Bizarre Furnace Problem - Periodic Smoke in House

[btt118]

I would greatly appreciate any suggestions/thoughts on the below problem. I will not be attempting any repairs myself. Hoping for some ideas to run by my HVAC tech as he seems to be stumped.

The furnace is a Frigidaire FG7TC 080D-VC 80,000 BTU 95% Two Stage variable speed natural gas furnace installed in June of 2015. Problem is as follows:

1. Last December (2017) was in my house in evening when smoke alarms starting going off. I noticed a haze of smoke suspended below the recessed lights near the ceiling and there was a strong burnt plastic smell. Didn't know what the source of smoke was, but I immediately turned the furnace off to stop any flow of air (heat was on but didn't notice if it was blowing). Ran through the house to determine source of smoke and found nothing. There was no additional smoke and the existing smoke began to dissipate. Went down into the basement looked over the furnace and it seemed fine and in fact the burnt plastic smell seemed less strong around the furnace than it was upstairs. The next day I restarted my furnace and it started without issue (except that there was a burning smell from the vents, which was clearly just residual from the night before).

Wasn't sure the source of smoke so I removed and inspected all outlets and other potential sources in the house and called the HVAC guy. Outlets were all fine, and HVAC guy visually looked over furnace and didn't see anything wrong. For the next few days, I ran the furnace only when I was home and awake. After a few days with no issues, I began resuming normal use.

2. After that, the furnace ran completely fine all winter of 2017, and in the coldest of months (I'm in PA). Then when summer came I ran AC all summer with no issues.

3. In late October of this year (2018) I am awoken at 4:00 a.m. by smoke alarms. There was the strong burnt plastic smell in the house again and a haze of smoke around the lights. This time I called the fire department (the other half gave me grief for not calling the last time). Before I called though, I ran downstairs to look at the furnace and observed nothing out of the ordinary. Team of like 15-20 firefighters was at the house for over an hour trying to determine the source. They did a superficial inspection of the furnace, inspected the entire house, and ran an infrared heat sensor around. Never determined the source.

4. The next day I restarted the furnace and it seemed fine. I continued to run the unit but only while I was home and awake. It ran without issue. HVAC guy came out to reinspect the furnace (not sure particulars of what he did) and could not determine anything wrong. For the next week or so I continued to run the furnace, but only while home and awake - all was fine.

5. This week (11/5/18) just before I turned the furnace off to go to bed I smelled the burnt plastic smell. I did note that the blower had stopped just before I noticed the smell because I happened to be waiting for it to finish its heat cycle so that I could shut it down for the night. However, at the time it stopped the tstat indicated that it reached its target temp. In any event, I went down to the basement noticed nothing unusual but I turned off power to the furnace just in case.

6. A few minutes later I restarted the furnace, and stood by to hear it go through its start up procedures. Inducer fan ran and then the burners lit. I stood by for several (3-5) minutes as the burners continued to heat, but the blower never came on. The area at the top of the system before the duct (the hot air plenum?) kept getting hotter and was almost too hot to touch. My fiancee yelled down that the burnt smell was now stronger upstairs. I shut the system down and turned off all power to the unit.

7. The next day, I fire the unit it up and it works fine. WTF...

We've considered that it may be the blower, or control board. My HVAC guy says that typically a blower either goes or it doesn't, and that it'd be strange for it to run fine like that and just have those intermittent issues. Based my very limited knowledge from reading online, I was wondering if it could be the limit switch sticking, but for some reason he doesn't seem to think that would be the issue.

Any thoughts, or suggestions would be greatly appreciated. Thank you.

[dandyme]

Smell/smoke may be coming from condensate drain pan overheating when blower stops prematurely.
Have them check the board carefully for crack in printed circuit.

[STEVEusaPA]

X2.

[pecmsg]

Your HVAC Guy needs to Figure Out whats going on. Safetys should trip long before any burning smells.

Might want to find someone else, Especially since this is an ongoing issue!

[btt118]

Thanks for the suggestions. The burning smell seems to come from within the system, rather than outside of it. The condensate pump is on the outside, but is it possible that it could be sucking the burning smell back into the system through the condensate line?

Was wondering, have any of you seen such an intermittent issue where the problem occurs, but then the system runs fine for an entire year before the problem reappears? It seems so bizarre.

Also last night, it continued to heat to 74 degrees, when the temp was only set to 70. It was operating fine other than that so don't know if it's related. HVAC guy will come out again hopefully this week. Parts are under warranty, but as pecmsg suggested - I want him to figure out what's going on rather than just swap out parts. Given that it ran fine for so long after it occurred the first time it'd be nice to confirm that we identified and corrected the problem so I don't have to continue to worry when it will randomly rear it's head again.

[pecmsg]

You dont want a parts changer you want someone that knows the "System" there components, sequence of operation.

You want an Experienced Tech. If they say lets try this, or lets see what happens, ask for a more experienced tech!

[bobboan]

You have at least two issues:
1. The blower is stopping because either the motor, the board or the wiring harness is bad.
2. The limit switch is bad and not shutting the burners off when the blower fails.

Pecmsg explained that the drain PAN was over-heating when this happens, not the external condensate pump.

YOU NEED TO LEAVE IT OFF AND GET A COMPETENT TECH IMMEDIATELY.