Heating & Cooling Maintenance, Repair & Troubleshooting FAQ from NortherHeat

Flame sensor



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Thermocouple





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Gas valve (Standing pilot model)




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Capacitors




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Purge and balance




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Circuit boards




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Blower turns on but there is no heat.

Thermostats only control a fan relay, fan center or circuit board to turn on the blower motor.

Although these relay's are necessary for cooling to work, they have nothing to do with the heating operation or outdoor cooling section of the equipment.

Electric furnaces may or may not use this relay controlled by the fan switch of the thermostat.



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What is a flame sensor?

The flame sensor, seen here is typically located in front of the pilot or one of the burner flames.
It is used as a safety device that insures a flame is present.

When a flame is present the sensor passes a small electrical current through the flame and tells the circuitry what to do.
If the sensor gets dirty the current passing through the flame is to weak for the circuitry to work properly.

To clean the flame sensor, disconnect power to the furnace, remove the flame sensor, clean it with some crumpled up aluminum foil or some very fine sandpaper or steel wool.
Reinstall it and reconnect the power to the furnace.




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What is a pressure switch?

Pressure switch's are a safety device.
They insure the flue pipe and inducer draft blower motor are functioning correctly.

If the flue pipe becomes obstructed, or the inducer fails to start, the furnace will not ignite.
Also be aware, if the vacuum hose or vacuum ports become obstructed, or fill with water, the furnace will not start.

Often furnaces are installed with improper flue pipe sizes and can take a year or two for the problem to surface.

Sometimes the pressure switch can simply go bad and need replaced.



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Where is the diagnostic light?

Diagnostic lights are typically found on the circuit board.
If the circuit board detects there is a problem the lights will begin to blink a code. Ex... 3 blinks and a pause, then 3 blinks-pause again, and so on.

The codes are then located on one of the doors, the circuit board or in the blower compartment, describing the problem.

Note... on most furnaces when the power is turned off, these codes are lost, you will have to wait until the furnace malfunctions again to retrieve the diagnostic code.




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