Full control of the A/C and heat (the hard way)

by Greg Zimmerman
Published: Updated:

I previously wrote how to do the “A/C mod” to have a digital thermostat control the air conditioning and heat through a relay. It works well and is doable for the average handy person. I also install the A/C mod for customers. This project however, is the varsity league super challenge.

What this mod does is convert the roof mounted A/C and furnace to the same full functioning wall mounted electronic thermostat that the big campers use. The system works just like in a house. I can select fan hi/low or auto which cycles on and off. I can select electric or gas heat. If the electric heat is selected and it’s 5 degrees colder than the thermostat is set for, it will automatically turn on the propane furnace to assist. Very clever.

The parts are expensive and the hardest part is running a 1/4″ diameter thermostat wire from the roof unit, under the ceiling carpet, to the thermostat location.

Credit for this mod goes to Casita forum member “Hotrock59” who is a certified HVAC technician. His instructions are here:

The Parts:

Coleman control module

The brains of the mod is the Coleman “ducted” install control module.I got mine from PPL motorhomes as well as the Coleman thermostat, I used the Coleman 65659. If your Casita doesn’t have an electric heat strip and you want to add it, it’s here. For wire, 5 strand thermostat wire is required and a hand-full of blue female spade crimp connectors.

The Installation:

The ceiling mounted part of the A/C has to be removed as well as disconnecting the wiring. The 4 mounting bolts for the A/C unit are taken out and metal hold down ring taken down.

By far the hardest part of this is running the new thermostat wire under the carpet to the thermostat location.

I used a GB steel “fish tape” to poke between the carpet and fiberglass. Trying to get around the corners, I ended up snapping the tape and breaking it. The solution was to cut little holes in the carpet and only run the wire in small sections before poking ahead and pulling the wire for the next section.

Once the wire was run to the thermostat location, the heat strip was removed and the control box connected to the power wires that went to the old knob. The plug going to the old temperature knob was moved to the control box as well. The heat strip was plugged back in and a thermo-probe inserted into the condenser fins and plugged in. Next the wiring from the thermostat was connected to the control box with the spade connectors. A ground wire needed to be screwed to the metal chassis of the A/C unit. It’s a tight fit.

To gain access to the back of the thermostat, the carpet had to be peeled back. The thermostat wire colors don’t match the thermostat or control box, so I made a diagram and printed out the instructions from the forum article to make sure everything was wired properly.

Once operation was verified, I used 3M headliner spray glue to put the carpet back. The holes in the carpet from running the wire are barely noticeable.

As mentioned, it’s expensive, time consuming, but in the end a fun and functional mod to greatly increase the enjoyment and functionality of our Casita.


Joey June 28, 2019 - 9:05 am

This MOD is in the future for me, thanks for the post. @Travelrn910

robert davis July 21, 2019 - 9:20 pm

Casita must have read your stuff, my 2019 has the gas heater and A/C cool and warm all on the same thermostat

Jim Booth May 12, 2021 - 8:50 pm

This is a great write up. I have a few questions. I have the thermostat wire run, finally. I’m at the step to connect AC to the new control module but the existing AC cable is a few inches too short to reach it, and especially to connect the wires inside the metal junction box. Did you splice onto the Casita wire to reach, and left wire nuts hanging? I take it the old brown thermostat wires can be cut away, but what are they connected to at the other end? How does the furnace control function get back to the furnace? Thanks much.

Greg Zimmerman May 13, 2021 - 2:12 am

I’ll send an email with explanations.

Dave December 3, 2021 - 10:11 am

Greg, would you send me the email and explanations, too. I am a “neviche” electrician – and, know black wire is positive and white is ground!

I have have 2021 Casita with exchanged Mach 3+ with turn style knobs and continuously running fan! I contacted Casita for help on install but no response. Thankfully, I saw your post!
Thank you for sharing your dyi.

Michael March 6, 2023 - 7:50 pm

Greg, I just saw your post! I, too, have a 2021 with replaced Mach3+ with turn-style knobs for heat strip and ac. We expected to be able to control hvac from thermostat. Disconnected wiring between the thermostat and ac controller does exist in our trailer. Are your instructions able to help me make the changes that lead to being able to control the furnace, ac and heat strip from thermostat? Casita wouldn’t help me.

Greg Zimmerman March 7, 2023 - 1:12 am

Yes, you will be able to control both the furnace and AC from the thermostat following my instructions.

Michael March 7, 2023 - 10:05 pm

Thanks Greg. I am going to try updating our hvac control box using this blog entry. You mentioned an “average handy person” can do it! I consider myself that person! The good thing is I don’t have to run the wire. Thank you for sharing.


Leave a Comment

Adblocker Detected

Ad blockers remove the social media links, contact us info, and the newsletter sign up button from our site.