Furnace and AC operating at same time

[Travelchief60]

First trip with our 2019 27RB. Have not needed to use furnace until this morning. Outside temp of 35 degrees. Using our Dometic thermostat we set the mode to "furnace" and set temp to 67 degrees. Immediately the AC kicked on and started blowing cold air. A few seconds later the furnace started blowing warm air through floor registers. This continued until we turned off the thermostat. Tried it a few more times with same results. Suggestions? Ideas?

[LabMan]

Our thermostat has a setting for the fan of either "auto" or "on". If we run the furnace with it in the "on" position, the AC fan only will run along with the furnace. Putting he control between auto and on or leaving it in auto prevents the AC fan from running with the furnace on. Our thermostat is a Coleman digital, don't know if yours work like it, but that's the only way the AC fan runs without it set to "cool" for us.

[o2silverado]

Originally Posted by LabMan

Our thermostat has a setting for the fan of either "auto" or "on". If we run the furnace with it in the "on" position, the AC fan only will run along with the furnace. Putting he control between auto and on or leaving it in auto prevents the AC fan from running with the furnace on. Our thermostat is a Coleman digital, don't know if yours work like it, but that's the only way the AC fan runs without it set to "cool" for us.

What he said, turn the fan to auto.

[ASTMedic]

Originally Posted by Travelchief60

First trip with our 2019 27RB. Have not needed to use furnace until this morning. Outside temp of 35 degrees. Using our Dometic thermostat we set the mode to "furnace" and set temp to 67 degrees. Immediately the AC kicked on and started blowing cold air. A few seconds later the furnace started blowing warm air through floor registers. This continued until we turned off the thermostat. Tried it a few more times with same results. Suggestions? Ideas?

Need to calibrate your thermostat. I do that everytime since when you kill the batteries and pole power there's zero power to your trailer. Its like your thermostat is a new install every trip needs to be calibrated as such

[Travelchief60]

Thanks for your guidance. Suggested resolutions proved successful.

[Lafayettemo]

How do you 'calibrate' a Domestic thermostat?

[jimp]

I have a Dometic thermostat as well, and had the issue of "short cycling" when using the AC, not the furnace. I was just told this past Friday by the service tech to turn the fan on either high or low (not auto) as the thermostat will flip the AC on to cool the unit down if the furnace heats it up too much. Tech stated the fan will eventually shut down/stop after the AC or furnace stops running when the set temp is reached.

I got the impression it would work similar to a residential heat pump unit (we don't have a heat pump on our RV) based on what the tech said. Will experiment soon with the unit. Tech also said if you use the unit below the Mason-Dixon line (south of VA/KY southern borders) to run the fan on low or high (and the fan will still stop blowing air when set temp is reached) and north of the MD line, to use auto.

I have heard/read to run the fan on constant to prevent the AC coils from freezing up during high humidity. I really think the Dometic thermostat is junk IMO. LOL

[JaycoJeff]

2020 SLX8 224BHW Dometic thermostat. Press mode to get to fan mode. Press arrows to toggle to auto. Press mode to get to furnace. AC fan should not come on now when using furnace.