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ComfortMaker Forced Hot Air - Sometimes doesn't heat

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Very odd situation on this one.

Most of the time it works OK... when heat is needed you can hear the small blower kick in and about 10 seconds later you hear the solenoid kick in (you can see flames in the flame box) and about a minute after that the main blower kicks in and starts heating the house.

BUT, sometimes (like once a week), the small blower kicks in and thats it. The small blower runs forever and the heater doesn't go.

If I either shut off the heater at the thermostat and turn it back on -or- power cycle the heater at emergency power switch then it will usually work.

If I open the back cover I see that there is a circuit that goes from the small blower, to the fan limit switch, to another switch that looks like a pitot tube (checks for air flow in the exhaust stack?) to a Fenwal box that appears to fire the solenoid and a spark.

Any clues (or troubleshooting techniques) to help me figure out why it doesn't work?

Its no fun to come back to a cold house....

Thanks in advance!

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sometimes just blowing through that small hose will clear up the problem, and it may be that the pressure switch itself is intermitant. the small fan is called a draft inducer,.it may need oiling if it can be.,the draft inducer causes a suction that enabling the switch to engage the main gas valve.

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