Help me fix my heater

October 18, 2006 12:22 PM

I have an old furnace in my home that I can't get working. Now that summer is over, I'd like to run the heater again, but the burners won't ignite. I've just replaced the thermocouple and that didn't seem to help. It seems it will cost ~$100 to just get a diagnosis. If I could narrow down what was wrong, I might be able to fix it myself. Any ideas? I can post photos if that will help.
posted by shaneflyer to home & garden (10 comments total)

At least put up the type (fan-forced / natural convection) and any nameplate data you can find.

Quite a few municipal gas companies will come light your furnace for free. If that is true where you live, call them out! Maybe they will just happen to know a technique you don't.
posted by jet_silver at 12:32 PM on October 18, 2006

Is the thermocouple close enough to the pilot light? Does the pilot light ignite/stay lit? When the thermostat engages, does the furnace kick on, and does gas come from the burners?
posted by stovenator at 12:39 PM on October 18, 2006

It's a forced air heater. Brand name is "Holly" (could find no info on the web). Pilot lights and stays lit just fine (it was lit all summer). The thermocouple and the pilot are both attached to the same holder, so the distance is sort of predetermined.

When I turn the thermostat up, it *seems* like it has some effect on the system (the pilot light decreases in strength a bit), but the main gas valve going to the burners doesn't seem to open. I thought it was a bad thermocouple at first, which was why I switched that out, but that doesn't seem to have solved it.
posted by shaneflyer at 12:54 PM on October 18, 2006

The first thing to try might well be to shut off the pilot, follow the instructions on the gas valve for shut-off and then wait the specified length of time to try re-starting it.

In operation what should happen (at least, this is what happens on the forced air heaters I've had) is the fan should come on before the main burners light - it's a safety item.

If the fan doesn't come on, you should see if there's power to the fan that switches on and off with the t'stat. If there isn't, trace the wires to the power source and verify they're connected. If there is, maybe the fan is stuck or the motor isn't working.

If the fan comes on, but the burner does not then light off, there is probably something the matter with the way the heater senses fan operation - maybe a pressure switch.
posted by jet_silver at 1:21 PM on October 18, 2006

I've found that in the past, when the thermostat kicks on, the burners would ignite and burn for a couple minutes and then the blower would start, rather than the reverse. I guess the blower could be out too, but I think the burners would still light up.

Thanks for the help though.
posted by shaneflyer at 3:36 PM on October 18, 2006

Depending on where you live, an old furnace can be quite poor in efficiency (< 80%) and actually cost more to run than just buying a new so if you live in a cold climate, consider investing in a new as a cost-saving measure. the gas company will finance for you if you're not so flush.br>
If you live in a warmer climate, you might just consider installing electric baseboard heat rather than mucking with an old furnace. Could turn out to be just as cheap as either fixing an old one or getting a new one.

Also, consider the risk that a poorly maintained furnace can spew Carbon Monoxide into your house and kill you and your family. Happens more than you think, unfortunately.
posted by drmarcj at 5:09 PM on October 18, 2006

An old furnace that's inefficient may cost you a lot more in fuel and repairs than you spend on an efficient replacement. There's a federal tax credit that looks like it applies to furnaces.
posted by theora55 at 5:54 PM on October 18, 2006

Sounds like the gas valve has gone south. If the thermocouple has been hot for more than a few minutes, and it still won't open when activated, it's probably the valve.
(Only two things that I know of go wrong to cause a Pilot works (so you've got gas), valve clicks when activated (So you've got low-voltage power to the valve) but no gas comes out (safety interlock is stopping it) - and you've replaced the thermocouple.)

Don't know about the efficency argument - when I did the math, replacing my 69%(Guess) efficient furnace with a 99% one, would take almost ten years to pay back, after having to run a new vent system (you can't vent the new ones out of roof chimnies - They don't generate enough updraft) and all the other hassles.

The (gas/carbon Monoxide) leak comments are valid. Get the thing working(Soap your connections to check for leaks!), and PAY a licenced HVAC person you've checked out with your BBB to come out and give it a seasonal tune up/check. Some HVAC companies will do it for free, but many of them will try to scam you, and all of them will hard sell you. TANSTAAFL.
posted by Orb2069 at 6:51 PM on October 18, 2006

I'm not sure I can help, but heres a thought re: thermocouple. Are you sure it is a thermocouple? My furnace has a thermocouply-looking thing, but is has only a single electrical connection. A thermocouple would need two (+/-) terminals on the connector. I concluded it was a measuring conductivity, not temperature, and I fixed my furnace by cleaning the surface of the "thermocouple" with steel wool.

Also, running the pilot light all summer if it is not needed might cost you $20-50, if I remember my calculation correctly.
posted by mediaddict at 7:29 PM on October 18, 2006

Fan? Is this thing a forced-exhaust furnace? Those require the exhaust fan to operate before the gas will come on. I've seen the fans go, probably due to condensation (efficient gas furnaces exhaust a lot of water vapor).

A forced exhaust furnace will have an exhaust pipe that exits the house directly, likely via a PVC pipe.
posted by Goofyy at 9:10 AM on October 19, 2006