Gas Furnace burner peters out after starting

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Old 12-11-07, 08:55 AM
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Gas Furnace burner peters out after starting

Hi All,

I'm stumped, all appears to work well. The furnace has an electronic ignition board (changed it out and it's OK) when the room is cold and thermostat turned up the furnace fires up immediately (pilot and main burner), blower comes on etc.
Then, watching the flames, as the heat comes up the flames will slowly decrease and eventually die out over a minute or so as the temperature increases. I've set the blower control to turn on early to make sure nothing is overheating..... I'm stumped. I've removed the filter and made sure the forced venting motor is OK etc. Two observations, the venting motor is running a little 'rough' but I've oiled it and it's just a bit noisy. I also appear to hear a bit of buzzing from the main valve.... note. about 24V ac remains at the main valve (MV) at all times.... still the main valve seems to be shutting down slowly after the furnace initially fires. If I wait a while and the temperatures drop, the main burner will again immediately light.... AAARRRGGHHH please help TIA

cheers, guenter
 
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Old 12-11-07, 03:49 PM
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Open or closed combustion chamber? (Closed will have 2 pvc pipes going to the outside of house, fresh air intake and exhaust; 'open' will have just the exhaust pvc and you could stick your hand into where the flames are.

When flame peters out, does it stay blue or does it try to turn yellow?
 
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Old 12-11-07, 05:29 PM
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Thumbs down

Hi ecman51,

Originally Posted by ecman51` View Post
Open or closed combustion chamber? (Closed will have 2 pvc pipes going to the outside of house, fresh air intake and exhaust; 'open' will have just the exhaust pvc and you could stick your hand into where the flames are.
When flame peters out, does it stay blue or does it try to turn yellow?

It's an Open chamber.... corrugated aluminum pipe going outside and the exhaust is moved out via a little fan.

the flames 'do' remain blue (more or less) just get smaller until finally they just barely hover above the pipe and then stop.... as if the main valve is shutting down slowly. the pilot remains brightly lit.... and the ignition does not turn on (ie the main valve remains energized). Checking I can still see about 23 V on the main valve solenoid, same as before. After observing, it seems that even initially when the burners are well lit, there is not as much gas coming out judging by the flame heights, as there used to be. I did change out the ignition module... no difference. All of this appears to have started at the end of summer when the furnace was being used again.
I just cleaned the pilot with no difference, and I checked to make sure there is a good amount of exhaust gas - ie the exhaust is not plugged. Baffeling.

It must be the main valve..... tapping it has no effect on the flames whatever......it does have that slight 'buzzing' though ....

cheers
 
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Old 12-11-07, 06:54 PM
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Natural gas or propane?

I almost suspect problem well upstream!, from the fact something is allowing normal pressure at your burners for a while, then it peters out.

If say you had an obstruction in your gas valve, the flame should always be small. But say it is out by the meter where the regulator/vent is. If say that regulator was blocked inside and allowed only 1/10th the normal flow through - what would happen is that during furnace "off" times, the gas line betwen your gas meter's regulator-vent would slowly charge the gas line between it and your furnace's gas valve with full pressure! And since gas compresses, it would be under full 7"w.c pressure inlet -at first. But when you turn on the furnace that pressure would soon dimmer down to that of the 1/10th volume I spoke of, and the gas pressure/volume in the line between the gas meter and the gas valve at your furnace would quickly come down to a trickle.

Follow this?
 
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Old 12-11-07, 07:28 PM
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It's a natural gas furnace. Additionally there is another furnace (one for upstairs one for downstairs) that is not having a gas flow problem.... good thought though. Wouldn't have occured to me. So, no, obstructions in the gas supply are not there. Strangely, the furnace heats to the same point irrespective of when the blower is turned on. At first I thought I'd put the blower on manual and see if the heat would go higher... it doesn't. the furnace seems to like to heat to some point and then..... slowly shuts down the gas supply.... At the moment I simply keep the thermostat at 18C (about 68F) and everything works sort of. If I turn the heat up, the furnace simply never reaches the set temperature. The burner will go out and the pilot remains lit and the venting motor continues to run.

cheers

cheers
 
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Old 12-11-07, 07:35 PM
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Huh. Maybe you have cheap overseas burners that when they get hot, the slits in them(if you have that type) start closing down or something. This is truly a good one and begs for an answer.

I presume you already know that when both furnaces run at same time, that both don't starve at same time?, or that the other one causes this one to starve.

Anything you can think of unusual about the gas line to where it becomes designated to this, such as having some abnormal 'trap' built into the line due to it's layout, where maybe junk collected in it?
 
