Furnace problems...

[Ned Flanders]

I am hoping someone has some experience troubleshooting furnace problems.

My furnace has just started doing the following...

The venting motor starts, then the igniter glows and the burner fires up to preheat the exchanger, but right when you would expect the fan to start the flame goes out and the process starts over again right away.

I know the fan is working (I can switch the fan on from the thermostat) and the filter is new. Because the fan is not coming on i know it is not a matter of too many vents closed.

If I monkey with it and reset it it seems to start working again?

this last time I blew it out carefully with compressed air and pulled the flame sensor and cleaned it (although it did not seem very badly soiled/oxidized)?

Circuit board, or maybe the internal heat protection cut off?

Thanks in advance for any help!

[DD_BobK]

Based on the behavior, I would suggest:

1) "malfunctioning" flame sensor. 2) logic board 3) over temp sensor bad (but unlikely since the unit only runs for a very short time)

Item 3) might be able to be tested by installing a jumper

cheers Bob

[Stormin Mormon]

Most likely a bad flame sensor. That, or bad circuit board. I'd go with the flame sensor, first. It's needed cleaning, once, you say. Cheaper, also.

Christopher A. Young Learn more about Jesus

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My furnace has just started doing the following...

The venting motor starts, then the igniter glows and the burner fires up to preheat the exchanger, but right when you would expect the fan to start the flame goes out and the process starts over again right away.

I know the fan is working (I can switch the fan on from the thermostat) and the filter is new. Because the fan is not coming on i know it is not a matter of too many vents closed.

If I monkey with it and reset it it seems to start working again?

this last time I blew it out carefully with compressed air and pulled the flame sensor and cleaned it (although it did not seem very badly soiled/oxidized)?

Circuit board, or maybe the internal heat protection cut off?

Thanks in advance for any help!

[trader4]

Is there a circuit diagram on the back of the cover? If not, probably can find one online. That would show all the sensors involved.

It could be a flame sensor or over temp sensor. But there are other sensors, at least one that is there to make sure the furnace is properly venting by measuring vacuum/pressure. Could be that sensor or that the vent is partially blocked. I'd start by verifying the vent pipe is clear.

[The Daring Dufas]

Does your furnace have a Honeywell Smart Gas Valve or is everything handled by the control circuit board? O_o

TDD

[Ned Flanders]

Well it looks like it might have been the flame sensor... as I mentioned I removed and cleaned it last night, and it has run all night properly.

I am not 100% convinced the problem has been solved but I am feeling better about it. If it makes it thought the next evening (as that seems to be when the problem has been cropping up) I will relax a bit more.

The last time it broke down was just before a long weekend with sub zero temps outside. That time it was the igniter element and I had to scramble and pay through the teeth for a replacement before it got dangerously too cold in the house (water pipes). I don't want to be doing that again.

(Note to self - weren't you going to pickup some spare parts?) LOL

Thanks again everyone!

[clare]

Possibly a draft problem too. If the pressure sensors sense a restricted stack, it will shut down - stack could be blocked, switch could be bad, or pressure tubes could be blocked (or full of condensate) Blocked tubing is actually a pretty common occurrence. Shuts off the eductor fan which also shuts off the gas.

[Home Guy]

Hey Ned - Looks like you got your furnace working:

Or maybe you had a meth lab?

[Tony Hwang]

Hi, Flame won't start if that's the case. Sounds like flame sensor which may simply needs cleaning. Sensor seldom goes bad.

[trader4]

I'm not so sure about that. If the vent is completely blocked, then the sensor should prevent it from starting. But what if it's PARTIALLY blocked? It would seem it's possible it's borderline and could get by starting, but trip later.

[Ned Flanders]

Somebody in that pic had a bad day, I am glad it wasn't me...

Well, it looks like it was the flame sensor. I am really surprised at how only a little buildup on the sensor could disrupt its proper operation.

I will have clean it again when I have some steel wool as all I could find at the time was a dishcloth with a nylon weave side used for scrubbing nonstick pots (so glad my wife did not see me using it). ;-)

[trader4]

I don;t know about flame sensors, but I recently had a check engine light come on in a car with only 20K miles. It was a running lean problem. After some investigating, it looked like the Mass Airflow Sensor could be one of the kulprits. I bought a can of cleaner and took it off. Upon looking at it, I thought for sure it had to be something else, as it looked perfectly clean and brand-new to me. I sprayed it and put it back in and voila, problem solved.

[Vic Smith]

My only experience with flame sensors was having to clean one 4-5 times every winter to get the furnace going. Starting the first winter after furnace was installed. It was a real PITA on cold nights hearing the furnace cycle, go "poof" and recycle. Sometimes it would light on the recycle and go for days with no problems. Bothered my wife more, because the "poof" would wake her. I just thought it was the nature of the furnace, and lived with it. About 5 years ago a fan relay on the motherboard stuck and I had a pro in. He replaced the motherboard and flame sensor. The motherboard comes with a new flame sensor. Haven't cleaned the sensor since, and it never recycles. I still don't know if replacing the sensor would have saved me grief, or if the motherboard reading of it was also flaky. Just know it was the best 4 bills I ever spent.

[trader4]

Sure. but the point is what if the vent is partially blocked, so that the pressure is close to the safety trip point? It would seem that it could then cut off at any time later when the pressure just went up .01 as part of some normal tiny fluctuation, pushing it over the edge.

[trader4]

You can make a DIY manometer that is as good as any pro model for a few bucks. All it takes is some clear vinyl tubing, a piece of board to mount the tubing in a U shape on, fittings as needed to connect to the test port, some water, a ruler and magic marker.