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I have a Intertherm model Mac 1199 natural gas furnace, for the past few weeks when it kicked on the blower would run just long enough to get up to speed and then kick off stay off for a few moments then repeat, The burner was staying lit through this. after it ran through this cycle of off and on for about 5 minutes or so, the blower would stay on until the thermostat got to temperature and shut off as normal. it would not do this every time, but I would notice it about once a day or so.

I tried to manually turning the fan switch off inside the furnace and found if i left the fan off for a couple minutes and switched it back on it would run its normal cycle. I was thinking a limit switch?

Last night the fan would not shut off at all, and the furnace continued to blow cold air after the burner had shut back down. I tried adjusting the thermostat up and down, with no change, manually turning the fan off which stops the fan and back on just goes back to full speed.

The pilot is still lit. I was still thinking that it was the limit switch, and started to pull it apart to get the switch out, and did an ohm test on the limit switch and it shows 0.0ohm, but it does say on the bottom of the unit that "this system is equipped with an auxiliary limit control" that I can not seem to find?

Any assistance would be greatly appreciated??

Hi Rocky,

Don't have old service book with me, will be tomorrow or Sunday late before I will be back at other home and office.


The limit controls the burner, since it stayed lit, not an issue.


Most likely a weak set of contacts on fan switch.


Thanks,
Robert

How do I test/check that?

Hi,

With ohm or voltage meter.


Ohm= remove wires and ohm across fan switch terminals, should be closed of blower should be running and show reading.


Voltage= Test across terminals with wires connected and see if you show a voltage reading for 120vac. If closed as it should be for blower to run, you should read 0vac.


Thanks,
Robert