intermittent burner problem

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  #1  
Old 11-19-08, 12:12 PM
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intermittent burner problem

15 yr. old natural gas furnace, HSI (hot surface ignition) with recent pre-season service.

I have an intermittent problem where the burners will not stay lit. They will continually light, go out, reignite, etc.
Inducer motor and blower motor work normal-ish (they do come on, but sometimes the blower runs the whole time and sometimes it turns off for a second at the end of the cycle but starts again).
This seems to occur in the mornings, but not every day, around 7-9am.
After reducing the thermostat and/or turning it off, after a few hours, the furnace usually works normally again.
The error LCD light has never come on (i.e. no lockout), which is supposed to indicate a problem with the pressure switch, limit switches, or control board.

What I've done:
new HSI (prior to all this)
new flame sensor
new ES filters
checked the tube from pressure switch to inducer for blockage


From what I understand, burners not staying lit can be due to:
flame sensor
gas pressure issues
limit switch

It sounds like there could be a bunch of other factors that could cause the burner problem, but since there appears to be a pattern to the problem, I don't think it's something like a ground or bad wire issue. I don't think it's the limit switch because there's no flashing error light AND the burner would not keep trying to ingite (would it? The temp would have to drop 40F, which would take more than a second).

My best guess now is that it has something to do with the pressure switch/inducer motor. The service tech said the inducer would eventually have to be replaced because that orifice is getting clogged up, but it is OK for now.

Since my furnace runs more overnite, could that heavier load cause a buildup of water in the tube from the pressure switch?
I did blow thru the tube the last time the problem occured. I could not tell that there was any water in there, but the furnace did run OK for a new minutes before I shut it off.

Any other ideas?

blue3
 
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Old 11-19-08, 04:36 PM
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Be prepared to catch it in the act, in advance, and be quick about it, and test for 24 volts leaving the pressure switch, test flame roll out switches (if you have them), at the gas valve, etc..

Make sure that 24 volts from thermostat is not quitting.

See if you lose power coming into control board.

HOW new is the flame sensor?

Make sure anything needing grounding is grounded good by freshing up any contact made.

Do you have any of those plugs with those pin connectors? They can be deceiving to where you think the jack is tight but the split pins have collapsed some and lose good contact. You can spread those pins wider. (I recently had this on a furnace that was making it run sporadically. Something very easy to overlook. Especially if anyone has replaced parts and unhooked those plugs in the past.)
 

Last edited by ecman51; 11-19-08 at 04:42 PM. Reason: added more
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Old 11-19-08, 08:30 PM
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Thank you for the ideas.
I have not looked for the ground wires yet. I hope the service tech would at least have noticed an obvious problem.

All the connections I can think of are spade(?) type. Maybe I will have to start checking those one by one.

The flame sensor is brand new, and the problem happened with my old sensor that the tech cleaned during maintenance. I suppose it could still be the wire/connection, but I would think that, as with any connector problem, would be a lot more random.

The last 2 times this happened, I have taken the tube off the pressure switch to check for blockage. I did it just now and was able to blow out a small amount of water from the tube. It has now run one heating cycle OK.

So my new question is about the pressure switch and inducer motor. If I am getting water buildup and this is affecting my burner, why am I getting the water, and how do I fix it? I have not seen this discussed anywhere, so I am clueless. The tech did say the orifice on the inducer was getting obstructed (with carbon deposits) but that it would run OK after they cleaned it. Maybe that is the problem?

Generally, the problem resolves after a few hours (I turn the t-stat down); but if it is water in that tube, is it really draining somewhere in a matter of hours without air movement?

blue3
 
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Old 11-20-08, 09:27 AM
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update:
After I blew thru the tube from the pressure switch, removing a small amount of water, the furnace ran one cycle OK. On the next cycle, the burner went out. I checked the tube again and it had water in it again. I cleaned it again but could not get heat all night
So the repair tech believes it is the pressure switch. I did have a leak because the vent stack had not been sealed above the roofline at the pipe collar (now fixed). The water ran down the outside of the vent stack; as far as I know, water did not get inside the vent pipe. But the repair tech figures that water got into the inducer assembly and thus got into the pressure switch.
I am set to have them replace the pressure switch. But now I am wondering if there is still water in the inducer assembly that should first be removed, so it doesn't get into the new pressure switch. How?

Would it be safe to assume that any water that was in the inducer assembly has now been removed over the few weeks that my furnace has been running? Does that thing get hot or would the air movement be enough?

blue3
sorry if I have used the wrong terminology; as you can tell, I am not an expert!
p.s. I did have the tech check the ground and a bunch of other things
 

Last edited by blue3; 11-20-08 at 09:28 AM. Reason: added p.s.
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Old 11-20-08, 03:28 PM
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Have you tried to blow out the condensate drain line both directions? May be plugged and is backing up.
 
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Old 11-20-08, 04:00 PM
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Do you mean the condensate drain that is for the A/C? Is this also used by the furnace? I know you get water from the combustion process but haven't figured out where it goes yet.

The condensate drain for the A/C had leaked/backed up because the installer apparently left a cap on that should have been open, but even before that was corrected I heard water moving thru the condensate drain pipe during A/C use.

blue3
 
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