Fix my (intermittently misbehaving) furnace, please!
March 26, 2022 2:50 PM   Subscribe

HVAC filter: We've had intermittent troubles with our furnace this winter. We've had our HVAC people out twice now and they haven't fixed it yet. Can you help?

Furnace is natural gas forced air, maybe Trane brand, but I'm not sure. It's at least 15 years old, but we get it checked/serviced yearly, with no major issues until this year.

It was serviced (yearly check-up) in October, and was determined to be in good working order at that time. And it worked fine until sometime in November/December. At that time, I noticed that it was blowing, but it was just blowing cold air. The furnace wasn't kicking on to heat up. Turned it off for a few hours; tried it later, and then it was fine. Then a week or so later, had the same trouble. Called out service person. It was working when they came in, and they tightened up the wires and couldn't find anything else. Furnace worked fine for a few months, so it seemed fixed.

Then in February, it happened again. Similar situation. Didn't work, didn't work, then it worked. By the time the service guy came out, it was working. Different guy this time. He didn't see anything obvious. He checked the batteries in the thermostat, and found a bit of corrosion, so he cleaned that. Furnace worked fine for about a month. Until about a week ago.

So last Saturday, it wasn't working again. We'd bump up the thermostat, which would just turn on the blower fan (no heat). Tried it intermittently through the day, without luck. Then it finally worked the next morning. Then it failed again 2 days later, so we installed a new thermostat. As soon as the new thermostat was in, the heat worked. Hallelujah! We rejoiced too quickly, as the heat stopped working yesterday, and we haven't gotten it to work since.

Here's my diagnostics: The sign of trouble yesterday was that I could hear the furnace turn on. Normally it runs for about 20-30 seconds then the blower kicks on to run out the hot air. Yesterday, I'd hear the furnace kick on for 30 seconds, then turn off for about 30 seconds. Then it'd turn on for 30 seconds and then off. It would just keep doing this without ever triggering the blower. I turned the furnace off at that point. THEN everytime I turn on the furnace, the blower kicks on immediately, and is just blowing cold air. The furnace itself doesn't turn on to heat the air.

It's possible that the furnace is getting old, but when it's working, it works just fine. The 2 or 3 different guys that have looked at it don't see signs that it's too old.

Have you had anything similar? Any ideas? YANM HVAC Person but I'll take any ideas I can get.
posted by hydra77 to Home & Garden (10 answers total)
sounds like a faulty flame sensor.

The flame sensors get dirty, coated with carbon over time causing them to not function properly

The flame sensor detects if there's a flame. If it does not detect a flame, it shuts off the gas.

If it shuts off 3 or 4 times in a short time span, the furnace will then shutdown and enter a lockout mode for a period of time.

There is a manual procedure to override the lockout enabling you to restart earlier, which usually involves turning off the power to the furnace a couple of times
It'll be in the manual

The symptoms I had were very similar to yours.
I thought the thermostat was toast. No display etc.

The furnace would run ok until it hit the preset temperature.
Then shut off.
When the temperature then dropped, it tried to restart, but it wouldn't succeed half the time,
Then entering lockout for a longish period of time.
posted by yyz at 3:00 PM on March 26 [7 favorites]

It could also be a pressure switch / pressure sensor going bad. Image search w/ examples.

Our furnace was running the blower but failing to start the burner, and I found that when I just tapped on one of the pressure switches with a screwdriver handle while it was trying to start, it would start just fine. I removed the bad switch and took it to the furnace installer for a replacement (free under warranty in our case). So it's a part that can still mostly work but be "sticking" or otherwise failing in a potentially intermittent way.

If you're okay opening up your furnace and can find the pressure switches, try tapping them to see if it helps the furnace start up.
posted by whatnotever at 4:25 PM on March 26

My furnace has an LED light that blinks for various things, when it stops working it repeats a particular pattern which I was able to lookup to help diagnose the issue. You might look to see if yours has something similar. Mine was viewable from the outside, I just had to count the blinks in the cycle.
posted by TheAdamist at 4:30 PM on March 26 [3 favorites]

My furnace has done this and it was a dirty flame sensor. I was able to reach in with a cloth and gently wipe down the sensor—problem solved.
posted by bookmammal at 5:02 PM on March 26

I also came here to say dirty flame sensor and to check the blinking led code on the control board when it isn't working. There is probably a little peephole you can look through to see if the furnace is lighting up and then turning off after 30 seconds. If it's not lighting at all, then you have a different problem.
posted by ssg at 5:24 PM on March 26 [1 favorite]

Seconding TheAdamist. I am not a HVAC expert by any means, but there are many things this could be and checking the error code is the first step to narrowing it down. When the furnace next stops working, go take a look and you should be able to find a light flashing in a particular pattern that indicates what went wrong. Then come back here or to your service tech or another source of expertise and try to diagnose the problem.

For context, I’ve had an issue recently that sounds quite a bit like yours (including showing no obvious problems when serviced). The code showed limit circuit lockout, which basically indicates the safety mechanism to prevent overheating was triggered. This could be because of a faulty sensor or because of insufficient airflow or because the gas levels are set improperly - but at least it helped narrow things down so I could continue diagnosing. From my research, it seems as though circuit lockout is far from the only error that could result in a furnace intermittently not working.
posted by exutima at 5:45 PM on March 26 [1 favorite]

It's possible that it's just a bad sensor, but a 15 year old furnace is at the end of its service life. Heat exchangers rust through, the induced draft motor can fail, etc. If you can afford it and live in a cold climate, it might be time to just pop for a new one.
posted by Larry David Syndrome at 5:54 AM on March 27

Best answer: I had one that I thought was a flame sensor (error codes pointed me that way), but actually turned out to be the control board (circuit board, motherboard, chip thingy were all terms used at the place where I got a new one).
It was easy to swap out, and mine cost about cad100.

It's probably the flame sensor, though
posted by Acari at 6:36 AM on March 27

I'll throw this out there, maybe it will help -- my furnace acted up a few years ago with similar symptoms as yours, and with similar solutions tried by my tech: flame sensor, circuit board, etc. None of which worked.

Turned out to be a clogged drain tube -- the water (evaporation produced during the heating cycle, I guess?) backed up into the heat exchanger, which shut the furnace off. (The furnace did fire up, but would stop after a couple minutes or so.)

The short term solution was to clean the drain line; the long term solution was to shorten the drain tube so the water had a more direct path down the drain line.

Good luck.
posted by Bron at 9:00 PM on March 28

Response by poster: Well, it seems to possibly be the circuit board. Fortunately (?) it was still not working when HVAC came out yesterday, so they were actually able to see the issue in action. They've diagnosed it as a circuit board issue, and are putting a new one in today. So fingers crossed that in a few hours I'll have heat again (and again... and again). I'll ask him to check the drain, Bron, while he's installing it, too, just in case!
posted by hydra77 at 9:46 AM on March 29

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