[rleescott]

I have a Heil combination hvac unit, gas heat/ac. Mod#npgaa60e1k6.It just passed the 10 year warranty.The furnace will not ignite. When the thermostat comes on, the exhaust? motor runs and continues to run but there is no ignition at the pilot, so no heat. The blower runs if fan is set to on, but, of course, not if set to auto, since the furnace doesn't fire up. I have no experience with ignitor pilots, only old style thermocouples. How do I proceed?

[SublimeMasterJW]

The furnace first clears out any fumes or lingering gases by starting a draft fan motor, a timing sequence begins, after a minute and ahalf the control senses the draft through the pressure hose to the proving switch and turns on the ignitor After 30 seconds the gas valve solenoid is powered up and raw gas comes out to the glowing ignitor. At this point the control does not know if the gas was ignited and gas gas is spewing out the valve. So the control has to rely on a flame sensor/electronic thermocouple to send back confirmation. If it does not get the confimation it shuts down. If the confirmation is good then the control starts a stimer and after aminute or so the fan comes on and throws warm air into the home. So what can go wrong?
No power to control
control fuse blown
tripped safety switch
faulty ignitor
faulty sensor
faulty control board
Here is a video I found on you tube that features howit works:

FoggyFacts: HVAC trouble shooting


FoggyFacts: Furnace Auto-ignition Furnace: Quick cycling

[rleescott]

This is a 10 year old gas elec combo unit, Heil. The rollout switch tripped. This makes me worry that the heat exchanger has a crack. Is there a video that shows burners with a mild crack. I can't tell how much dancing is too much dancing for the flame. Any techniques for proving a crack? It is the torpedo style jet burner, not ribbon style.

[SublimeMasterJW]

If you think their is a leak in the heat exchanger you could get the gas company to check that. The equipment used is costly to the DIYer. I found an interesting page on this :

Guide to Furnace Heat Exchanger Inspection, Testing, Leaks - Hot Air Heat Troubleshooting & Repair Guide

[rleescott]

I now have followup that I think will prove interesting. I reset the rollout switch and turned the unit on. Within a few minutes of constant observation of the burner pattern I observed no rollout of flames and I periodically touched the sensor surface. It was certainly warm, but not 300F, and it tripped! That suggested to me that it tripped without reaching the critical temp. I got a new 300F switch and the unit has worked ever since without shutting off.
Doesn't this mean that, although uncommon, the switch failed and caused all of this trouble? If the new switch hasn't tripped, doesn't this mean that I have no more reason to question the heat exchanger that anyone else whose heating is working without a shutdown? I am interested in your thoughts about this scenario and await your reply. If I have missed the mark, please let me know. Other thoughts also welcome.

[SublimeMasterJW]

Yeah those rollout are constantly in the heat of the battle so to speak. So after a while they get weak and in need of replacement. Luckily they are not that expensive, you should keep a couple of spares on hand. Glad you could git er done