Honeywell TH5220D Non-Programmable 2-Stage Heat/Cool Digital Thermostat

After a weekend of wild temperature variations from my electric wall heater, I started looking for a digital thermostat that I could use to control my LP furnace, electric wall heater, and the air conditioner. My plan was to use the electric wall heater (when power is available) to keep the trailer warm in mild weather and use the LP furnace as back-up heat for colder weather. I also wanted the same thermostat to control the air conditioner, and preferably, allow for two-speed fan control. I found this unit on the Honeywell website and it does it all.

What is unique about this unit is that it is a two-stage control for both heat and cooling. When connected to CG or generator power, I use heat stage-1 to operate the electric wall heater and heat stage-2 to control the LP furnace. When running on battery power, heat stage-1 runs the LP furnace and heat stage-2 is not used. For the air conditioner, cool stage-1 turns on the air conditioner compressor and operates the fan on LOW speed. If the air conditioner does not meet the cooling demand on stage-1, then stage-2 is activated, which switches the fan speed to HIGH. If the thermostat fan switch is set to ON, without calling for cooling, the fan runs on low speed.

Like the Hunter thermostat, this unit is battery powered and uses internal relay control to operate the heating and cooling units. This allows the thermostat, which is intended to be used for 24 VAC HVAC systems, to be used on 12 VDC RV systems. What is really great about this unit is that it can be powered from the 12 VDC RV electrical system and use the internal batteries for back-up. The R and Rc terminals are connected to 12 VDC RV power (I used a 3 amp fuse) for the control and power functions and the thermostat C wire is connected to DC ground to activate the power function.. Although this feature is intended for 24 VAC operation, it appears that the unit has a small power supply that converts the 24 VAC to 3 VDC internally. By hooking the unit up to 12 VDC, the DC voltage is simply passed through the power supply rectifier diodes and then regulated down to 3 VDC. Believe it or don't, it does work.

Making all of this work required adding 3 external relays: one to reverse the heat stage-1 and stage-2 lines depending on if 120 VAC power is available, one to apply 120 VAC power to the electric wall heater when commanded, and one to control the A/C fan speed.

Note that in my system, the A/C circuit breaker is manually switched between A/C and the electric wall heater. I did this because my circuit breaker panel did not have any spare slots for the heater, and since there is no reason to run the A/C and heater at the same time, this was not a problem.

After getting this unit installed and wired, I am happy to say that it works exactly as I intended it to. The suggested price for this unit is around $80 and is intended for professional installation only, so it's not available in retail stores. I got it on ebay for around $38 including shipping.