Heil N8MPN. Fires up properly. Shuts down circulating fan and burners after 15 seconds to several minutes. Draft inducer continues to run for short period, kicks out then immediately kicks back on and then unit goes back through same sequence. Honeywell valve shows proper function with heartbeat geen light. Checked chimney for proper draft and even pulled flue from chimney and trtied with flue freely venting in basement. Same results. Filter is clean and vaccumed out around burners. Sensor tube looks good with no visible cracks. Unit worked fine to end of last heating season.

I don't have the answer but am having the same issue. The only difference with mine is that sometimes it works perfectly, others it does exactly what yours is doing. Checking with some HVAC guys. If I find a solution I will post it.

Hi guys. Try cleaning the flame senser with some steel wool, or fine sand paper.

Thanks for the reply. I tried that the other day but still same results. The tip of the ignitor is fairly burnt looking even after cleaning (looks like the diameter is smaller, wore at the tip). Do these go bad but still work intermittently?

Doesn't happen very often, but flame sensers can go bad. Usually cleaning them does the trick, but sometimes they do need to be replaced. Normally they aren't all that expensive, so its worth a shot.

Ordered a flame sensor/ignitor. Will get it on Monday and try it out. If a high limit is tripping would the furnace blower (exhaust and squirrel cage) kick back on to cool the unit down?

Not comletely familiar with your exact unit, but yes in many units the draft and main blower will kick on when a high limit opens. Usually the draft will shut down after a while and the blower will stay running, but there are certainly variences from brand to brand. If a limit is opening, you should be getting a code from the diagnostic led.

Sounds like that may be the problem. I do not believe I have a physical board in my furnace. I have tried to locate it but all I can find are a bunch of individual components. I know it's not common for a furnace to not have a board but my furnace seems to fall under that category. Is there a way to test to see if the high limit is faulty? Again, I appreciate all your help on this.

Check for voltage across the limit. IF you get 24, or 120v the limit is open (tripped). If no voltage, it is closed like it should be. Or you can disconnect the wires to the limit and check for continuity between the terminals on the limit. You want continuity, if none the switch is open.

OK. So after internet searching etc. I found 2 possible causes (I changed 2 things so not entirely sure which actually fixed the problem, but I suspect the thermostat). I changed the old Honeywell Mercury thermostat to a new pogrammable thermostat. Also changed the fan switch settings on the fan relay on the furnace.
In the last week everything seems to operate perfectly! Thanks for all that helped. And special thanks to one of my co-workers for coming out and helping (15 years residential furnace work) for additional help/ideas.

Glad you got er going. Thanks for the update. Happy new year.

And, I think your correct. Probably the stat fixed the issue. The spring in the old mercury stat wears out over time causing the furnace to short cycle. Good job.

Hi,
I have and IPC gas furnace. About three years old.It has from day one been giving me problems.The problem is if you set thermostat at 59 at bed time the heater will not fire until I hit the breaker. After a few minutes and playing with the thermostat it will fire up. The code is six + one.It does this on a regular basis.I've had a gas furnace guy look at it and he found nothing wrong. All the testing showed
It to be okay.He was supposed to follow up , but no word. I'm at my wits end .If someone has any clues I would greatly appreciate the help.