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Old 01-17-2020, 04:27 PM   #1
Junior Member
Brand: DRV
Model: MS 40KSSB4
State: Colorado
Posts: 27
THOR #10148
WiFi Thermostat For Heater in DRV Mobile Suite

Last winter, I installed a Cheap Heat electric add-on to the heater in my Mobile Suite. It has worked great eliminating the worries of frozen pipes during the summer at 9,000 feet in the Colorado Rockies, and during those chilly mornings in Arizona in December and January.

I found the thermostat for the dual AC / Heater system that came standard in the rig inadequate when switching between the air conditioning and heater modes. In the late spring in Arizona, it can still be cool in the evenings / early mornings, and warm inside the trailer in the afternoons. This situation is routine during the summers in the Colorado mountains. I found myself adjusting the thermostat several times during the day and switching between heating and cooling modes two times every day.

Also, when we leave the rig in Arizona in late September, it is still hot, so we want to keep the interior of the ring at 80 degrees or cooler while we are gone. But, some years we don't get back to Arizona until late December or early January. During that period, it can get near or even below freezing (we had a few mornings in December, 2018 where the temps were in the mid 20's). So, we want the heater to be active at a 60 degree set-point for freeze protection and to limit the temperature extremes for the extensive wood paneling and cabinets in our unit.

The programming capabilities of the factory HVAC thermostat are inflexible (to be kind) and hard to use. The connections between the thermostat and the two AC systems are digital -- this limits the ability to upgrade the installed thermostat.

To gain more flexibility and automation, I decided to install a wifi thermostat to control just the heater. I will use the existing factory thermostat exclusively for the two multi-zoned AC units. While this is not the perfect solution, it solves the issues for our use-cases.

NOTE -- The install is not overly difficult, but should not be attempted by anyone who is not comfortable working with AC line voltages and HVAC systems. While I'm not a professional HVAC tech, I do have a working knowledge of the systems in my unit. PROCEED AT YOUR OWN RISK. THIS APPROACH WORKS FOR ME, AND IS PRESENTED HERE TO PROVIDE IDEAS FOR OTHERS WHO MIGHT HAVE SIMILAR ISSUES.

I will leave the factory thermostat for the two AC systems set at 80 degrees for most of the year, then utilize the limited programming it has during the warm weeks to achieve afternoon cooling that is more comfortable.

The new wifi thermostat will control only the heater, and will be set at 60 degrees for most of the year. During cool weeks, I will utilize the programming to wake-up to a warm living area, then return to the 60 degree setting in mid-mornings.

This automated schedule mimics the manual changes I routinely make.


I selected the Ecobee 3 Lite Wifi thermostat. It is designed to control complex HVAC systems. It is overkill for my situation, but it was less expensive than other thermostats which work with Apple Homekit, and provides the functionality I wanted. This thermostat requires 24 VAC; however, the factory heater utilizes a 12VDC control system.

To solve this problem, I selected an isolation relay utilizing an approach suggested in the Ecobee documentation. I used this one: Emerson 90-380 Fan Relay 24 Volt Coil.

To power the system, I selected a standard 24VAC HVAC transformer: White-Rodgers Class 2 Transformer, 40 VA Rating, 120/208/240VAC Input Voltage, 24VAC Output Voltage - 90-T40F3.

These two parts are readily available (about $25 for both on Amazon). I mounted the two parts in a 4x4x4 NEMA 5 Plastic box (available at the blue and orange big-box stores for about $10-$15. 25 Feet of standard 18-5 Thermostat wire completed the required supplies.

I've attached the Ecobee suggested wiring diagram which calls for three leads for the thermostat: 24VAC supply, 24VAC Common, and a 24VAC output when the thermostat calls for heat.

I mounted the thermostat just above the factory zone 1 temperature sensor, and snaked the new thermostat wire down the wall, behind the fuse panel, and extending out near the heater.

The factory thermostat actuates the factory heater by closing a connection between a 12VDC supply line to the thermostat and a control wire also connected to the thermostat. On my unit, the two wires at the heater are blue and blue with a white tracer and are connected to the factory thermostat via a low-voltage white/orange duplex wire. YMMV. BE SURE TO VALIDATE THIS CAREFULLY SO YOU DON'T DAMAGE EITHER THE FACTORY THERMOSTAT OR HEATER. I ran a short piece of thermostat wire (utilizing two of the five connectors) from the heater.

The NEMA 5 box is mounted in the pass-through screwed just above the sliding doors. The newly connected wires from heater and the new thermostat are terminated in the box following the attached diagram. The line voltage side of the transformer is connected to a new AC pigtail which is connected to an existing plug which was unused since I disconnected the factory convertor (in favor of using the Magnum inverter to charge the trailer house batteries and provide 12VDC to the entire trailer).

So far, everything works as designed.

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2016 40KSSB4
2019 Ford F450 DRW Regular Cab
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Old 01-17-2020, 04:29 PM   #2
Junior Member
Brand: DRV
Model: MS 40KSSB4
State: Colorado
Posts: 27
THOR #10148

Pics of the transformer and relay I used.
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2016 40KSSB4
2019 Ford F450 DRW Regular Cab
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Old 01-17-2020, 05:16 PM   #3
Brand: DRV
Model: 38PS3
State: Idaho
Posts: 37
THOR #2022
Nicely done and good contribution. I hooked up an additional thermostat in the bedroom because the main thermostat only pays attention to the living room. The thermostat is not wifi and is run with its own batteries. No relay or power supply needed.
I used the same two blue wires that you did to activate the heater/cheap heat system.
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Old 01-17-2020, 06:41 PM   #4
Junior Member
Brand: DRV
State: California
Posts: 4
THOR #17807
Thank you for the very informative post
2018 Mobile Suites 38 RSSA, 2017 Ford F450 Lariat Ultimate
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