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Jack of most 01-21-2009 05:19 AM

Furnace lites but won't run
I have a forced air gas furnace.
The thermocouple
glows fine.
The burner lights fine.
The thermocouple goes off. (burner still burning)
Then as soon as the blower starts the burner shut off.:censored: It will keep doing this over and over until I manually shut off the unit.
I've wiggled wires and tapped on the metering valve.

JohnH1 01-21-2009 12:49 PM

What do you mean the thermocouple goes off? Do you mean the pilot goes off IE: no pilot flame. Or are you describing a hot surfice ignitor/ glow bar

beenthere 01-21-2009 02:31 PM

Wonder if he's calling the HSI the thermocouple.

kennzz05 01-21-2009 11:13 PM

sounds like your cycleing on the limit probably a bad board leave it on fan on till you get it fixed

Jack of most 01-22-2009 06:02 AM

Yes, I was calling the HSI a thermocouple. Does cycling on the limit mean, thermostat is set to a particular temp. which happens to be the temp of the room? What's a bad board? Thanks for all your help!

JohnH1 01-22-2009 06:37 PM

It sounds as if you eather have a bad curcuit board or a bad Heat Exchanger. I have seen a curcuit board act that way. Look at the burner when it begins fireing and watch the burner near the flame senser. When the blower starts do you see the flame pattern change and fluctuate around the senser. If this happens the senser may thing the burner went out and shut the unit down and restart again. This is a sign of a bed heat exchanger.

A/C Williams 01-22-2009 07:29 PM

I have a thought of what might be happening. There is a flame sensor that looks like a metal rod about the size of a metal coat hanger. This is going to sense that the burner flame is present. The flame sensor will get a carbon buildup on it and not allow the sensor to detect that the flame ignited. It can run for a few miniutes like your describing and then shut down the burners. The sensoe needs to be removed and cleaned with steel wool or sand clothe. Put it back on and try it. If this is a Ruud or Rheem furnace there should be a amber colored LED on the curcuit board that will light up if the flame sensor is clean.

beenthere 01-22-2009 08:52 PM

Dirty flame sensor won't let it run long enough to bring the blower on.
What he describes, sounds more like the blower is blowing the flame away from the sensor due to a cracked HX.

Jack of most 01-24-2009 10:16 AM

The flame pattern didn't change. Thank goodness! You all scared me with the whole cracked HX talk. I'm sure I need a cleaning and service though.
More info: If the thermostat is set at 68 and the inside temp is 68 it does what I explained. If I raise the thermostat to 2 degrees or more above the inside temp it will then run and raise the house temp, but then go into the off and on cycle again. Could this be a bad thermostat? I will also try to clean the flame sensor and report back.

Again, thanks to all.

beenthere 01-24-2009 11:10 AM

Bad stat, or low air flow, and tripping the high limit.

Dr Heat 01-24-2009 02:36 PM

check your filter then check dampers to be sure everything is open.

A/C Williams 01-24-2009 02:54 PM

You may also want to check if the thermostat has batteries. If they run weak the t-stat can do some strange things sometimes.

A/C Williams 01-24-2009 03:01 PM

Also if your control board has LED lights, it may be giving you a code of whats happening. The LED will flash a certain number of times and then have a pause. Look on the back of the blower door and see what to code number says. The blower door will usually have a plastic window to see the LED because if you take the blower door off it will kill the power to the heater and lose the code.

JohnH1 01-25-2009 11:00 AM

If you raise the stat and it runs long enough to reach the set temp and then starts short cycling again your heat anticapator or cycle rate on the t-stat may be set slightly to short. What type of thermostat do you have?

Jack of most 01-25-2009 12:14 PM

Lux 500 programmable
I cleaned the flame sensor and replaced the batteries in the stat.(Didn't know they were there) No LEDs. I change the filter every 3 months or less.
I think it's working! If it was just the batteries, I feel like an idiot with 25+ years of home repair experience. Always shied away or referred out HVAC situations. THANK YOU, THANK YOU, THANK YOU! I learned a lot and I feel like I owe you all something but all I can do is leave you with this... I hope I made you smile cause I am.

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