furnace not always turning on

Question

I have a Carrier gas furnace.

For some reason, when I switch it on via thermostat, it SOMETIMES turns on, but SOMETIMES, it gets stubborn and refuses to turn on.

These are the symptoms:

• When it's working, it seems to work ok. It turns off when it reaches the right temperature, then turns back on when it cools down. It works fine like this all day.
• Other times, it just won't turn on. No matter how much I switch the thermostat on/off, the furnace won't turn on.
• Even when the furnace won't fire up, I can turn on the fan.
• I took off the air filter. It was very matted with dust, but I pulled off 95% of what I see. I'm sure there's invisible dust on there, but is that enough to prevent the furnace from turning on?
• Sometimes, it won't work for an entire day, and the next morning, it turns on (via the scheduler on the thermostat). I didn't need to touch anything.
• This morning, it refused to turn on again. I went downstairs, and checked the door was fully closed and tightend. The doors looked ok. Then, I flipped the power switch on/off, then nudged the air filter to see if it was all the way in. Within seconds, the furnace powered back on.

Does this sound like a correct diagnosis?:

• Thermostat is probably ok.
• gas, and pilot is probably working.
• Since it turned on by nudging the filter and flipping the power on/off, it's PROBABLY one of those two?
• I'm guessing it's NOT the electrical, since I can always turn on the fan?

I'm guessing replacing the filter is probably the cheapest thing to try first, THEN maybe call an HVAC person or electrician?

Thanks. I would like to eliminate the simple solutions before paying someone to inspect, so any input would be welcome.

Answer 1

If removing and reinstalling doors and covers causes it to turn on, it's probably an interlock switch. Do replace the filter - it won't fix your problem, but if it was plugged with dirt it's due. Otherwise this will quite likely take a call from an HVAC service company. Your determination that it's not electrical sounds accurate, because the fan works at all times.

Answer 2

I actually have what sounds like almost the same problem. Basically, heat once in a while does not turn on, and the only solution is to hit that main power switch off/on. What I suspect is happening is that once in a while some critical sensor reports a problem (or doesn't report what is supposed to report, which the control board then considers a problem) - e.g., flame sensor doesn't detect a flame, temperature sensors reads too high, etc. It makes sense then that the heating system will not work until a power cycle forces a full control board reset. Based on your symptoms, it sounds almost the same except that it times out and resets (12 hours? 24 hours?) and tries again. I know in my case, once it has the problem it will not restart on its own for at least 24 hours, because I have had some situations like that where I was unable to power cycle for a full day.

Some newer equipment will report an error code that tells you what failed. Unfortunately, older equipment like mine generally doesn't - extra displays cost too much in the olden days.

The fan is controlled independently so that you can have "fan on all the time", "fan on when furnace wants it to be on" and "fan on when air conditioning is on", so being able to turn on the fan just tells you it isn't a total power supply failure but doesn't tell you why the furnace isn't running.

Unfortunately, troubleshooting this type of problem is not easy unless it happens very frequently. If you happen to be near the furnace when it happens (not an hour later) then the sequence of clicks, flames, etc. can provide a lot of clues. But if it happens only once every few weeks that is not so easy - and usually I just want to get it running again to heat up the house.