Gas fired forced hot air furnace flames-out and recycles

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  #1  
Old 01-12-09, 09:44 AM
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Gas fired forced hot air furnace flames-out and recycles

I have a natural gas forced hot air furnace that flames out while running. I am not getting error codes. It starts normally. Sometimes it has trouble keeping the house warm.

Thermostat turns on:
- Exhaust vent fan switches on immediately.
- Flame igniter begins to glow after about 10 seconds.
- Flame lights normally.
- Blower kicks on after about 15 seconds THEN THE FLAME IMMEDIATELY TURNS OFF EVERY TIME.
- Blower stays on and the flame ignition cycle begins after about 10 seconds (while the blower keeps running). The flame stays on the second try but goes off after a while and recycles until the thermostat turns off.

Sometimes the house can't keep warm so I turn off the furnace for about 15 minutes and then turn it back on. That seems to do the trick and will allow the house to warm up.

There are no obvious adjustments or adjusting screws.

The vacuum switch that is operated by the blower vent was recently replaced (2 weeks) but it is the ONLY thing that the technician touched while working on the furnace. I watched him and he did it in 5 mins (& charged me freaking $240 on new years day!!!)

I'm guessing that it may be a bad temp limit sensor. There are about 5 of them (the size of a thick nickle) around the flame box They are wired in series back to the main circuit board. I don't think that it is an actual overheating situation because the flame goes out the first time after 15 seconds. The furnace has not had enough time to overheat.

The furnace is about 15 years old.

Please advise!
 
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Old 01-12-09, 10:49 AM
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I'd say that the two most likely reasons for the burner to shut off are:

1) Flame rectification failure --- the circuit board isn't detecting that the main burner is lit

2) Pressure switch opening, which would also shut off the burner.


A common problem is that the flame sensor that detects the main burner is lit gets an oxide coating on it and doesn't permit the flame to reach the sensor.

Cleaning the flame sensor with a wire brush would be worth trying. The flame sensor is usually on the opposite side of the burner compartment from the hot surface ignitor, and is a narrow rod sticking up into the burner flame. It has a single wire running from the bottom back to the circuit board.

Try removing and cleaning that ---- are you feeling lucky today?
 
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Old 01-12-09, 11:08 AM
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Cleaning the Flame Sensor

I will try to clean it. I did clean it at the beginning of the 06/07 season when I was having trouble. It ended up being a bad circuit board (for $550 on a Saturday).

I need to be lucky, the temps are forecast to drop into the single digits by week's end.

Is there a sensor in the plenum that may be bad?
 
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Old 01-12-09, 11:52 AM
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Limit switches in a fan compartment would open and keep the burners off if that were the problem.

Better than just cleaning the flame sensor is to measure the DC microamps (uamps) flowing along the wire. That's the real way to identify if the circuit board has failed again, or whether the flame sensor might need cleaning.


Usually on late model furnaces there would be an error code helping to identify what's happening. Is there no light or is it just no indicating a problem?
 
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Old 01-12-09, 12:23 PM
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Error Codes

I'm not getting error codes. The 2 red lights on the circuit board are on solid and not blinking.

Does it normally take about 15 secs. for a the flame detection to time out? It just seems that it would be a lot of gas. I wouldn't want to be smok'n near that thing. Although I wouldn't doubt that the circuit board is cued-up to check for flame at the same time it starts the blower. But if the flame isn't on and the blower sparks there might be fireworks. On the other hand the blower would dilute the gas quickly.

What the heck do I know?

It just seems ironic that when the furnace starts up cold that the flame goes out at exactly the time that the blower starts.

I'll clean the sensor when I get home and get right back to you.

With regard to measuring DC microamps - How would I check that? I own a volt-ohm meter. Would I need to open the circuit at the sensor and insert the meter. At what scale? Or should I just punt and call a technician if cleaning the detector doesn't work... (I took vocational electronics 30 years ago and my dad worked for IBM for over 30 years. He taught me a little bit but I ended up in healthcare. He died 3 months ago. He is probably trying to get even! Ha)

I greatly appreciate your help!
 
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Old 01-12-09, 01:18 PM
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Multimeters commonly measure amps and milliamps (1000 milliamps=1 amp). A good meter will measure microamps or uamps (1,000,000 uamps= 1 amp, 1000 uamps =1 milliamp).

You'd have to check to see if your meter has a current range for uamps.

If the circuit board doesn't detect the presence of the main burner flame within 3-5 seconds, it will shut off the main burner. Since there is no reason for the burner to shut off, I am merely guessing that the flame sensor current is marginal and that it's shutting off more or less coincidentally with the fan turning on.


