Furnace T-shooting. Help!

[Dave Richards]

I'm hoping there's someone who knows something about gas furnaces. Reminds me of a joke about a skydiver whose chute didn't open.

Anyway, here's the scoop. Carrier gas furnace. Piezo igniter to light pilot. Electronic set back type T-stat.

Furnace won't trigger igniter when t-stat calls for heat. If I shut the main power switch off at the furnace for 10 seconds or so and turn it on, it will fire the igniter and the furnace starts like always.

It's -8°F as I type. I woke up to a 58° house. Little boy was cold and so was mamma. Even the dog was cold! :eek:

Any ideas? Thanks.

[Tyler Howell]

Dave Wrong time of year for this kinda work. But you knew that already:o .
I'm thinking igniter modual. Replace the whole thing.
Does it have a thermo couple?? Try brushing it off with a stiff brush.

[Dave Richards]

Thanks Tyler. You're right about it being the wrong time of year. ;) I was starting to think it's the igniter module. I'll have to see if I can find one to put in there. I assume there's a thermocouple (although our gas fireplace has a thermopile,according to the manual) to trigger the opening of the main gas valve after the pilot has been on.

Alright, the search is on.


Thanks again.

[Scott Donley]

Dave, this sounds odd but I had close to the same problem. The T-stat looked like it was working. Had a repair guy out, he replaced the 2 AA batteries, all was good. My new T-stat has a battery low warning on it :)
Good luck ( and I thought 20 deg. here was cold )

[Dave Richards]

Scott, batteris in the T-stat were my first thought yesterday so I put in new ones. Thought about the T-stat being the problem but I wasn't screwing around with it when I finally got the pilot to light.

Another thing that makes me think Tyler is on the right track is that long after the main burner has lit, the igniter continues to spark. Seems to me that it shouldn't do that. I would think that at least once the main valve is triggered to open the igniter should stop firing the spark. Actually, I would think it should stop once the pilot has lit.

[Frank Hagan]

I would recommend calling a service company out. The electronic spark units are more complicated than the old standing pilot models, and there are quite a few components that "could be" the problem. By the time you replace all of them, not only will you spend more than with a qualified repairman, but your family will be unhappy for being so cold!

Here's a few things it could be, and some background on intermittant spark ignition systems:

Look at the electronic spark module ... usually a "grey box" type of thing that has the ignitor wire on it that runs from it to the intermittant spark ignitor. See if it has the words "100% lockout".

I don't know the codes for furnaces, but on boilers, you often have a 100% lockout ignition module. When the appliance fails to fire after a set number of tries the system goes into lockout mode. Cutting the power to the appliance restores operation on some of these (on others, there is a "non-volatile lockout" that requires you to push a reset button).

The ignition module has to know there is a flame present at the pilot before it opens the main gas valve operator (or, allows the gas to continue flowing on systems without a pilot). Sometimes its a separate flame sense rod, and sometimes the system uses the same rod that sparks. The way the system knows there is a flame is that the ignition control sends AC current down to the ignitor, and the current flows through the flame to the flame sense rod (or to the pilot hood, or other source of ground). Flame conducts electricity, but poorly, and the current sine wave gets screwed up, and it ends up DC current (this is called flame retification). The ignition module monitors the ground circuit to see if there is a DC current on it. If there is, there is flame at the pilot.

It could either be that ignition module, the ignitor itself, or the wire between them. Or something interfering with the ground circuit back to that ignition module. Or any number of other things: intermittant blower operation, pilot tubing cracked and leaking on a LP unit, gas valve pilot operator sticking, etc. All of these things have to be working correctly for the heater to continue to fire.

One possible scenario: The heater lights and works OK while you are watching it. You can turn the thermostat down, let the furnace go out, and then back up, and it lights fine. You can do this multiple times. You announce to the LOYL that you have fixed it, and you prepare for bed. The heater has worked all day. About the time you go to bed, the setback on the thermostat lowers the temp from your daytime 68°F to 55°. The heater goes off for an extended period of time, and cools down. When the temp gets below 55°, the heater's components have cooled too. The ignition sequence starts, but doesn't light in the number of tries, and locks out. If there is a weak flame signal ... caused by a weak ignition module, flame sense rod that is dirty, pilot light that is weak, wire connection in any of the components that makes connection when hot but not when cold, etc. ... then the system won't fire all the time when cold. Its out until you interrupt power and start it up again.

If that's the case, you can either keep the temp higher until the service man comes, or lower the temp manually, but more slowly.

[Jim Dunn]

Dave my son's was doing pretty much the same thing yesterday. Service tech said it was a gas sensor. Cleaned it with a little steel wool and it's working fine. It's less than 18months old. Their installing a new one tomorrow. Good luck fixing it.

On a side note we had our granddaughter for the whole day and nite cause parents were afraid the furnace would quit again. Our luck, as it's fun being grandparents.

[Matt Meiser]

I assume that you mean that the ignitor isn't even glowing? If so it may be the ignitor, but we had similar behavior from ours for a while and it turned out to be the gas valve. It was opening too slow and the furnace wouldn't light during the timeout period. After that, it wouldn't do anything until turning the switch off and back on. It'll probably be a lot cheaper to call for service tomorrow rather than today if you can limp through until then.

[Bill Turpin]

Once the burner lights the igniter becomes the flame rod as referenced above. The DC current flow is only 2 to 4 microamps. That is millionths of an amp. This current flows through a ground wire. The least amount of rust, corrosion, or loose connection will cause this current to be lost. The AC from the igniter gets rectified by the flame and flows through the charged ions of the flame to ground. The igniter should stop popping within one or two pops after ignition. If the ground is not the problem, then the igniter board is probably bad.

Bill in WNC mountains

[Dave Richards]

Frank, thanks for that information. I'll take all that into consideration.

Jim, I doubt it's the gas sensor since the problem seems to occur before the gas is involved.

Matt, I have a piezo type igniter not the glow type. Once the igniter module finally fires the piezo igniter the pilot lights immediately and after a short period the main burner lights. This would all lead me to believe the gas valves are operating correctly. It seems to be getting the module to initiate the spark in the first place.

Bill, I thought about those few microamps from the thermocouple. I reseated the Molex connector on the module and the Molex connector in the line between the module and the thermocouple. It seems to be functioning alright now. At least the house is at the temp set by the T-stat. I haven't been around for a few hours so I don't know if the igniter was continuing to spark after the main burner lit.

Frank's point about the variation in temperature between the daytime set point--68°--and the nightime set point--62°--is something I'll keep in mind. I guess I'll see how it goes tonight.

If I wasn't living in BFE, MN I probably would have already gotten my hands on a new module and installed it. Maybe it's a good thing I couldn't get one. Maybe reseating connectors cleaned them enough to keep the thing working.

I'm still puzzled though because shutting the power off to the furnace and turning it on again seemed the trick to get it started this morning.

Thanks all.

[Bill Turpin]

A Piezo is a crystal (named after Mr. Piezo), that when struck emits a high voltage spark. These are found on BBQ gas grills (the little push poppers). Your module that operates with the thermostat is a high voltage transformer that steps up 120 or 24 volts to ± 10,000 volts. The igniter then becomes the flame rod sensor after ignition. The spark module contains this transformer for ignition, an amplifier to step up the microamps from the flame to enough to hold in the safety circuit, and a logic circuit to not allow gas valve opening if there is flame when there is not supposed to be. These modules are fairly cheap to build, but furnace manufacturers RAISE the prices because of liability requirements. :(

Bill in WNC mountains