum
With Initial Feedback the Crews Learned Rapidly.With Continued Feedback They Maintained GoodSavings
When Feedback Was Removed the Savings Droppedto 30% of the Maximum
Measured Savings
as a Percent of Achievable Savings
(running average)
0%
20%
40%60%
80%
100%
Program Startup, Learning Curve with Technician Feedback
Production with Feedback Maintained
Disaster when Feedback was Removed