Furnace Info:
-Upflow Condensing Gas Furnace is a Heil 7000 (model# NUGK075DG09)
-Electric spark or host surface ignitor ( I think it's spark)
-Installed Oct. '96 (when we built the house)
-90% efficiency
-wall thermostat is honeywell programmable. Don't know model but was also purchased in fall of '96


Problem:
I noticed the blower on the furnace hadn't shut off and was still blowing air when it should be off (thermostat fan setting was "auto").
Later, when heat should have been on, I felt cool air blowing out of vents, so then realized it was not blowing heat.
Checked out furnace and burners were not lit.
Then shut off power to furnace so blower would stop.

Called local service. According to my wife who was home, service tech stated he couldn't find the problem and that the furnace started for him.
He then showed my wife a limit switch that could be used to ignite the burners by touching what looks like a small post on the switch. My wife thought he used a screwdirver to touch a post on the switch. I'm not exactly sure which limit switch it is. It is above the burners. I have the manual, but it isn't real clear to me which switch it is (there are 3).
I attached a couple of images with the limit switch circled in red.

By the next morning, furnace wasn't working again, blower was constantly on and no heat.
I used a screwdriver as indicated by the technician on the limit switch to see if the burners would ignite. When touching the switch, it sounded like the exhaust fan started, and a few seconds after that, I could see the spark ignite the burners. But, the burners don't stay lit. As soon as I take the screwdriver away from switch, they turn off. THis was done when the thermostat temp was set high enough that the furnace should be operating.

The description on the invoice left just stated "fix limit on furnace & check operation".
Tech had noted if you had to start the burners more than a couple times by touching the limit switch, we probably need to get a new furnace. That seems a bit extreme and wonder if some component needs to be replaced.

Any suggestions? It's starting to get cool here in ND, so need to figure out soon if I should make another service call to see if it can be repaired or start shopping for a new furnace.

Thanks in advance!

Attached Images

  

That switch that you have circled in red is a flame rollout switch. Do the burners touch or come close to that switch when they ignite? That switch is a safety and should never be bypassed. The service guy is obviously clueless, He should never advise you to jump anything with a screwdriver. You either have a weak rollout switch, or the burners are rolling out and it's doing what it is supposed to do.

Thanks for the info. Now that you mention rollout sensor, I looked closer at another pic I found in the maunal, and yeah, I guess that does appear to be the flame rollout sensor. When I did "jump" that switch and the burners ignite, it does appear flame rolls out on those 3 burners beyond the box above them and I could definitely feel the heat above the switch, but then they zip right back in under that box and go out as soon as I break the "jump".
If I were to let the furnace try to ignite on it's own though, ignition never even starts. The blower is constantly on and that's it. The exhast fan (blower beneath the burners) never sounds like it even starts and ignition is never even attempted. So, it would seem that the rollout switch doesn't come in to play because nothing gets ignited when the furnace operates on it's own.

I wonder if I have a bad thermostat that is keeping the blower on? And maybe the flames are only rolling out when I try that manual ignition?

If that rollout switch is bad, can that keep the ignition process from even starting the cycle?

Thanks,
Gary

Does the combustion fan start when you jump the rollout switch?

Yes, and shuts down when breaking the jump.

Thanks,
Gary

Does that rollout switch have a little red button on the top of it? If so it's a manual reset, if not it has to be bad, it's just a normally closed switch once it is exposed to a certain amount of heat it opens the switch. If you jump it out and the furnace works it has to be the problem.

Thanks again for the suggestions!

There is a small square post sticking up between the 2 vertical tabs on that switch (this is what I was touching when jumping the furnace).
I pressed down on that post and did here a faint click sound, but couldn't really tell if anything physically moved.

Then I tested the furnace... here's what I observed

Thermostat was on the off setting. Room temp was below what the thermostat setting was.
I turned the power back on to the furnace.
Blower didn't turn on.
My wife turned the thermostat to heat, while I watched the furnace.
Once the thermostat was switched to Heat, blower turned on.
A few seconds after that, the burners ignited.
The furnace stayed on until reaching the set temp.
Burners turned off, and blower kept going.
Blower continued to run and did not turn off.
Room temp dropped below set thermostat temp.
Burners did not ignite. Blower kept going.

I then did the same test and before starting pinched down on that post again. Heard the same faint click.
The furnace did the same behavior... started as it should normally do, shut off heat when it should, and blower kept blowing. Same behavior as above.

Doing the test, it seems like the furnace works properly until it gets to the end of the cycle. The blower keeps blowing, but the burners never re-ignight when they should.

Could that still be an indicator of a bad rollout switch (i.e. the furnace will go through a cycle and not come back on while leaving the blower running)?
And can that cause a blower to not shut off?

Interesting stuff for sure!

So you are manually reseting the rollout switch each time?

Something is strange, Have you watched the burners during a cycle? Do they seem to rollout anywhere close to that switch during a heat cycle?

Yeah, I was starting the tests by pressing that post.

I watched the flames for a cycle and I don't think they rolled out, certainly not like they did a little when jumping the burners.
The flames are blue and I could occasionally see an orange flame (the front cover was off, so that may have impacted a little). All the flames seem contained. I watched it shut down, and that seemed OK as well.

The last time I started another test, I pressed that post while power was still on to the furnace (burners weren't on though), then had my wife adjust thermostat so burners would ignite. It ignited as expected and ran until we adjusted the thermostat to shut it down. I then put the cover back on the furnace, waited a few minutes, had my wife turn up the thermostat to engage furnace, and everything started as expected without me pressing that switch.

So, I'm going to see if it will operate normally on it's own for a bit and see if it goes back to it's behavior of shutting down, but the blower continuing and burners not igniting when they are supposed to. I'm guessing it will go back to problem behavior.


... actually, it just did. I heard the thermostat "click" a little and the "Heat On' on the display had disappeared but the blower is still running, blowing cool air again. I'll let the house cool down and see if the burners will ignite again on their own while the blowers continue to blow.


well, in the few minutes since writing the previous paragraph, thermostat went to "Heat On" again. Burners are not on though, and of course, the blower is still blowing.

Time to call it a night I think :)

Thanks,
Gary

If the flames are not rolling out,replace the rollout switch.

We have exactly the same furnace model as yours. Yesterday, we got a similar problem: blower not stopping but burners stopped when temp was about 62F. We had to turn off the power to stop the blower. By reading your post, I tried pressing the small square post (though without trying putting "heat on" last night). Then this afternoon, the furnace started after we set thermostat to "heat on" . But again, burners stopped when temp was about 62F and blower still blowing.

Finally, we reset the thermostat (by pressing the small reset hole button using a key) and found out that its default temp is 62F (it seems that thermostat somehow was not aligned with the furnace). Now burners and the blower work together (twice so far). Hopefully, you have fixed your problem as well.

Thanks for your post.

Our issue was the flame rollout switch.
I emailed the company that was out to check it originally, explained how it was behaving and what I tested, and they sent another person out to re-inspect.
Burners, heat exchanger, and everything else was fine, but the flame rollout switch was bad. He replaced that and tested the furnace operation, and everything checked out fine.

Thanks again hvacservicetech_07!

No problem at all. Glad to help!

Where is the flame rollout switch