Automatic Digital HVAC Thermostats

Since my coach uses a single thermostat controlling a roof A/C and a propane furnace changing out the original Coleman for a residential automatic change over was pretty easy.

Honeywell RTH5100B

Powered by 2 AAA batteries that last a year. Non programmable but will do auto change over for heating and cooling. In auto mode the preset heat and cooling temps must be separated by 3 degrees. No over lap. High temp for heat of 68 and a low temp for cooling of 71 as an example. I have mine set to 68 and 78 which has worked well.

Any owner considering the option will need to dig deep into the capabilities and limitations of any thermostat they are considering and the need to match it to the HVAC hardware installed in the coach.

The Dometic digital thermostat in both our Newmar coaches work that way. Set to auto mode will apply heat or cooling as needed to maintain setpoint. Programmable to change temp setting twice daily at set time. The hysteresis is not user adjustable (the difference between set point and when the climate control activates). IIRC it's 2 or 3 degrees.

Sometimes to be more energy-efficient I will set one zone to cool ~80 and the other zone to heat ~68.

chuckftboy wrote:
Since residential thermostats are designed 24 volt ac you would need to use one that has alternate battery power as DrewE said. Motorhome thermostats are designed for 12 volt dc operation. I have 2 heat pumps and 2 furnaces so in my case, I would need 2 thermostats, one for each a/c furnace combination. Also most newer motorhomes use remote sensors and home thermostats typically sense at the thermostat, making location difficult.

To answer Doug's question. Modern home thermostats have a 3rd selection "AUTO" for auto change over from heat to cool with a built in differential between the heat and cool setting of no less than 2 degrees differance. If heat is set to 70, cool can't be set lower than 72. Referred to as the thermostats "dead zone".

I have AUTO also in my home. But that is to control the whole house blower fan. You set to Auto and the whole house fan shuts on and off with the tstat when met. If you put on FAN, the whole house fan runs all the time and the AC/HEAT just quits running. I also looked up the hi dollar NEST home tstat for $250. IT also does not have the capability of auto switching between Heat and COLD. You MUST tel,l it HEat or Cold. Doug

Since residential thermostats are designed 24 volt ac you would need to use one that has alternate battery power as DrewE said. Motorhome thermostats are designed for 12 volt dc operation. I have 2 heat pumps and 2 furnaces so in my case, I would need 2 thermostats, one for each a/c furnace combination. Also most newer motorhomes use remote sensors and home thermostats typically sense at the thermostat, making location difficult.

To answer Doug's question. Modern home thermostats have a 3rd selection "AUTO" for auto change over from heat to cool with a built in differential between the heat and cool setting of no less than 2 degrees differance. If heat is set to 70, cool can't be set lower than 72. Referred to as the thermostats "dead zone".

The "poor man's" way to automate the changeover would be to use separate heating and cooling stats in each zone with a buffer set between the heating and cooling settings to prevent any operational overlap. One of our previous sticks & bricks homes was actually set up like that when we bought it. It worked well enough, but it was eventually ripped out when we upgraded to a heat pump and natural gas system.

this link is for 4 zone RV system. This is for a Dometic system.

http://www.forestriverforums.com/forums/attachment.php?attachmentid=28112&d=1366084072


My home AC/Heater works the way you want but will not work with another system. Heater/AC units mother board talk to each other and the thermostat. I have not seen a rV system that does that.

Doug, system work just like you want, if you set the hot/cold temp to close you could get what you said. I would have to go look to get the temp balance numbers, but that also is build in so you would have to over ride the numbers to get a conflict. Just two weeks ago here, we had a February warm up. House got up to 78 and AC came on cool it back down to 76. We had this happen several days. No one touch the thermostat. Our heat is set to 72/73 if a lot of wind.

I would think a wall stat that would switch between COLD and HEAT operation automatically would be counterproductive. It would HEAT the RV lets say to 75 degrees. But, with the variance on tstats it may heat to 78 or 79, then the AC unit would start and cool it down to the preset 75 or even colder depending on the variance of the tstat. You would have a constant up and down as each system competed with each other. BTW, my home programmable tstat(18 years old) has OFF and HEAT and COLD modes. In summer you switch to COLD. In winter you switch to HEAT. Now, maybe newer home systems have just one mode--- you set the temp and the system determines to HEAT or COOL, but I would think there would be competition between the modes. Maybe someone with Home AC/Heating qualifications can clear my question up. Doug

Most relatively inexpensive digital thermostats (that I've seen) will not automatically switch between heat and cool modes, which seems to be the gist of what you want.

Assuming your RV thermostat system is compatible--that it is not some keypad driven one with proprietary digital control signaling between the thermostat and the air conditioner-- pretty much any digital thermostat that can run on battery power (batteries in the thermostat) will work. If your home thermostat that apparently can switch between both heat and cool modes automatically meets that description, it should also be workable in the RV.

Another approach would be to have two thermostats, one dedicated to heating and one to cooling, and make sure the temperatures are not set so that they fight against each other when they're both on at the same time.

We have two furnaces and two A/C's. All are controlled by two Honeywell 7-day programmable digital thermostats, one in each zone. The original thermostats were Duo-Therm analog units. The installation just involved some minor wiring and the addition of a mini-toggle switch to control the hi/lo A/C fan speeds.

If you had one AC and one heat unit, almost any of the digital thermostats would work.
With 2 or more AC units, in different zones, along with energy management, it might be a little more difficult finding a residential thermostat that will work.