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Old 12-11-07, 07:39 PM
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the furnaces are about 10 to 15 years old and have worked perfectly until recently. The other furnace coming on has zero effect on the one slowly dying....... I'm going down ther to stare at it some more....
 
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Old 12-11-07, 07:49 PM
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The part that puzzles me is that I would think that if the gas valve were screwy it would always have low pressure. Why should the electric hold open the valve, say all the way, to begin with, and then no longer can hold it open much after a bit? You have already said the volts do not go down, as it runs - right?
 
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Old 12-11-07, 08:02 PM
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absolutely right. no change in the voltage to the coil. I'm going to take the cover off the valve. I looked at the gas valve a little closer a while back and remember that there is a bit of a circuit board that connects the MV, PV and common connection to the coils themselfes. Maybe that connection is dirty. Seems the little board should slide right out of the valve body. However after a couple of beer, I'll wait till tomorrow so that I don't mess things up.

cheers

re beer and fiddling, learned my lesson about easily screwing things up when I exchanged the boards between the two furnaces. When I pulled the high tension off one of them (still pretty hot since furnace just quit) I managed to pull the male pin right out of the high tension coil and broke it right off the potted wire from the coil.... 'that' did take a while to fix until I scratched away at the potting compound so that I could solder back onto the broken wire again.
 
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Old 12-12-07, 12:05 PM
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In your OP, you said that the flame SEEMS to peter out. So does it actually diminish AFTER the flame cones have been established? The reason I ask is sometimes with burners, at the moment of fire there is some turbulance at first until it sort of 'settles in'. I'd like to know that once the flames are no longer jumpy, and settle in, that the cones and overall flame height INDEED comes down, and is more than "SEEMS" to.

I love a good mystery, as long as there really is one.

Keep us updated.
 
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Old 12-12-07, 12:24 PM
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I know very little about furnaces, but is there a thermo limiter switch that could be acting up?
 
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Old 12-12-07, 12:29 PM
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Originally Posted by gary s. View Post
I know very little about furnaces, but is there a thermo limiter switch that could be acting up?
The flame would shut completely off then, even during a call for heat. He has another issue where his furnace starts doing this from the get go before the furnace would even have hardly enough time to heat up good inside.
 
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Old 12-12-07, 02:06 PM
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Replace the gas valve.
 
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Old 12-12-07, 07:41 PM
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Originally Posted by ecman51` View Post
In your OP, you said that the flame SEEMS to peter out. So does it actually diminish AFTER the flame cones have been established? The reason I ask is sometimes with burners, at the moment of fire there is some turbulance at first until it sort of 'settles in'. I'd like to know that once the flames are no longer jumpy, and settle in, that the cones and overall flame height INDEED comes down, and is more than "SEEMS" to.

I love a good mystery, as long as there really is one.

Keep us updated.
Absolutely, the flames indeed come down from an initial height of a several inches to half an inch at which point the exhaust fan draft extinguishes them. The temperature at this point has tyhpically increased enough for the blower to turn on for a while to deliver a bit of heat. I found that if I keep the thermostat at about 67F the furnace will shut off and cycle. That is, if I set the temperature heigher the furnace has not reached the thermostat temperature limit and will keep the pilot and vent motor running.... 23V or so is still at the main valve. Sooooo, what can cause the gas valve electromagnet to shut off slowly in time even though power is still applied to the coil..... seems to me the electromagnet should either work or not..... and this is the part I can't understand

cheers

cheers
 
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Old 12-12-07, 07:46 PM
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Originally Posted by daddyjohn View Post
Replace the gas valve.
sigh.... probably very true. But I can't pass up a good challenge. My motto: repair not replace...... to the frustration of my wife

The DIY repair probably will be more expensive in the long run though considering the wife went out and bought the most expensive space heater she could

will take a closer look at the valve asap ...

cheers
 
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Old 12-15-07, 07:30 AM
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Success!!

the furnace is working but I'm still stumped. Checked out the gas valve by comparing to the one in my second furnace. Everything check out perfectly.... so, the spark ignition module is good the valve checks out good.... being a
bit frustrated I whacked (gently, kind of) the black gas pipe leaving the valve and feeding the burners. Voila, the flames sprang to life. So what the heck is going on????? any guesses?

cheers
 
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Old 12-15-07, 10:17 AM
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Take off the INLET pipe to the gas valve and see if the inlet screen is clogged/dirty. Hitting the pipe jarred something- perhaps it dislodged a small piece of debris blocking a passage in the gas valve. You'll also want to check the manifold for debris/scale. Gas valves are operated by the pressure of the gas and have small internal orfices and passageways. I still think that, eventually, you're going to need a new gas valve.
 
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