1

HVACR115 – Mechanical for Gas Heat

HVACR115 – Mechanical for Gas Heat

Gas Heat ExchangersGas Heat Exchangers

2

Heat ExchangesHeat Exchanges

• Heat exchanges are constructed to provide efficient heat transfer from the burner to the air in the exchanger while keeping the flue gases separate from the air circulated in the duct system.

• The heat exchanger is the difference between the life and the death of the occupants.

• Heat exchanges are constructed to provide efficient heat transfer from the burner to the air in the exchanger while keeping the flue gases separate from the air circulated in the duct system.

• The heat exchanger is the difference between the life and the death of the occupants.

3

Heat ExchangersHeat Exchangers

• May be composed in the following forms:– Tubes– Clamshells– Drums

• May be composed in the following forms:– Tubes– Clamshells– Drums

4

ClamshellsClamshells

• Two pieces of metal, formed into tubes and stamped and welded together.

• Come from factory with some leakage, probably about 5%.

• The pressure created by the blower motor keeps the flue gasses in the heat exchanger and out of the space.

• Most common

• Two pieces of metal, formed into tubes and stamped and welded together.

• Come from factory with some leakage, probably about 5%.

• The pressure created by the blower motor keeps the flue gasses in the heat exchanger and out of the space.

• Most common

5

ClamshellsClamshells

6

TubesTubes

• Newer heat exchangers are tubes, the flame and combustion air is blown into one side and then the hot gas takes many turns through the air flow and then out the flue.

• Has a lot more surface area for better heat transfer.

• Newer heat exchangers are tubes, the flame and combustion air is blown into one side and then the hot gas takes many turns through the air flow and then out the flue.

• Has a lot more surface area for better heat transfer.

7

Drum TypeDrum Type

• Almost like a 55 gallon drum,• Fire and combustion air on the inside• Space (conditioned) air blows across the outside.• Older and going away in gas, still popular in lower

efficiency oil equipment.

• Almost like a 55 gallon drum,• Fire and combustion air on the inside• Space (conditioned) air blows across the outside.• Older and going away in gas, still popular in lower

efficiency oil equipment.

8

Diagnosis Of Heat ExchangerDiagnosis Of Heat Exchanger

• A cracked heat exchanger must be replaced, it can not be repaired or ignored.

• The ONLY way to find a cracked heat exchanger is to measure CO2 in the flue gas.

• As the blower comes on the CO2 in the flue gas will go way up.

• A cracked heat exchanger must be replaced, it can not be repaired or ignored.

• The ONLY way to find a cracked heat exchanger is to measure CO2 in the flue gas.

• As the blower comes on the CO2 in the flue gas will go way up.

9

Cracked heat exchangersCracked heat exchangers

• Be careful when diagnosing a cracked heat exchanger, document everything. If any doubt get another technician from your company to give a second opinion.

• Cracked heat exchangers most often mean a replacement of a furnace.

• Be careful when diagnosing a cracked heat exchanger, document everything. If any doubt get another technician from your company to give a second opinion.

• Cracked heat exchangers most often mean a replacement of a furnace.

10

Cracked heat exchangersCracked heat exchangers

• They do not crack on there own. Something has caused the problem.

• Usually a furnace that is to large for the space, or restricted duct work.

• Find the problem, or it will happen again.

• They do not crack on there own. Something has caused the problem.

• Usually a furnace that is to large for the space, or restricted duct work.

• Find the problem, or it will happen again.

11

Air FlowAir Flow

• To diagnose an air flow problem:– Temperature of supply air– Temperature of return air– Subtract the two to find ∆T– Use the output BTU from the name plate in the

following formula:– CFM = OUTPUT BTU ÷ (1.08 x ∆T)

• To diagnose an air flow problem:– Temperature of supply air– Temperature of return air– Subtract the two to find ∆T– Use the output BTU from the name plate in the

following formula:– CFM = OUTPUT BTU ÷ (1.08 x ∆T)