Usually I'd expect the fan to start 60-90 seconds after the burner turns on, so starting after 15 seconds strikes me as unusual.

Also, I'd siggest you post the make and model of the furnace which you can get off the rating plate in the burner compartment.

Are the codes for the lights posted on the furnace? If so, what does it say about having solid red lights? That you aren't getting some kind of error code again makes me suspicious of a bad circuit board. It doesn't prove that it's bad, but it raises a suspicion.

Another thing you can do is disconnect the wire to the flame sensor and operate the furnace --- see if you get a flame failure error code under that condition, which should shut off the burner after 3-5 seconds.
 
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Old 01-12-09, 05:12 PM
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Update

The furnace is a Goodman GMPN0800-4 80,000 btu in and 72500 out (I believe that is what it meant.) Category IV for efficiency. It was made in 1993.

First - You were right on all accounts. The timing was ~60 secs between firing up and blower on. When I unplugged the flame sensor and started the furnace, the flame went out after 5 secs. I also removed what I thought was the overheat sensor in the heat exchanger and it seemed to be taped on with a metal tape(?). I blew into it. It was clean and not unusual. I put it all back after I blew into it. The first time the furnace wouldn't start up after I reassembled it. I restarted it a second time and it was fine. No problems. Didn't flame out. I put it all back together and watched.

After about 5-7 minutes it flamed out while I was screwing the gas supply/burner cover back on (vs. the circuit board/electrical area cover). Then, at the same time 'the blower cycled off for 1 sec then on 10-12 seconds and off for 1 sec. It did this about 8-10 times. Then the blower stayed on and the furnace fired up and started running again.

Completely different than this weekend.

I think that I heard it laugh at me.
 
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Old 01-12-09, 06:21 PM
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You'll have to describe what you are calling the "overheat sensor." It might be the limit switch that turns the furnace off if it overheats, but I don't recognize it from your description so far.

Also, you might try pulling the rubber tubing coming from the pressure switch where it fits on a barb by the inducer motor housing. That fitting not infrequently gets plugged with gunk which can open the pressure switch and turn off the burner.

Turn off the furnace and use a small diameter drill bit to stick it down into that fitting and clean any debris that might be constricting that hole. You need quite a small drill bit since a larger one wont fit in far enough to do any good.
 
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Old 01-12-09, 06:49 PM
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Definitions

You are correct. What I describe as the overheat sensor is what I believe is the limit switch. It went into what I call the heat exchanger. The end of it had what appeared to be metal tape that was deliberately perforated to allow air to circulate through. It was holding a small ~1/2 to 3/4 inch cylindrical sensor at the end of it. The rest seemed to be black plastic with two wire connectors on the outside.

Also, what I call the blower is the large fan that blows warm (and cold in the summer) air throughout the house. The vent blower (is that the inducer?) seems to stay on properly and does not cycle.

I will check the hose anyway.

Thank you again!
 
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Old 01-12-09, 08:58 PM
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Yes, that's the limit switch.

Sometimes it pays to check and be sure.
 
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Old 01-13-09, 05:43 AM
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Smile Bingo!

This morning I woke up and the furnace was going wacko. Everything was cycling on and off. Including the inducer (vent blower, whatever...) The house was getting cold and not able to keep up with the 18 degree weather outside.

I looked under the inducer where the new vacuum switch was (the technician literally tossed it under there). Lo and behold there was a kink in the hose. The new vacuum diaphragm is different than the old. The old stood on legs. The new one was on it's side. It doesn't have legs. I unkinked the hose and was happy. This would do it...

Then I thought about the potential blockage problem you mentioned and remembered that the technician never checked. I took the hose off the inducer and the vacuum switch and blew through it. A little fluid came out. I was glad I did that. Then I thought that if there is fluid in the hose and the new diaphragm inlet was below the vacuum connector on the inducer, there may be fluid in there. I turned the vacuum switch on it's side without the hose and A LOT OF FLUID (water?) came out. The diaphram must have been nearly FULL OF FLUID!!! UNBELIEVABLE!!!

I emptied it. Moved the vacuum switch so that it was above the vacuum hole in the inducer. Made sure it wasn't kinked (cheap hose). Restarted everything and it worked fine. Even 15 minutes later.

Only open issue is whether there is some other problem causing excess fluid build-up but I think not.

This was a difficult, intermittent problem. Thank you. You did it! I would NEVER have fixed this without your help and guidance. You are a great man! Please let me know if I can do anything for you.

Take care,
Rick
 

Last edited by RickW; 01-13-09 at 07:14 AM.
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