Help With Gas Furnace

We bought the house in May and now need to start using the furnace. Well, we don’t have to, but we wanted to start testing it and made sure it worked properly before it really was needed. So, the other night I went to turn it on and no go. Blower on instantly, but no burner.

Luckily, the seller paid for a year of a home warranty. So, I got in touch with the warranty company and got a tech to look at it. It turned out the safety was tripped, so no gas was actually getting to the burner. He reset the safety, adjusted the gas flow down a bit, got the burner to light (electronic ignition) and checked the system, turned it off again until I could get a new filter, and left. I put a new filter in later that day when I finally managed to find a store that stocked our weird size and turned it on. Success. The burner lit and then a minute later the blower kicked on. Except it never got any warmer. After a few minutes, the burner shut off and I have not been able to get it back on. When the heat is turned on at the thermostat, the blower kicks on instantly but the burner doesn’t. And I don’t really want to mess around with the safety, since I have only a vague idea of what he did.

So, I’ll be calling the warranty company again tomorrow. I figure either there’s something else wrong with the gas flow settings or there’s something wrong with the electrical in the furnace. But I thought I’d ask here for any possible insight so I can hopefully give some more useful information to the next tech. What I really don’t want to have to deal with is four or five visits trying to figure out why the burner is shutting off.

What safety was tripped? High limit? Low gas pressure? Flame roll-out switch? Is the induction blower running? Is it a spark ignition or hot surface igniter? We really don’t have enough to go on here.

Did the blower really come on instantly even though the burner didn’t light? It shouldn’t come on that soon. Just a wild guess, but I wonder if the high limit is what is tripping and it’s wired to try to run the blower.

All you can do is call the warranty company again. Good for you that you decided to check it out before it got really cold.

I’m not sure. I think it’s a high limit, but unfortunately he wasn’t as clear as possible.

Yes, now that the burner doesn’t light the blower comes on right away. That’s using the AUTO setting on the thermostat. It seemed like the burner ignited, then the blower came on, and then at some point after that (probably a few minutes), the burner tripped off and didn’t reset.

As I said, I wasn’t looking to solve the problem myself. I was hoping I could get some insight on what it might be. I just really don’t want to go poking around something connected to both electricity and natural gas.

There’s two blowers in your furnace. What you’re probably hearing is the inducer motor. It’s a small motor that turns on to blow any residual gas out of the pipes and check to make sure there aren’t any blockages in the exhaust. After that’s done, you can hear the gas valve open and the see the hot surface glow red.

This is when you can figure out what the problem is.

If you never see a flame (or the hot surface). You have some type of issue with the flame (gas/ignitor etc)

If the flame lights and then just a few seconds (like 1 or 2 seconds) the whole unit shuts down, you probably have a bad or dirty flame detector.

If the flame lights, stays lit for about 20 or 30 seconds, then shuts off without the main blower turning on OR the main blower turns on and but the whole unit shuts down a few minutes later, it can be the high/low sensor (depending on your model).

If you really want to poke around, you can pull the upper, then lower panel off and look for a box with a very small window. In the window you’ll probably see a blinking light and somewhere inside the unit will be a legend telling you what the blinking light means. If you don’t see a blinking light or it says everything is good (sometimes called a ‘heartbeat’), you may have shut power off to the furnace for a minute or two then turn it back on and tell the t-stat to call for heat so that it fails. Also, if ask for heat with the bottom panel off you’ll have to hold the safety switch down.
ETA, FYI, if you’re playing with it to diagnose it on your own (even just for fun), once it’s failed enough times (usually three), many furnaces won’t trying turning back on until you’ve killed the power for a few seconds. I think some may try again after some set amount of time, I’m not sure, but that’s probably why yours ‘tripped off and didn’t reset’. But you don’t even have to pull it apart or poke around. I’d just want to confirm that the blower you hear turning on right away is the inducer motor (that is, no air is coming through the vents in your house) and some other things, like does the flame come on and if so, for how long 1 or 2 seconds? a minute? Does the main blower ever turn on?

I had something very similar happen to me last year. The HVAC guy cam out, replaced a capacitor and everything worked fine. Cost about $100, and it was sooooo worth it.

The main blower is definitely coming on, even though it probably shouldn’t be. I’m saying that because the switch is set to auto and it keeps blowing air through the vents and will do so continuously until turned off at the thermostat. I haven’t tried unplugging it (I can’t find a switch).

For what it’s worth, it’s a Rheem Classic Super Quiet 80.

If your fan limit switch is broken, sometimes it can break in such a way that the fan never turns off after the furnace is done heating. I’ve found that giving it a good smack (the limit switch) will turn the fan off. I suppose if could be possible that having that switch broken could also make the fan turn on as soon as the unit powers up as well, but ISTM that even with that switch not doing it’s job properly (at that phase) you’d still get warm air eventually.

It could be possible you have multiple things going on which is making it harder to diagnose (over the internet).

I dunno if this applies to your furnace but every year I have to get down and clean the flame sensor on my furnace. It is the absolute easiest thing you can do on a furnace aside from vacuuming it and it always fixes my problem. Google “clean furnace flame sensor” for many excellent how-to links. Even if it doesn’t fix your problem, it’s a good thing to know how to do!

There should be a small light on the front of your furnace, typically orange.

If there is a fault, that light will be blinking and the pattern will tell you the fault code.

You count the number of “fast” flashes and then the number of “slow” flashed to read the code. For example “blink-blink-blink -(pause)- blink” is code 31, normally a pressure switch but it can depend on the manufacturer. Most furnaces have the trouble code posted on the interior of one of the doors.

Well, the tech was out today and replaced something. So it was working for a while, blowing nice hot air, and now it doesn’t work. Same behavior as before in that there is air blowing through the air vents instantly and the burner doesn’t light. So it’s time for a third call to the home warranty people.

I wonder how many times I’ll have to annoy them before I get a new furnace.

Did they give you any kind of bill/receipt/work order that says what they replaced or what they did?
Without that we can really only take guesses. Such as:
It’s something as simple as a loose connection that got bumped back on a bit tighter when he was monkeying around in there and after it ran for a while it vibrated back off and the unit shut down.

Pressure sensors can be finicky. The one on my parent’s (year old) furnace ‘went bad’. The code was saying that the pressure sensor was why it wasn’t firing. I pulled the sensor off, but before replacing it I tested it and it seemed fine. So I put it back on and haven’t had any more issues with it. I did also check and the intake and exhaust lines had no blockages.

Your board could be dying. It wouldn’t surprise me if they dropped a new board in next if the unit is giving the trouble codes for parts they know are good or faults they know aren’t present.

If you have a condensing furnace, I’ve seen issues where the condensate doesn’t drain properly and backs up into the system. That’ll shut everything down as well.

And then, of course, there’s always a small chance they’re just milking you (or rather the insurance company) for the service calls. Are you dealing with them directly or the insurance company? You might want to call the insurance company and tell them that these guys have been out here X times and haven’t figured it out yet it’s far too cold to be without heat and ask them if they work with anyone else in the area.

Well, it appears that the safety kept tripping because the gas flow was set way too high. It may or may not have needed the replacement sensor, but it’s in there now.