Anyone have any thoughts regarding a problem I’m having with my Carrier Weathermaker SX furnace? It seems to run normally for the most part, except when it runs for 2 hours straight or so. Like when I get home in the evening and turn it up after it has been turned down all day. After running for a long period of time, it seems to have difficulty starting again after the thermostat shuts it down when the house finally warms up. Here’s what I’ve found. The thermostat is calling for heat. The induced draft blower motor comes on, the pilot flame ignites fine and stays lit. Then the gas valve clicks/thunks but the burner does not light. Gas valve thunks again (I assume shuts off) and waits several seconds and then repeats the cycle. Eventually, the burner will light. If not, I can shut the furnace off for a few minutes and then turn it back on and every thing works fine. It appears to me that the gas valve is not confirming all conditions necessary to allow the burner to light and is then shutting off the gas after a few seconds. The burner is not igniting even for a short time. Only the pilot flame lights and it seems to work fine. I cleaned out the burner, intake and exhaust as they had some deposits of Asian lady beetles. Also the condensate drain appeared to be gunked up so I blew it out and it now appears to be clear. The furnace was installed new in 1991. It is model 58SXA080-GG, series 100 and has been converted to propane fuel. Is this furnace known for any particular weak areas? Any comments or thoughts would be appreciated.
It ain’t pretty fer nice, but it’s hell fer strong.
Replies
I had a similar problem recently with my York Diamond 80 furnace. It has electronic ignition (no pilot light) and would light up the flame ok. But a few second laters, the gas valve would shut off. It would repeat the cycle a few more times. The furnace guy diagnosed the problem with the flame sensor. This is a sensor near the flames that is supposed to generate millivolt current to keep the gas valve open. In my case, there was some dust on it thereby causing a weak current where it would sometimes work and sometimes not.
The repair man showed me how to read the diagnostic code on the furnace. There's a small port hole where you can see a blinking LED light. The number of blinks will correspond to a trouble code. This sort of information can also be found in your owner/service manual if you can locate it.
Without being able to put my meters on it , I would guess that the pilot assembly is getting weak.You have a bi-metal pilot safety that has to heat up before the main gas valve can open.
I think that you're probably right. It has a pilot flame sensing switch. I thought that the pilot would go out if the sensing switch failed and that's not happening, but I could be wrong. The furnace is 12 years old so it certainly wouldn't hurt to replace the ignitor-flame sensing switch assembly as a start. Thanks for your help.It ain't pretty fer nice, but it's hell fer strong.
I'm not familiar with this particular furnace but looked round a bit and can't find any specific info abut what type of pilot flame proving device you might have. It might be a readily available simple thermocouple that needs replacing, but whatever type of sensing device you have it sure sounds like that's what is most likely at fault here. It would be the cheapest place to start if you don't have the specs for your unit and/or a meter capable of reading current in the ranges delivered by these proving devices to confirm faulty performance.
Since I don't have the specifics on your furnace, it just might be that it's a Honeywell gas valve with a flame rod sensor ignition like GT described. There would be a small hot surface igniter which is responsible for igniting and establishing a pilot flame which must then prove itself to the main gas valve before ignition of the main burners is allowed. If so, then you would have a device that looks like the pic on the bottom of this page involved in the pilot ignition.
http://www.pdqsupplyinc.com/oemheating/heilquaker/quakerpages/flamesen.htm
There would be a pair of wires from this little unit that plug into the Honeywell valve control.
If this is so, then it might well be time for a new ignition module. Not very expensive (around $20 at most any HVAC place) and fairly easy to put in.
A couple of cautions here. Handle the thing with the greatest of care. Don't bump or handle the hot surface igniter itself or the flame rod sensor. The hot surface metal is very brittle and so breaks without any real provocation and if you touch the flame rod and don't clean it again, you've likely just substantially shortened it's life expectancy.
As most control units do these days, yours needs a good bonding wire connection to ground. While you're in there, make sure this connection is clean and sound.
Turn the power off to the furnace while you work in there.
FWIW, the term "flame rod sensor" here is really a misnomer. That rod doesn't actually sense flame at all but rather delivers current to the top of the established pilot flame. It's the flame itself that transforms and acts as a conductor for the AC current delivered into the top of the flame. The resulting current at the base of the pilot flame then arrives in DC milliamps. It's this current that the control valve senses and then subsequently allows the valve to the main burners to open. Therefore the metal at the base of the pilot flame must also be clean for conduction purposes and the size of the pilot must be adequate for allowing sufficient conduction of that current.
Knowledge is power, but only if applied in a timely fashion.
Edited 2/13/2004 10:34:20 AM ET by GOLDHILLER
Edited 2/13/2004 11:53:08 AM ET by GOLDHILLER
It actually has a bi-metalic flame sensing switch rather than a rod. I think that I'll replace it as a start and go from there.
Thanks for your help.It ain't pretty fer nice, but it's hell fer strong.
Dmarty,
Had a similar problem with my current furnace when first installed. It was caused by a build-up of moisture in the horizontal exhaust vent tube. Just enough to create a high enough pressure to kick the unit off upon restart. It was not plumbed arrow straight, and formed a sort of trap, thus restricting flow.
Is yours a high efficiency, ie. vented out the side of your house with PVC?
Jon
The condensate drain was what I first suspected. That would explain why it runs fine at first but not after a few hours of use. The water is not draining as well as it should and is backing up. I'll check it again. As I said before though, it wouldn't hurt to replace the pilot switch considering the age of the furnace, so I'll do that too. Thanks for the info.It ain't pretty fer nice, but it's hell fer strong.