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uavpilotsmitty · Jan 5, 2007, 11:12 AM

I have a Coleman 7900 Series LP gas furnace in a 1988 mobile home that is used as a weekend cottage. During the winter, the furnace works fine when we are in the home and the thermostat is set to 68. When we leave, the thermostast is lowered to 45. When we return 1 to 2 weeks later, the furnace is not operating and the upper limit switch has tripped. I reset the switch and set the thermostat to 68 and the furnace works fine. This has happened 3 or 4 times. My first guess would be to replace the switch but I'm hesitant because I haven't been there to witness the switch tripping and I'm concerned something else may be wrong with the furnace and that the switch is just doing it's job. It doesn't make sense that the switch should just trip on its own just because the temperature is lower.

Anybody got any other suggestions or should I just change the switch?

Thanks!

NorthernHeat · Jan 5, 2007, 01:45 PM

First I would look at ductwork and filters. It makes no sense to me that such a cold return air temperature and a fairly constant temperature rise, that it would trip when you are gone and not when you are there, say at 68 degrees. I have a feeling you may have another problem going on. Also is this a limit to the heat exchanger or the flue pipe (a spill switch)?

uavpilotsmitty · Jan 8, 2007, 08:42 AM

Originally Posted by NorthernHeat

First I would look at ductwork and filters. It makes no sense to me that such a cold return air temperature and a fairly constant temperature rise, that it would trip when you are gone and not when you are there, say at 68 degrees. I have a feeling you may have another problem going on. Also is this a limit to the heat exchanger or the flue pipe (a spill switch)?

The limit switch is located in the blower housing (heat exchanger?). The blower is actually a Nordyne unit that replaced the original Coleman blower as part of an air conditioning add-on. The air conditioning add-on instructions called out transferring the switch from the original blower and the installation looks good. One thing I have noticed is that there is a lot of dampness (real high humidity) inside the home when we first go in and the furnace is off.

Thanks for your help.

labman · Jan 8, 2007, 11:18 AM

I don't like throwing parts at a problem, but if you replace the switch with a new one, it too should trip if Northern Heat doesn't steer you to the problem.

juggallojed · Jan 8, 2007, 03:30 PM

IF the power was interrupted when the heat cycle was in operation,, it is possible that the heat rose into the area where the safety is... this would cause the manually reset safety to open...
I've run into this several times after a power outage

Felrio · Jan 8, 2007, 04:06 PM

I am not very familiar with how these switches function (internal design, function and such)

Let's say that the switch it is doing its job... which is what?. trip at low temperatures?. if that is the case... have the temperatures being low enough to make it do that?. if not, then the switch could be defective; if yes, then it is doing its job.

When you are not there, you leave the unit without power... so the only difference when you are not there is temperature...

I hope this helps.

uavpilotsmitty · Jan 11, 2007, 09:45 AM

Originally Posted by juggallojed

IF the power was interrupted when the heat cycle was in operation,,, it is possible that the heat rose into the area where the safety is... this would cause the manually reset safety to open...
ive run into this several times after a power outage

Thanks for passing on your experience. I have been wondering about the effects of a power outage on the system since I have noticed other signs of power failure while we were away (melted ice cream, clock with the wrong time, etc.). I'll check with some of the locals about power outages as well as experiment with this the next time we're up there.

Thanks for your help and for all the other people who responded!!

T-Top · Jan 11, 2007, 08:01 PM

The blower limit is the last line of defence on your unit that's the reason its manual reset. The high limit is a auto reset. The blower should run from 90 seconds to 120 seconds after the heat shuts off. To me it sounds like a bad blower control switch or blower motor going bad. It maybe that it was not wired right from the get go when the new blower unit was installed.

labman · Jan 11, 2007, 08:38 PM

The power failure sounds likely to me. I have never had problems with my furnace coming back on after one, but I could see it happening. Guess you could check it out by flipping the breaker off mid cycle.

An additional problem could be when our power goes off, it often cycles on and off several times. That could trip the over load in the blower causing it not to restart when the gas came back on tripping the limit switch.

Maybe a cigar for Juggallojed.

windstar · Jul 3, 2008, 12:47 PM

The problem is when you turn the tstat way down when you leave. When you return and turn the furnace back on the 45 degree return air going back to the furnace causes the blower to go off and on.

Your burner continues to operate and no way to expel the heat from the heat exchanger fast enough. This is why your blower deck limit trips.