A/C Mod (thermostat for the A/C)

by Greg Zimmerman

The “A/C Mod” is one of the most popular upgrades for 2018 and older Casitas. From the factory, the roof mounted air conditioner fan runs constantly and the compressor cycles on and off. It’s noisy and wasteful. Also, the mechanical temperature dial is hard to set because there are no numbers. The “mod” is to install a digital thermostat to control the roof top unit and the propane furnace (if equipped). The thermostat cycles power to the A/C via a relay so the whole thing shuts off when it’s not called for.

I can’t take credit for inventing the mod, that goes to Gene from the Casita forum, and Larry from Little House Customs for installing hundreds of them.

This article summarizes the installation. For detailed instructions, see this PDF document.

The Parts:

I use a battery powered Honeywell non-programmable thermostat model RTH111B. It’s available from the Amazon link, Home Depot or Lowes. I tried the fancier backlit model, but it’s more expensive, the batteries don’t last as long, and I had a bad one out of the box once.

The “magic” relay takes 12 volt DC current from the thermostat and cycles 120 volt AC power to the roof A/C unit is available from Mouser. Also needed are a pair of yellow 12 gauge female spade connectors and a pair of blue 16 gauge .187 female spade connectors as seen in the photo.

The blue outlet box is a standard “old work” style box with the tabs and screws removed and the bump drilled out of the bottom so the relay will lay down flat. Stainless screws are used to secure the relay in the box. Not pictured is a solid outlet box cover screws in place on top to seal it.

A 12 volt toggle switch is used to kill power to the furnace when using the electric heat strip. If you don’t have a propane furnace, it’s not required.

Not pictured are 12 gauge solid wire, 12 gauge stranded wire, 16 gauge stranded wire, and a 12 gauge wire tap.

The Install

I drill out the rivets and remove the old furnace thermostat. The 2 wires need to be tested to see which is positive and negative. Holes are drilled for new wires and the toggle switch. The new thermostat is mounted and screwed into place. In the microwave cabinet the wiring takes place. With a built in microwave, the round vent rivet is drilled out and pivoted out of the way for access. If the space is a pantry, the glued in carpet needs to be peeled back for access. The thermostat is wired according to the directions. Of note a second jumper pin has to be made (thermostats only come with one). I use sections of solid core thermostat wiring bent into little “U”s. It’s tedious, but works.

16 gauge wire from the thermostat is run to the upper rear cabinet in the “TV corner”. That wire goes up into the cabinet above the microwave and makes it way to the back corner with some 1/4″ holes, lots of tucking into carpet seams, and even more patience.

With power shut off to the Casita, the back corner electrical outlet taken out. One of the 2 black wires on the outlet is the wire to the A/C unit. Once it is identified and removed, it is extended to the relay box with a section of new wire, attaching to the relay with a stranded jumper and wire nut. The second AC wire is run back to the outlet and put in place of the removed wire. Now the relay can interrupt the AC circuit to the roof unit.

You can’t use a crimp connector on solid core wire, which is why we make a jumper from stranded wire and it connects to the solid core wire via a wire nut.

Wired relay

The thermostat wire runs to the relay, and a new ground wire is run from the relay and tapped into the white ground wire of either the 12 volt outlet or TV antenna amp in the back corner.

Once everything is tested and works, the relay box cover goes on and the box is strapped to the existing outlet box in the upper back corner.


There are 2 ways to run the system. The new thermostat can cycle power on and off to the unit so the fan stops when not needed. Or the thermostat can be “maxed out” and use the knobs on the unit to operate the old way with constant fan.

Air Conditioning: Roof dial to “cool low” or “cool high” and temperature knob full cold. Now set thermostat to “cool” and set the temperature. Since the roof unit is always asking for cooling (knob at full cold), the thermostat handles when it runs.

Electric heat (if equipped): Roof dial to “heat” and temperature knob to full hot. Toggle switch “off”. Set thermostat to “heat” and set temperature.

Propane heat: Roof dial to “off”, toggle switch “on”, thermostat to “heat” and set temperature.

Now say you’re in a noisy campground and you want the fan to run all night. To use the roof unit the “old” way, do the following:

Air Conditioning: Thermostat to “cool”, and set to 60 degrees. Roof knobs to “Cool low” or Cool high” and set temperature knob to your liking.

Electric Heat: Toggle switch “Off”. Thermostat to “heat” and set to 90 degrees. Roof knobs to “heat” and set temperature knob to your liking.


Gary Langley April 21, 2019 - 10:13 am

We love this mod. No more fan running all night. And propane furnace cycles when needed.

Dan Casey April 24, 2019 - 5:09 pm

Nice write up ! 👍…..only problem is my 2012 Freedom does not have the furnace…..what part would change for me then..?

Greg Zimmerman April 24, 2019 - 6:38 pm

The only difference is you don’t need the furnace cut off switch and you have to source 12 volts+ from a different source for the thermostat. So likely you’ll run 2 wires from the TV corner to the thermostat location. Or maybe the furnace wiring is in there anyways that you could utilize for the power.

Dan Casey April 26, 2019 - 6:08 am

Thanks for the reply and help…..you have done a great job on your write ups…..I found you on the Casita forums…. “kcdaniels”……

Full control of the A/C and heat (the hard way) - The Handy Camper June 2, 2019 - 4:24 pm

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Jim May 14, 2020 - 10:01 pm

Is there a way to thermostat control a non thermostat controlled rooftop AC unit on my 2000 5th wheel

Greg Zimmerman May 15, 2020 - 7:01 pm

Depends on what model A/C it is. Coleman has a new bluetooth remote unit. https://www.airxcel.com/rv/coleman-mach/products/accessories/bluetooth-ceiling-assembly


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