I have an International Comfort fan assisted warm air furnace with an issue:


The unit does not heat - burners and pilot do not light.
The flame blower runs when heat is commanded from the thermostat.
There seems to be no gas as the pilot.
There is no pilot light, and no spark when re-powering the unit, or when the flame bower starts.
I am assuming gas pressure is available since other gas appliances are functioning.


Any thoughts would be welcome.

Good observation so far. Model# would be helpful, or see if you can find it here. Get a pic.International Comfort furnace - Google Images (http://images.google.com/images?hl=en&source=hp&q=International+Comfort+furnace&gbv=2&aq=f&oq=&aqi=)

Thanks. I think this is a NTG3/FBF (NOx) model.

I did some more trouble shooting. The pressure sensor had no 24V AC on either side.

There was 24V at the output of the controller so I assumed this was a problem with the axillary limit switch.

Before pulling out the main fan I felt around looking for bad connections or resets on these sensors.

Then I slid out the main fan on its tracks to bell check the switches - they were both "on".

Perplexed, I re-powered the unit and found the 24V AC was now reaching the pressure switch and the unit lit up!

The good news is the unit is working. The bad news is that I don't know why - a loose wire, intermittent, component, or reset I did not see?

Go here. Google Image Result for http://www.sustainablefacility.com/EUN/2000/04/Files/Images/5494.jpg (http://images.google.com/imgres?imgurl=http://www.sustainablefacility.com/EUN/2000/04/Files/Images/5494.jpg&imgrefurl=http://www.sustainablefacility.com/Articles/Products/9a0ebca4aae38010VgnVCM100000f932a8c0____&usg=__Q-YJDuuLRquAu-dSOWWiQ-ReC3w=&h=160&w=90&sz=6&hl=en&start=1&itbs=1&tbnid=gQYQRoMdi8KYnM:&tbnh=98&tbnw=55&prev=/images%3Fq%3DInternational%2BComfort%2Bfurnace%2BN TG3%26gbv%3D2%26hl%3Den%26sa%3DG) I'm lookin' too. Look/sound like yours?

That is the one.

Before pulling out the main fan I felt around looking for bad connections or resets on these sensors. Then I slid out the main fan on its tracks to bell check the switches - they were both "on".
Check auxiliary limit switches for evidence of moisture exposure. While not obviously "tripped", internal oxidation may have been causing some resistance. Moving wires, etc. around may have had a "scrubbing" effect, restoring the circuit - for now.

Thanks. Since at one time the circuit through the two limit switches was broken I think it is reasonable to assume one of these was (is) bad.

I notice that there are some universal parts that are much cheaper:

http://www.pexsupply.com/White-Rodgers-3L05-1-Snap-Disc-Limit-Control-135-to-175-Degrees-F-Range-Open-on-Rise-14759000-p

Do you think these are a legitimate replacement?

So long as they have the same thermal response value and mount correctly, yes.

Thanks very much for your help.

Happy Thanksgiving!