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mystic_cobra 10-07-2012 09:46 PM

Gas Furnace No Heat
Decided to start a new thread.

I've been reading through the "no heat" threads and have not found one with the same problem as mine...
Turned the Honeywell programmable thermostat to HEAT today for the first chilly day to make sure the system works and got nothing. The thermostat is working as I get the little fire icon on the screen and I hear it click but the furnace does not light. No fan unless I switch the switch from Auto to ON. AC works fine.
I had the wife switch on the thermostat so I could listen for any activity and I did hear a fast click that sounded like a pencil rattling. It was a fast click, not a slow click. It stopped after five seconds or so.
I've never lit a pilot light on this furnace in the three years we've been here-it normally lights itself.
It is a forced air natural gas central air system installed in the early 90s.
Decal says: Lennox Model C16-41W-00-1 MFD AT: 58 MLC LB-49075DC

A separate WARNING type decal has lighting instructions on it but says DO NOT TRY TO LIGHT BY HAND as it has an ignition device which automatically lights the pilot.

Any ideas where to start before I call for service?
I'm a DIY'er, rebuilt my own kitchen, and a car guy but have never fiddled with a furnace, so please take it easy on me. :)

Nickhvactech 10-07-2012 09:57 PM

Sounds like the automatic spark ignitor is attempting to ignite but failing thus preventing any further operations. Verify (if accesible) if you see the sparks at the ignitor(means gray wire from board is probably ok), if so.. verify if you have gas going through valve supplying gas to ignitor..

Also look for L.E.D code on Control board once failed.. helps

mystic_cobra 10-07-2012 10:50 PM

I've never had the cover off and didn't know that I had an LED board. Going to pull the cover and take a look now. Thanks for the tips.

Nickhvactech 10-07-2012 10:54 PM

Unless its the lennox with the large door cover showing the board and electrical compnents if you have to remove the blower door cover it will reset board(safety to keep supply voltage from parts when servicing) not all older lennox furnaces do have leds but some do.

mystic_cobra 10-07-2012 11:07 PM

Here's what it looks like under the cover. I think the model number I posted is for the AC portion. There's another Lennox sticker inside where the heating parts are located.
I wiggled all the wires, made sure they are were connected, and tried again and same thing. It clicks but no fire. There were a few other quiet sounds that sounded a little like the gas may have been moving (flowing) but I might be wrong there.

how 10-07-2012 11:59 PM

It sounds like the pilot assy might be debris restricted.
Can you see the spark happening through the inspection port at the bottom of the picture when there is a call for heat?
Does the pilot light for a short while and then go out?

Having a dirty flame sensor in the pilot flame is a pretty common fault.

Marty S. 10-08-2012 05:59 AM

That's an older Lennox G20 furnace. Call a qualified service tech to check it every year. They are prone to heat exchanger cracks but the part has a 20 year warranty.

mystic_cobra 10-09-2012 08:34 AM

Thanks for the tips guys. (I forgot) We have a home warranty with AHS and have already called in and have a service call for tomorrow. Will post an update with the outcome.

mystic_cobra 10-09-2012 12:14 PM


Originally Posted by Marty S. (Post 1026342)
That's an older Lennox G20 furnace. Call a qualified service tech to check it every year. They are prone to heat exchanger cracks but the part has a 20 year warranty.

Hopefully, these cracks don't cause carbon monoxide poisoning or houses to burn down. :eek:

mystic_cobra 11-13-2012 07:14 PM

Just to close the loop on this...It turned out the control board was bad.

The tech checked out the system while here to verify everything else was good. We talked about the heat exchanger. I'm considering replacing the furnace with a new high(er) efficiency unit but will have to finish up some other projects first. He said this one was about 60% efficient and the modern ones are 80-96%.

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