Help me with my home furnace and thermostat

Factual Questions
1

My gas furnace won’t go on, and it’s getting cold. The pilot light is fine, and everything else looks okay too. I pushed the thermostat up to 80 F, and still nothing.

The thermostat is of the old-fashioned mechanical type. No electronics. I disconnected it, and I see the two wires leading downstairs to the furnace. I figure that if I put the two wires together, that will simulate a message from the thermostat telling the furnace to go on. So I did that, and still nothing is happening. My guess is that this experiment proves that the problem is somewhere other than the thermostat.

Am I on the right track? Any suggestions? Thanks!

2

I think you’re on the right track, an old thermostat is nothing but a temperature controlled switch, it’s either open or closed. If nothing happens while the wires are separate, and nothing happens while they touch, it’s not a broken thermostat.

3

Is there a master switch on or near the furnace? If that got turned off, a standing pilot will stay cheerfully on year-round, not knowing that the power to the furnace was off - meaning that the main gas valve will not open.

4

Damn! Thank you!

The truth is that the switch WAS in the on position. Despite it being a low-tech device, I figured it couldn’t hurt to turn it off, wait 10 seconds, and back on again. Sure enough, a minute later everything is fine!

In retrospect, I did the same thing yesterday, and that must be what fixed it yesterday, I just didn’t connect the dots.

I suppose I might need a new on/off switch? Well, the emergency is over, and I’ll discuss it with my plumber and electrician, but if anyone has any more ideas, I’d love to hear them. Thanks again!

5

I had a similar problem last year. It wasn’t the thermostat or anything it was uhm…

Dammit, it was a big piece that had a metal cover over it. Maybe a heating element? Something?

Anyway, the way this whole piece worked was by vacuum. The metal cover had a crack in it - so no more vacuum. It would actually work just fine if the entire unit was COLD and hadn’t been run for a while - but once the metal case was good and warm, the crack would expand and ruin the vacuum and it wouldn’t be able to kick on again.

I called my HVAC company and they came out and diagnosed it right away, put in a new piece (about $200?) and it’s been fine since.

I dunno if that is anything related to your problem - but just note that “doesn’t turn on” doesn’t always mean there’s an electrical problem.

6

Essentially you have “rebooted” your gas furnace. Something inside is causing it to need to be reset.

Gas appliances tend to go BOOM very loudly if they are tinkered with by those without training. Find out what professional trade in your area services gas appliances and call one of the licensed professionals. It will most likely not be a plumber or electrician. Your gas supplier can give you some recommendations if you’re not sure.

7

An older furnace will likely have a 24 volt control transformer external to the heater, which supplies voltage to the thermostat. Bad transformer = no operation. That said, turning the main power off and back on wouldn’t repair a bad transformer.

Newer furnaces often incorporate one or more safety circuits, including the door which one removes to change the return air filter. If the door is not correctly reinstalled, the safety switch stays open, and the unit will not run. Other safety circuits can include diaphragm switches to verify positive draft inducer operation, cadmium sulfide flame verification eyes, high limit and flame rollout sensors, to name a few of the most popular.

Often, the operation is a sequence, and any failure puts the control board in ‘safety fail’ mode, which is reset by temporary removal of power.

Example, describing a 80% + AFUE standard chimney unit: Thermostat calls for heat > draft inducer fan starts > flue pressure differential switch closes > main gas valve on (with standing pilot) > main burner ignition > Cad cell flame sensor satisfied > heat exchanger warms to fan control lower limit > main fan on > thermostat satisfied > orderly shutdown.

Some more information about your unit, and pictures would help us diagnose with greater accurately.

8

If you’ve got an old style round-base thermostat (I’ll wager it’s a classic gold-tone Honeywell) with only two wires, your furnace is about as low-tech as possible, and there’s nothing I’d expect to need a reset - no digital controls or microprocessors here. At least I wouldn’t expect you to have a modern furnace with a logic board and related whatnot if you have a mechanical thermostat - usually people put in electronic thermostats when they get fancy new furnaces. :smiley:

For the two-buck fix, I’d replace the switch - it’s usually just a standard light switch and is an easy DIY job. In truth, the switch itself may be fine, but a funky connection at the switch could get just enough jiggle out of flipping the switch to come alive for a while.

9

Thank you, danceswithcats, but gotpasswords figured out the exact model of my thermostat without pictures! :smiley: His only error was that I got a new switch for only 99 cents, not a whole two dollars. I plan to install it tomorrow, after I figure out which circuit breaker controls the furnace.

Thanks everyone!

10

You’re gonna post and let us know you didn’t blow yourself to Kingdom Come, right?

11

Yes, I promise. :slight_smile:

Meanwhile, what I’ve already done is to unscrew the box and pull the switch out so I could see it. I had expected to see one of the wires screwed down but only loosely attached. Instead, there’s a piece of black electrical tape around the area where the wires are screwed down. This leads me to suspect that the screws might be missing. :eek:

We’ll see tomorrow.

12

I’ve seen that done to cover the screw terminals, I do not know why.

