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

by Greg Zimmerman
Published: Updated:

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.

Graeme B Cohen June 21, 2021 - 9:03 pm

My unit has a heater that reverses to produce cool. Will this hack work?

Greg Zimmerman June 21, 2021 - 9:21 pm

It depends on how your heat pump knows to switch from cold to heat. Is it a setting on the unit or in the thermostat? This is fairly “dumb” mod that only cycles power on and off to the A/C unit based off the digital thermostat. It won’t be able to tell the unit to go from cool to heat without some fancier wiring. But if there’s a “heat/cool” switch on the unit itself, then the mod can be used to cycle power to it.

Dan Casey April 24, 2019 - 5:09 pm

Nice write up ! đź‘Ť…..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

[…] Casita Projects […]

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

Randy Edwards October 25, 2020 - 4:38 pm

Greg, I did that exact mod on my 2007 Casita 17FD. I have been very pleased with it working well to cut off the fan when the proper (set on thermostat() is reached. However, I have had an issue that perhaps is related to the mod.

The issue is that my Casita drains the battery very fast when it is not plugged into AC but with nothing running or on in the trailer. I am thinking that the problem may be that when the trailer is not being used, the thermostat energizes the relay. Since there is no AC pluggedin, the relay stays energized and adds a drain to the battery.

Am I correct in my analysis? And am I correct that now that I think about it, perhaps if I turned the thermostat to OFF, the drain from the relay would not exist.?

Further, sunce you seem to be an elecltical guru, could you please tell me what you estimate the normal battery drain. Should be. I measured the current at the battery to be about 400 mA when the battery is at about 80% charge (determined by a digital battery charg meter I plug into the DC outlet by the TV corner). Thus between 100 and 60% charge on average about 1 A Hr (or Amp) is used per day.. I plotted % charge over time for about two weeks, and the points closely follow a line that is plotted for total Amp (100 A nominal fo rmy battery) declining by 1%/day.

Thanks for any comments,

Greg Zimmerman October 26, 2020 - 8:13 am


Initially I was going to say no. However if you leave the thermostat set to cool and have the trailer unplugged, the thermostat will in fact energize the relay. I’ve always turned the thermostat to “OFF” when in storage that was never an issue for me. The biggest culprit for battery drain is the propane detector. It’s hard wired to the battery and always drains power because it has a small heating element in there. That’s why I install a battery disconnect to unhook the battery while in storage. Otherwise the battery will die in about 3 weeks. Casita started installing disconnect switches in 2019.

Randy Edwards October 26, 2020 - 11:32 am

Thank you. Rather than install a switch (because the space is so limited in the battery compartment) I did install a terminal block that allows me to disconnect the positive battery terminal on the block with a battery wing nut. However, it is easy ro forget to disconnect it after returning from a trip. Lately folks have figurted out how to install a switch, so I might do that also. I take it that you agree that without a switch or disconnect, the thermostat should be put in the off position.

Frank Stern November 2, 2020 - 8:20 am

Thanks for your helpful site. On the Casita forums there is much discussion of using Hunter thermostats — any thoughts on that versus the Honeywell one you suggest?

Greg Zimmerman November 2, 2020 - 9:43 am

I have no personal experience with them, but no reason why not to. My local Lowe’s carried the Honeywell in stock so it was easy to buy when doing a job.

Don..T February 17, 2021 - 9:51 pm

I have a question.. Do I need the toggle switch if I don’t have electric heat with the Ac unit? I have propane heat that goes to a two wire stat.. I just installed an upgraded Honeywell T stat and hooked the furnace up to that.. Im now thinking about doing your mod and thinking I wouldn’t need the toggle.. What do you think?

Greg Zimmerman February 19, 2021 - 3:41 am

Yes, the toggle switch is there to make sure the furnace doesn’t come on at the same time as the A/C. You can only omit the toggle switch if you do not have a propane furnace.

Don..T February 19, 2021 - 11:12 pm

Ok thanks Greg

Dave June 4, 2021 - 1:13 pm

I caught the battery drain comments. My question is that if running off the battery and using the propane furnace the thermostat will kick on the relay and sap extra power from the battery draining it faster as well. I use a cpap machine so all battery charge is needed. Would maybe adding an additional on/off switch to AC/heat strip be needed?

Greg Zimmerman June 4, 2021 - 7:37 pm

I’m no electrical engineer, but the current drain on the relay is minimal. According the specs it might only be 1 watt max if I’m reading the chart right. If it’s a concern, you could always add a second switch that kills power to the relay when you don’t need the ac/heat strip. Or make it a 3 way switch, but then you can’t run furnace and heat strip together (not sure why anyone would need to…)

Here’s the spec sheet on the relay: https://www.mouser.com/datasheet/2/418/4/NG_DS_1308242_T9A_0312-199557.pdf

DP July 23, 2021 - 11:02 pm


I have a A/c where the fan runs all the time. and compressor kicks in and out as needed per ceiling knob cold adjustment. So I dont have to listen to fan run all night and once trlr is cool. Im thinking I can keep a compressor thermostat knob on full and put a 120 volt elect baseboard heater type that is double pole to 120v line in. BUTZ I need a 120v cooling thermostat, not a 120v heating thermostat. Do they make a 120v cooling thermostat AND will controlling the 120v line in work w/o damaging other a/c components???

Greg Zimmerman July 23, 2021 - 11:18 pm

Honestly I don’t know if they do or not. I wouldn’t trust a high voltage thermostat because it’s hard to run the wiring up to code (that stuff is dangerous). I am comfortable running 12 volt lines through the cabinets and modifying the AC outlet just enough to run the AC mod though.

DP July 23, 2021 - 11:20 pm

I feel very comfortable work w/ 120v. and wiring to code.

geekifier June 1, 2022 - 2:15 pm

Another option is to use a thermostat that works with heat pumps. That way the Y terminal is used both for heating and cooling, and there is usually an L/AUX terminal that is energized for “emergency heat”. Use that one for the gas furnace.

Geoffrey Porter September 2, 2022 - 3:20 pm

where does the new 12 gauge solid wire go?
where does the 12 gauge wire tap go?

Greg Zimmerman September 2, 2022 - 8:49 pm

The new solid wire goes from the TV power outlet to the relay box. The reason for the wire nut is to put a short length of stranded wire to the solid wire because you can’t put a crimp on connector onto solid wire. The crimp connector goes onto the relay. Hope that makes sense.


Leave a Comment

Adblocker Detected

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