Running hot and cold

[Al Fresco]

I recently had occasion to use the Dometic thermostat (single zone, LCD) to run the furnace in my 2011 Sunseeker 2300 for the first time. The temperature in the coach was approx. 50 degrees. When I set the temperature on the thermostat to 72 degrees and toggled to the furnace mode, to my surprise, both the furnace and air- conditioner came on and ran simultaneously. (Only the furnace indicator light was lit on the thermostat.) Even when I set the temperature to 90 degrees and toggled to the furnace mode, the air-conditioner again came on and continued to operate together with the furnace. I finally dealt with the problem by shutting off the air-conditioner breaker.

Unfortunately, my motor home is currently sitting in an RV park and is not immediately available to me. I will be returning to the RV park next week and am wondering if anyone can suggest a likely cause for this anomalous behavior and what I might do to correct the situation. I suspect that some kind of wiring problem might be involved, but thought I would check to see if this could be related to something I am doing wrong with the thermostat. I am wondering if anyone else has experienced this type of problem?

[ChrisZazzara]

Actually, Al, you've stumbled upon a nice feature of the high end Dometic thermostat you appear to have in your unit. The way I've seen it demoed and explained shows that Dometic will start the furnace and the cooling system fan. Furnace to heat the coach and cooling system fan to pull the pooling warm air from the ceiling via your cold air return and distribute it further.

A friend of mine with a Cedar Creek 5th Wheel has the same feature and I think it's a pretty slick use of the ventilation system basically all managed by a higher end thermostat.

[grhodes50]

I have the Dometic Single Zone LCD thermostat in my tt. Does yours look like this;


If so, set the fan to auto before selecting cool or furnance. If you leave the fan on the high or low setting the AC fan will run constantly whether you have cool or furnace selected.

Hope this helps.

[Al Fresco]

ChrisZazzara,

Thanks for explaining this feature of the Dometic thermostat. Instead of a problem, I am happy to learn that it is actually a beneficial function.

grhodes50,

Yes, that is the thermostat I have. And thank you so much for explaining how it works. I did have the fan set on the “low” setting which accounts for the fact both the furnace and air-conditioner were blowing air. (Is this function specified in the instruction manual? Or did I just miss that part of the instructions?)

As usual, knowledgeable folks on this forum have come to the rescue.

[grhodes50]

Originally Posted by Al Fresco

ChrisZazzara,

Thanks for explaining this feature of the Dometic thermostat. Instead of a problem, I am happy to learn that it is actually a beneficial function.

grhodes50,

Yes, that is the thermostat I have. And thank you so much for explaining how it works. I did have the fan set on the “low” setting which accounts for the fact both the furnace and air-conditioner were blowing air. (Is this function specified in the instruction manual? Or did I just miss that part of the instructions?)

As usual, knowledgeable folks on this forum have come to the rescue.

The fan settings while set in furnace mode are explained on page 9 of the manual.

One other thing I would like to point out is on page 11 of the manual.
When using the AC and the fan is set on auto, the fan will run on high when the room temperature is 5° or more above the set temperature. The fan will run on low when the room temperature is 4° or less above the set temperature.

So when first starting the AC and say the room temp is 80° and you set the thermostat to 72°, the fan will run on high until the room temp reaches 76°. At that time the fan will shut off for a couple of seconds and start back up on low. The fan will shut off when the room and set temps are equal.

When you set the fan on high or low, the fan will run continuously while the AC compressor cycles on and off.

The first few times it did this I thought something was wrong.

[Al Fresco]

grhodes50,

Thanks for further clarification on the operation of the Dometic thermostat/heating/cooling system. I am not sure the instruction manual I received with the thermostat even had 9 pages. As I recall, it was rather brief. I will check when I get back to the motor home.