Gas Furnace - Pilot on, Blower on, Burner not on


Old 05-27-04, 09:34 PM
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Gas Furnace - Pilot on, Blower on, Burner not on

Carrier gas furnace. At least 14 years old. Cannot determine the model #. Circuit board diagram shows a number: 310417-401 Rev A.

Current outdoor temps around 10-20 Celcius.

Digital thermostat shows 20 indoor temp. Heat set at 23 degrees.

As the title states, the pilot is on, thermostat indicates it is turning on the heat, blower turns on, burners do not light.

Some initial testing - determine that gas valve is not getting 24V AC.

Some more searching, there's a suggestion to jump the limit switch.

Can't seem to get anything to happen when I jump at the limit switch itself. Perhaps the points I'm trying to jump are just dirty and not making contact. When I jump the connections on the circuit board that wires to the limit switch connect to, the burners turn on but the blower turns off. What's causing the blower to turn off?

How should I proceed? What to check for now?
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Old 05-30-04, 04:25 PM
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Hello: Half_Handy

Jumping the wires on the circuit board are be bypassing the high limit switch and the circuity. Which means one of two possibilities exist. The high limit switch is defective or there is yet anothe safety device that is defective or keeping the gas valve shut down.

Try removing the thermostat (T-Stat). Than connect only the heat wires and note what happens. Doing this verifies the T-Stat is not the cause.

The blower shuts off because the normal cycle is about to begin. But in this case it cannot do to the above possibilities. Most likely that model has a time delay relay to run the blower fan. Which means fan turns on set by time not be sensing heat, etc.

Best bet is to remove the circuit board, have it tested at a local heating parts supply store or at a dealers store for that brand name product. May consider replacing the limit switch.

Possible the pilot flame isn't hot enough to heat the safety element in the pilot assembly. Blow out the dust and lint in the pilot assembly. Use a can of keyboard compressed air cleaner. Blow out assembly fully, than allow it to relight by plugging in heater or turning power back on.

Also possible the safety element the pilot flame heats is defective. That is not a thermocouple but looks like one. May be flame sensor or flame prover.

An excellent source for original replacement parts is your local retail appliance parts store, the local heating dealers and agents retail part stores. Bring the make, model and serial numbers. The info will help to determine the possible problem causes.

Heating Agents & Appliance part stores sell replacement parts for almost every heater brand. The locations for the dealers, agents and retail part stores are listed in the phone book.

Retail heating parts dealers, heating agents and part supplers can also help determine what the possible problem may be. Bring the make, model and serial numbers. Dealers are an excellent source for original replacement parts and are listed in the phone book.

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Old 05-31-04, 05:14 PM
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check for manually resettable limits at the burners and at the fan
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