Gas Furnace


Old 12-11-00, 06:23 PM
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Bryant Gas Furnace; 6 years old; digital Honeywell thermostat (to keep the wife honest!); new filter every month.
When my thermostat is demanding a warmer house, the blower and burner come on fine. After ~45 seconds, the burner shuts off even though the demanded temperature has not been achieved. ~2 minutes later, the burner comes back on and continues to try to heat the house. This is very repeatable and does so all night long. The system worked fine last season.

Is it wrong for me to expect the burner to stay on until the desired temperature is achieved?

I'm hoping that it is something easy to replace like a temperature sensor but really dont know where to start.
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Old 12-11-00, 07:30 PM
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There are several possibilties that may be causing the problem you so well discribed. It's referred to as cycling.

You mentioned keeping the filter clean, which is great. That helps to maintain a full volumn of intake air flow. However, have you checked to be sure the intake vent or vents are not restricted?

Sometimes furniture gets rearranged and an intake <return air vent> vent gets partially blocked causing the furnace to cycle the burners off and on.

Another possibility could be a reduced fan speed do to motor drag or squirrel cage drag. Verify the motor and fan are turning without resistence.

Still another possibility could be too many registers are closed or not opened fully.

Another possibility could be a defective fan switch or a defective high limit switch.

<<<The fan switch turns the fan ON.>>> <<<The high limit switch turns the furnace burners OFF.>>> Often these two componets get mixed up by name and function, therefore, for clarification purposes only, this paragraph was included.

Another possibility could be restricted A/C coils in the plentium. However, I doubt it in your case since you change the filter often.
Old 12-12-00, 02:54 AM
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Additional Info for Tom

The intake vents are not restricted and are the same as the past 5 heating seasons.

The exhaust fan freely spins for minutes after it's motor turns off.

All registers are open and are the same as the past 5 heating seasons.

About the defective fan switch or a defective high limit switch: Where are they and are these easy to replace? (I am an engineer and have a good mechanical aptitude)

Thanks for taking the time help with this -- its getting cold here in Indiana!
Old 12-12-00, 08:54 PM
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Hi: blittle701

They fan switch and the high limit switches are located in the middle of the furnace. About half way up from the burners and half way down from the exhaust vent on a vertical heater. You'll need to remove the front decorative panel to access them.

On a horizontal furnace there isn't any front decorative panel but the switches will still be located in about the same area on the furnace.

These parts usually stick out and often are covered with a plate or cover of some sorts. LOOK for the wires attached to them.

You can tell the fan switch because it will have the thick wires. Exactly the same type of wires that come from the wall plug. A fan switch is operating on full house current. <120 volts>

Which means you MUST "pull the plug" out of the wall or turn off the main breaker before attempting to remove the fan switch.

The high limit switch runs on low voltage. <28 volts> but it also has house current going into it. Same "pull the plug" safety rule must apply here.

The high limit switch and fan switch could be combined into one switch on some units. The high limit switch will have the rotating dial with temperature markings on it.

Unplug the furnace, unscrew the holding screws and remove the switch. Remove the wires and LABEL them so you can be sure to replace them exactly as you found them.

You can then take the switch to the local appliance parts store for testing and to be sure you get an exact replacement part. Bring the make, model and serial number information too.

Old 12-13-00, 04:02 PM
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Sounds as though you have an induced or forced draft system. I would go to the flame sensor or the draft proving swich. The flame sensor is located on the oposite side of the spark ignitor or hot surface ignition. The draft proving switch is at the end of the 1/8" tubing that comes off of the induced draft motor housing. I'd replace the sensor first though. Unless you have a multimeter that can measur micro amps to test the sensor.
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