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Old 11-16-2013, 07:58 PM   #1
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Suburban furnace not restarting

Furnace is a Suburban NT-16SE, and the thermostat is also a Suburban 161654 (at least it looks like that one; there's no model number on it that I can find). I took the burner out, and it has hardly any rust on it. The burner chamber looks clean, and the vents on the outside appear to be clear of debris.

Took the SMB out this week for a 2 nighter, and it was cold enough to use the LP furnace. First time really using it, other than briefly testing it when we got the van.

So I turn on the propane switch, and move the thermostat control to the middle of the range. Furnace fan kicks on, and after a few seconds, the burner fires up. Got the van nice and toasty in a few minutes.

Van warms up, and the thermostat correctly shuts the furnace off. After a while (15 minutes?), I notice it's getting cooler in the van again. Check the thermostat, yep, still set to middle of range. Wait about another 10 minutes, and it still hasn't turned the furnace on.

Turned the thermostat back to Off, and then back to the middle of the range. No fan, no heat. Tried this a few times, still no fan or heat. Turned thermostat back to Off.

After about 30 minutes, I try again. This time, the fan comes on, and then the burner fires up. Okay! Good!

Then the van warms up, and the thermostat shuts the furnace off. Then, as before, a lot of time goes by and the furnace won't come back on by itself. I have to wait quite a while with the thermostat Off and then try again. Sometimes it works, sometimes it doesn't. I took the cover off of the thermostat, and everything seems connected and moving as it should.

Back home today, I spent some time watching it work and trying to figure out the problem. All the wires seem secure, there's about 3/8 of tank of propane on the gauge, the thermostat coil moves as the temperature changes, but the furnace won't kick on after the first run. After waiting a while with the thermostat off, it will fire again. At least the problem is consistent!

I don't think it's the propane, but you never know. The tank was filled on the east coast, elevation about 300 ft. Now it's in New Mexico, and the elevations I've been trying to use the furnace at are between 3900 and 5300 ft. I did notice that when the burner is firing, the flame has a bit of yellow in it, but it had that at the low elevation, too.

Tomorrow, I'm going to try purging the air out of the LP tank, but I don't think this is the problem. After all, the furnace will fire the first time very easily. Also, with the propane switch off, the furnace fan will still start up. So I'm guessing that there isn't a propane sensor in the furnace.

Before I spring for a new thermostat, does anybody have any other ideas?
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Old 11-16-2013, 08:48 PM   #2
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Re: Suburban furnace not restarting

It sounds like an issue with the thermostat.

The start sequence that allows the furnace to start is; 1. Thermostat contacts close, 2. The fan starts, 3. The air flow switch inside the furnace closes, 4. The gas valve opens, 5. The spark igniter fires, 6. The flame sensor verifies gas ignition and you have heat.

The off sequence; 1. When the temperature is satisfied the thermostat contacts open, 2. The gas valve shits off, 3. The fan runs through the cool down cycle and shuts down.

Hope this helps you troubleshoot the problem.
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Old 11-17-2013, 09:11 AM   #3
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Re: Suburban furnace not restarting

It sounds like the thermostat is fine. I try to keep our propane tank always at least half full. I'm wondering if the tank is getting colder while the propane is being being burned, which might reduce the pressure in the tank and the burner is getting starved. Just a guess though.

I have had the opposite problem...filling the tank to the "full" mark and then driving through the central California valley in the summer and the outside temperatures warmed the tank so much that the tank then showed it being "overfilled" and the water heater wouldn't fire up until the tank/propane cooled down.

If you do replace the thermostat, get a programmable one (even though those in the hardware store are listed for 24v, they will work with our 12v systems). It needs to be one that uses batteries. You can then set the time that you want the heater to turn on in the mornings rather than get out of bed to do it. Nice when if you both sleep in the penthouse like we do.
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Old 02-12-2014, 10:36 PM   #4
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Re: Suburban furnace not restarting

Finally got back to this problem. Turns out that replacing the module board has fixed the problem...so far. The module board is screwed to the plenum of the furnace, and the heat slowly kills it.

I saw that Dinosaur Electronics sells an Extend-A-Life kit that allows moving the module board outside of the furnace box.

I also replaced the limit switch, hoping the cheap route would fix the problem so I wouldn't have to buy a board, but it didn't. Replacing the limit switch is apparently a recommended maintenance item, so that didn't hurt too much.

Thanks for everyone's suggestions! Much appreciated.

PS: Of course, I had to fill the propane tank just after the polar vortex hit the East, driving up propane prices....
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