13

It insulates the bare wires/screw terminals on the switch from the surrounding environment, so it reduces the chance of creating a short circuit when you shove the switch and wires into the outlet box. I’ve seen electricians on home improvement shows do that sometimes.

It seems to be more important if you’re installing in an older, metal outlet box rather than one of the newer plastic (i.e., non-conductive) outlet boxes.

14

The picture request concerned the furnace, not the tstat.

15

(danceswithcats, I can easily take a photo of the furnace, but I’m still trying to figure out where I can upload it to, so that others can see it. Should I email to the address in your profile? Not that I really think it would show you much…)

Okay, everyone, here’s the latest. Since my last post (about 2 weeks ago) I have replaced both the ancient mechanical thermostat (with a cheap but electronic one) and the master switch (with the 99c one that I already mentioned). Neither change has helped.

But I’ve noticed a pattern: The heat never fails during the day. The problem almost always occurs only at night. And not every night, only on the really cold nights. It seems that if the outdoor temperature stays in the 40 F range, I’m okay. But if it dips into the 30s, that’s when it fails to cycle back on. And so the house is cold in the morning, and I go to the basement, turn the master switch off, wait 20 seconds, turn it back on, and every single time the furnace comes back on, all by itself, around 50-70 seconds later.

The one exception (to my “only at night” observation) was this evening. Today was probably the coldest day so far this season, and about 2 or 3 hours after sunset, we noticed that the house was getting cold. A look at the digital thermostat confirmed that the room temperature was about 3 or 4 degrees below the setting. So I flipped the switch, waited, and right on schedule, the furnace came back on.

Clearly, the problem has something to do with the cold. But it seems very different than what ZipperJJ described above (in post #5). Whatever it is that I have that is too cold, it doesn’t need to be warmed up. It only needs to lose the electricity for a few seconds. What could that possibly be?

For now, this is what I did: I removed the switch from the equation, even though it is brand-new. I disconnected one of the wires from the switch, and put it on the same terminal as the other wire. The switch is now doing absolutely nothing except serving as a base for a screw which is holding the two wires together.

It is cold out tonight. If the furnace is still working in the morning, that will strongly suggest that there was something wrong with the new switch. But my guess is that the problem is elsewhere, and the furnace will be off in the morning. (I’ll be able to restart it by cutting the circuit breaker for a few seconds, but that doesn’t really help me for the following morning.)

That’s it for now. I’m going to bed, praying that nothing goes BOOM in the near future. Thanks, all.

16

There comes a time when you should give up and call a HVAC guy.

17

And you’ll be wise to do it on Friday for straight time rather then Saturday or Sunday on double or triple time.

18

Yeah, I know. Trouble is, that’s what I’ve done in previous years, and I can’t shake the suspicion that the “repairs” done in the past didn’t really fix anything, and that all along, the true repair was the “resetting” of some obscure piece.

Ever since my first days as a computer programmer several decades ago, I have gained a real respect for repairmen of all types, including car mechanics, physicians, and HVAC guys. One example is putting up with customers/patients who insist that they know what the problem is, and lead the repairman on a wild goose chase of inaccurate symptoms. I confess that I might be doing that here, but more relevant is The Case Of The Occasional Symptom. Like the “odd noise” that a car makes only sometimes but not always, it is very difficult for the repairman to diagnose a problem if it is not a constant one. In my case, the problem seems to exist only when the temperature is very cold, and I’m not inclined to ask the HVAC guy to come to my home at 4 AM, especially if when there’s no guarantee that it will be cold enough to have the problem recur.

Which segues nicely into a report of today’s conditions: The furnace did stay on last night. We woke to a confortable home. But I don’t know if that’s because I bypassed the switch, or because the 6 AM temperature was 43 degrees. If it had gone down to 35, that would’ve been different.

Meanwhile, since I do seem to have a sure-fire short-term solution (turning it off and on), I’m considering this problem to be extremely curious and frustrating, but not an emergency. We’ll see what happens over the next week or so.

19

One thing you might do before calling a tech - jot down the make and serial number of the furnace and look for info online. A surprising number of manufacturers are putting their manuals online (usually PDF). The thing to look for is some sort of electronic control device. If there is one, it could be that something is latching up in it, which is why “rebooting” clears the problem. If the electrical system is nothing but switches, relays and the blower motor, then I dunno.

Odd that it worked with the power switch bypassed. Maybe just a coincidence. Maybe you beat the million-to-one odds and bought a bad switch. Can you tell if the wiring feeding the switch is copper or aluminum? If the latter, oxidation on the wire could be causing the problem. I hope it’s not that - it might mean your whole house has aluminum wiring, and that is not a good thing.

20

I’m a little unsure if the switch you are talking about is switching 120V line power or 24V control power.

I’d look around for a schematic of the wiring for the furnace. That, along with a multimeter and some thinking about the control sequence of operations, and you should be able to track down the fault. You are only going to be able to fix this if you understand both how things are actually working and how things are supposed to work. If you don’t think you can do that, it’s time to call the HVAC man.