I had a new Suburban forced air furnace installed in December 2009. It hasn't been used a lot since then, but fairly regularly, and has worked flawlessly until a few days ago when the fan came on, but shut off again after just a few seconds. I tried several times, but it appears the burner isn't lighting.
On the old furnace, the clicking sound of the peizo-electric ignitor was loud enough to hear easily over the fan, but this one has always been too quiet, so although I've listened for it and can't hear it, I can't be sure it's not sparking. I've seen no flicker, flash, or flame in the burner window. It does exactly the same thing every time-- fan comes on a few seconds, then it shuts down without ever lighting.
All my other gas appliances are working fine. I has just filled my propane tank immediately before this happened, but that seems a probable coincidence, not a cause.
I find no chewed wires, dirt, hair, spider webs, dust bunnies, etc. in the unit or around it. As far as I can see (without removing the unit), it looks normal.
I have limited tools & knowledge and can't find this particular problem in any troubleshooting guide I own.
Ideas?
* This post was
edited 10/29/11 08:13am by dragonflyspit *
I would bet either the sail switch or the limit switch are bad- on the newer fan control ignition module boards the blower will shut off if either of these is bad (the board know the blower is on, but gets no power on that circuit).
How is the battery voltage? Low voltage causes reduced blower speed, reduced air flow, the sail switch doesn't activate, and the propane valve doesn't open, so no gas to the burner and nothing happens when the ignitor sparks.
Tom and Lynne
Tom is an Electronics Engineer, Lynne a retired teacher.
2003 Foretravel 38' U295
Voltage is good. The same thing happens whether the RV is parked on 12-volt, plugged into 110, or the engine is running, so I'm all but sure it's not a voltage issue unless for some reason voltage is good everywhere but at the furnace. Blower doesn't sound slow.
Sail or limit switches going bad on such a new, and lightly used furnace seems unlikely, but stranger things have happened. My old one used to go through limit switches regularly for no reason anyone could determine, but it was the original furnace. Even a new "dinosaur" board didn't fix its problem. That's why I replaced it. This one didn't act like that one did. That one would blow very hot, trigger the limit switch, and eventually burn it out. This one never blows anything like as hot as that one did, and has never tripped the limit switch as far as I know.
Can I check those switches myself? No way can I afford another %$#@ RV repairman in my current financial situation.
Any of you furnace guys still reading this? Since voltage would be very unlikely, is the sail or limit switch the only possible cause? I don't know much about this stuff, but it seems to me those shouldn't go bad on a lightly used 3 year-old furnace (would kind of piss me off if Suburban expected people to pay $95-$150/hr plus parts to have someone replace those every few years).
What else might cause it? What can I check for? I have a multimeter & a few tools. Can I replace sail & limit switches myself? How do I test to see if they are working?
Help me out guys. I know some of you know this stuff really well.
The tests would be to ground the meter at the furnace, and test voltage at the thermostat wire coming in, and after the limit switch, then sail switch (or start at the board and work backwards).
*If* you have an SF series model, you can test the limit switch easily by removing the inside cover, the sail switch through the outside cover. I find the most common problem on that model is oxidation on the limit switch terminals (just remove the wires and burnish the terminals).
The exact model number is really needed for more detailed help.
I have a 1989 copy of Bob Livingston's RV Repair and Maintenance Manual, and I have the owner's and installation manuals for the furnace. They are all helpful, but not helpful enough. None say how to actually get into the furnace (they don't want ignoramuses like me doing that).
I can take the inside cover off the cabinet and see the external portion of the ignitor. Wire looks great. No dirt or pet hair anywhere inside the cabinet that I can see. Wire nuts all seem clean and firmly attached. Everything I can see looks lovely and new.
Thermostat on the wall causes the furnace to come on, so I'm sure that's not it. The blower starts first, as it always has, but it shuts off after less than a minute (maybe only 30 seconds) and the exhaust air outside never even begins to warm. I'm sure it never lights at all. I've tried looking in the burner window for a spark, but can't see anything. On my old furnace, I could see lights indicating whether the ignitor was arcing, or the flame was on. I could always see one or the other if it was running. I can't see a thing in this window (but never looked while it was working okay)-- don't know if that's by design or not. Also, I cannot hear the ticking sound of the ignitor arcing over the sound of the blower, but on this furnace, I never have been able to hear it like I could on the old one (old one was quite loud and obvious).
It occurs to me now that it might have been indicating a problem for a while but I hadn't recognized it: it would light and heat normally for what seemed a proper amount of time (room would warm). However, it would often immediately click back on as soon as the blower stopped, and it would run through another heating cycle. It's not so cold here that the room cools that much in the few minutes it takes for the blower to blow out that last bit of warm air, so I'd assumed that I'd adjusted the thermostat on the wall a bit too narrowly. I kept meaning to check it, but I never actually looked to see if the contacts were open or closed when the furnace shut off before restarting. I still suspect the two things are totally unrelated, but I might be wrong. Thought I'd mention it in case it's important.
The first answer to your question is all you need. One of those two switches is bad. Period. Use a test light, multimeter or disconnect the wires and use an ohmmeter to figure out which one it is.
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