Furnace blower spontaneously came on

[GreggH]

Problem: Furnace blower came on w/o turning it on.

Last night camping on SC beach, temp around 70, windows open, a little humid, thermostat control was in the off position, the furnace blower started spontaneously. I went over to the thermostat control, turned it to the on position and nothing happened, including not lighting up as normally turning it on. Did this a couple of times w/o any results. Went under couch and turned 12v disconnect off and blower, along with every other 12v, went off as it should have. I then turned 12v back on. The blower started on again, went over to T/control, turned on and this time the control lit up. Turned it to fan only which did start the a/c fan and the blower went off a couple of seconds after.

Now the furnace never kicked on but I don't know it ran long enough for the normal delayed firing to occur. Same with turning off, there is always a delay from when the blower receives a signal to shut off to when it actually goes off so I'm not sure if it was the turning off the 12v system or turning the thermostat to fan that turned off the blower. I tend to feel it was turning the 12v system off then on that did it.

Wasn't able to do any work on it today. I have the thermostat control unplugged right now but everything else is normal.

Any comments. Are you out there Howie?

[trekerboy]

What make and model furnace do you have?

[GreggH]

Sorry for the delay, we moved today.

The furnace is Atwood, 34,000 btu which is 8535-IV model.
Thermostat is Duo-Therm "Comfort Control Center"

[Foiled Again]

I hate electrical problems - but this sounds like a short somewhere. If your furnace is on a separate fuse/breaker, pull it until you get to a destination that allows you to have time to really trouble shoot it. literally sounds like a floating short somewhere.

[OTRA15]

I'll go with the short-circuit suggestion also. FYI here is another thread in which a crazy short (furnace blower and radio -- go figure!) was the problem:

http://www.airforums.com/forums/f44/...ue-148768.html

Sorry I don't have the expertise to help with the specifics of your problem.

Good luck!

Peter

[Gene]

Or change the thermostat, the easiest thing to do. You can test the circuits on it with a multi tester if you have wiring diagram for it. It may be online.

Gene

[6PackCharlie]

If I am reading your post right, you were not connected to shore power? If you were using batteries, you may not have had enough juice to spin the blower fast enough to make the "sail switch" activate.
I have a problem when I boondock. The first couple of times the heater comes on it works fine. But then the batteries get a little weak and the blower (probably need to get a new blower motor) does not spin fast enough to get the sail switch to connect. The blower will run non stop because the gas does not come on. When I am connected to shore power, the heater work fine.
I sometimes carry a third battery and connect it to the other two or more conveniently I leave my tow vehicle plugged in and that keeps the heater going all night.

[JCWDCW]

A spontaneous start sounds like a set of contacts that shorted or maybe did not open fully on last shutdown. The outside temp might have warmed things up enough to allow the contacts to touch and power the fan. If it cleared itself it may never happen again or it may become intermittent and the only way you might find it is to disconnect power and then go looking at contacts with a continuity meter....or obviously send it for service. A good look at the circuit diagram might give a lead as to where to start trouble shooting.
JCW

[guskmg]

Try rebooting the T-stat. Look it up under Dometic. However, I am inclined to think a relay is sticking too. Ask lewster, he knows Dometic in side and out.
guskmg

[A W Warn]

If the thermostat was calling for the furnace to come on, there would have been ignition. This rules out the thermostat. (unless there is a "fan on" function for your furnace. My CCC fan function only turns on the rooftop fan, not the furnace fan)

If the thermostat wiring short circuited so that the furnace fan came on, there would have been ignition. This rules out a short circuit in the thermostat wiring. (the furnace control wiring is two blue wires, so it is either on or off. There is no furnace fan only option on any I've seen)

I think the problem is internal to the furnace. I would suspect moisture on a circuit board, an insect on a circuit board, or a bad circuit board. If you continue to have this problem, get someone who is knowledgeable to diagnose it.

[Boxite]

Some furnaces have a thermistor which powers the fan until the plenum cools down after the thermostat shuts off the propane. I.E., if the thermistor fails it may think the furnace had been running previously (whether it had been or not) and the plenum had not yet become sufficiently cool to shut off the fan.
Just thinkin'....

[GreggH]

Thanks for the replies.
I've had the t/control unplugged until today. It got too hot so needed the a/c on and plugged the t/control back in and all has run as normal. We've been on the road for 3 weeks and haven't needed the furnace heat or ac until today. Earlier we did have the heat pump on at night and it ran normally. As a matter of fact, I can't remember the last time we've had the furnace on, honestly it might be 2 years or better. Short or relay sounds right. I had to replace the t/control 2 summers ago so that's fairly new, but not impossible. If there is a next time won't be so quick to shut it off to see if the furnace would have fired up and after shutting off the 12v, give the blower a chance to shut off as I turn things back on to see if involves the t/control. I checked what I could w/o major disassembly and nothing obvious. Will investigate better when home. The pain about phantom elect problems is you never know when they pop up again or if they will.
Thanks again.

[A W Warn]

I doubt you could have reacted fast enough to stop the furnace from igniting if the thermostat called for the furnace to come on. It takes only ~15 seconds for the fan to run enough to move sail switch to allow the furnace to ignite.