I have a Trane XL90 furnace (high effic, two stage) that goes into a fault condition every once in a while and will not generate heat until reset (a big pain especially if it happens in the middle of the night). Was wondering if there are any furnace gurus out there who might be able to advise me. The fault code displayed means either a grounding or igniter problem but I have checked both and nothing seems amiss. It sounds then it may be a controller board problem but I don't wish to replace it on spec as they want about $500 for the thing. Actually stopping a Trane is pretty easy in my case :)
How old is the unit? It might be a warranty issue.
If it has a silicon carbide hot-surface ignitor, go to your local supply house and get two more - one to replace, and a spare on hand. Much cheaper than a new control board - but remember that they are as fragile as porcelain. There's a reason they are shipped in a sturdy box well packed in foam.
When I last did HVAC full time these were just coming on the market, and I always had a few on the truck. Now we just put in a new furnace here that has one, and I need to go get a spare for the house.
The failure mode is for them to crack across the element and work part of the time - then after a few cycles they get hot and expand a bit and the crack opens before the gas lights off. You can see the sparks across the crack. The controller sees the open ignitor and trips a fault code blinkenlight, and that's one you have to unplug the furnace to reset.
Thanks for the suggestion Bruce - sounds like it might be worth a try. I've been able to watch it once or twice when it faults and the condition occures just before the ignitor is supposed to heat up. OTOH however, I measured the ignitor resistance and it showed in the normal range. Are there any precautions to replacing the ignitor other than fragility?
I just wish the dang fault codes were more explicit - at least then I could tell if it's a grounding *or* ignitor problem. I've checked the grounding all the way back and even ran an extra ground wire directly from the panel to the furnace - no improvement. A furnace guy here told me I might have to install a separate ground rod for the furnace but I find it hard to figure out why that would help if it's grounded properly to begin with.
One other thing I've noticed is that it has trouble starting once in a while in that the inducer blower has to cycle two or three times before the flame kicks in. It does this even in calm wind conditions. If it does not succeed in three tries, it quits and gives a code but this has happened only once since I installed it (in Sept 2000).
Before you spring for a control board check the flame sensor. Modern glow plug ignited furnaces don't use a thermocouple for flame sensing as was done in the older standing pilot scheme. There should be a short metal probe that is placed so one of the burner flames is in close proximity, a circuit on the control board senses it's resistance to ground which will change in the presence or absence of flame. These tend to get a light coating of dirt on them over time and will hold out the gas valve.
With the power to the furnace off (you'll probably have to cycle the power anyway to clear the fault condition) remove the access panel adjacent to the burners and look for a small, maybe 1/8" stainless steel appearing rod near one of the burners. Wipe it off with a soft cloth or paper towel. In a pinch just rubbing it with you fingers will probably do the trick.
On one of our furnaces (American Standard) I have to do this about once a heating season. This failure is characterized by the blower fan running continuously, but no heat. If I watch the start cycle the forced draft fan runs, the igniter comes on, the burner lights but then goes off. It tries to light three times then gives up and generates an error code. If the sensor fails after a successful start cycle the burner shuts down and the blower continues to run. Killing the power, wiping off the sensor, and turning the power back on solves the problem for maybe a year or so. My furnace guy gave me a replacement sensor but as it's so easy to clean and usually happens in the middle of the night I have not replaced it yet.
This seems to be a common problem and is easy to fix in the above manner. There are other failure modes in newer electronic controlled furnaces, if in doubt call your furnace guy.
Thanks Paul but I don't think that's my problem as the furnace does not get to that stage before the problem occurs. What happens is the inducer starts and ramps up/down normally, the fault condition then occurs just at the point the ignitor would normally begin to heat up. At that point, the inducer stops, the blinkenlight flashes (9 times) and the blower comes on and stays on.
I think I'll try replacing the ignitor on spec but it may be months before I know if it has done any good as the furnace can go that long now without a problem.
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