Perplexing Furnace Issue

[Jim Stoltz]

I've searched the 'Net for a similar problem but have come up empty.

The furnace fan will start and run fine, furnace produces heat, from a cold start. When commanded to come on again, the fan will run for half-a-second or so and quit. The thermostat checks out fine. There is a noticeable "click" at the furnace when the fan is commanded to come on, and again when it fails. If I shut it off and turn it back on it will fire up without problem until it reaches the set temp and needs to come back on again. I'm leaning toward the blower motor at this point. Any ideas?

[david craft]

most of those furnace motors have a thermal overload.  later model furnaces have a fan relay built into the board...a bad idea from my perspective. im going with the motor, unless it has the board with a relay...

[Jim Stoltz]

I'd swear that when I removed the thermostat and put the 2 wires together that I had the same problem (fan runs for half-a-second). But my 14-year-old son was having a bunch of friends over for a birthday celebration and wanted to "camp" in the RV (driveway is covered in ice and snow so the "camping" was in the driveway). I needed to figure out this heat thing.....

I moved the thermostat from the front furnace (the one misbehaving) to the rear, and moved the rear thermostat up front. Whaddya know? The problem moved to the back furnace. The front one is 35,000 BTU and will do the whole RV no problem. Without the rear furnace the back bedroom gets a little chilly, but bearable. 

What's interesting is that the fan strategy for the rear furnace is different than the front. The rear waits a while after the thermostat calls for heat before the fan runs. The front fan runs immediately. Whatever this difference is allows the rear furnace to sort of work with the bad thermostat. It heats, but cycles on and off a lot. The front one would try to come on once and throw in the towel. 

I ordered a pair of new Suburban thermostats from Amazon - they're pretty cheap.

[david craft]

thermostats havs an anticipator. its supposed to match the coil draw of the gas valve. if there is a slight short in the circuit, it will preheat the thermostat faster than its supposed to. i would bypass this fortesting purposes. the furnace with the delay has a time delay relay. the other prolly not. maybe one was replaced at one point. the delay function is the same that keeps the motor running during cool down. the relay without the delay uses a fan switch which has to cool down to shut off the motor.

[Matthew Tritt]

Very interesting, David. I had been under the impression that the time delay function was part of the furnace circuitry. The Suburban "Trail" furnace in my GTC has a pre-ignition time (with blower running) of about 30 seconds and a post-burn blower time of about one minute. Because it's from the early 70's it has a pilot, which has to be lit by the user. If the pilot goes out for some reason the fan will run until the thermostat setting is lowered below the inside ambient temperature. Do you know of any retrofit electronic ignitors that would work with this unit? It's a bit of a bother to lie on the floor to light the pilot with a match since the electronic pilot ignitor doesn't function.

Matt

[david craft]

interesting Matt, the pilot model should have no delay to ignite. the burner should ignite as the fan speed gets to the point where the sail opens the gas valve. not sure what nt model u have. a newer nt should drop right in. i do have a new orifice for the old pilot model..

[Matthew Tritt]

I could be wrong about the start fan sequence. It's been months since I ran it, so I'd bette check  and make sure. Is that new unit a peizo-electric????

BTW, the sail switch works fine. I had the whole thing apart to fix a problem with the fan making intermittant contact with the shroud. Ting - ting - ting, etc....

[david craft]

sorry matt..i missed this earlier, just happened to catch it on a reread. dinosaur electronics "fan 50" board will have a relay and will rewire fan power through board. if it doesnt ignite, the board cuts power to the fan motor. this board replaces the existing board for a few more bucks than the standard board. i thought you had a pilot model...if thats the case, you need a thermostat that will lock in the off position for there is no board im aware of that will solve the motor run on issue.

[Jim Stoltz]

Yes - that's exactly how it works. The front fan will come on immediately, but it will run for a while to cool down. The rear one has a time delay. The circuit board in the front one was apparently replaced by the PO - it's new looking and I can tell the furnace has been removed in the past. Hopefully the new thermostat will fix it.

[Jim Stoltz]

Fixed! The new thermostat had the terminals on the front rather than the rear so I had to extend the wires. Other than that, no issues and it looks a lot better.

[Rich Thomas]

I had to replace mine in the middle of my 2nd camp with the palace. It didn't have the symptoms yours had but I used the same thermostat worked well and still does. Glad you got it fixed.