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Old 05-27-2024, 04:28 PM   #1
3 Rivet Member
lemley's Avatar
1988 29' Excella
Lorena , Texas
Join Date: Aug 2017
Posts: 231
Adding a Honeywell thermostat to a Dometic Brisk II AC

Early during COVID, the Coleman Machine 10 air conditioner on our 1988 29’ Excella went out. Air conditioners were hard to find. I finally found a Dometic Brisk II 150 miles away and drove to pick it up. The only distribution box available was the manual model for non-ducted systems. This matched what we were replacing, so that was not a problem.

One issue with this setup is that the distribution fan does not cycle with the compressor. The loud fan is always on. Another desire was to be able to be able to monitor the temperature inside the rig when we were out and left the dog inside. So my solution to both issues was to install a WiFi thermostat.

Before you read this saga, let me start out by saying that there is a better way. You can purchase an alternate distribution box, a thermostat interface and a Micro-Air WiFi thermostat for just north of $500. That far simplifies the control interface as 1) the MicroAir is set up for 12 Volts DC as opposed to 26 VAC like the Honeywell household thermostat that I used, and 2) the MicroAir uses the Dometic data interface as opposed to the discrete control lines for fan, compressor and heat used on a standard home system. But either way you have to do the work of running a thermostat wire.

I took a different approach. I kept the existing distribution box, replaced the AC control with a Coleman part designed to interface to the discrete control lines on a household thermostat, and added a 26 VAC inverter to power the thermostat.

Here are the parts I used.

Controller. Coleman-Mach 8330-752. $72 Amazon
Thermostat. Honeywell RTH6580WF. $34 Amazon
Inverter. Power Stream PST-INV1224. $50 EBay
Adapter cable Coleman to Dometic. Coleman RV Air Conditioner Adapter Wiring Kit for Furrion Chill, Dometic, or Advent Air Units. $44 etrailer.
Wire, switches, etc. $20

My process:
Route the thermostat cable.
Mount the Coleman-Mach controller
Install the inverter and a back box for the thermostat.
Install the thermostat.
Hook it all up and test it out.

Some details…
1.Running the thermostat wire. This was the hardest part.
This is a rear bedroom model and I wanted the thermostat there, replacing the Suburban furnace thermostat. Most of the routing can be done through cabinets. The challenge is getting from the AC in the roof to the cabinets. I located a route that was not blocked by horizontal or vertical ribs (except the roof rib).
I drilled 1/2” holes through the interior skin and through the roof rib. Note on drilling the roof rib: there are several wires running between the AC plenum and the roof rib. I used a long screwdriver to reach between the AC and the roof skin and hold those wires out of the way.
All I needed to do at that point was to fish the 3 feet from the roof to the hole I’d drilled in the skin inside the vent fan space. Should be easy, right? I done quite a bit of wire fishing, but this was ornery. Along the way I attached a small nut to the end of my push rod with a string. I was able one time to catch the string with a magnet, but it would not pull back through the hole in the rib. I should have removed the nut and attached to the string, but I failed in getting lucky enough to catch the nut again.
It then occurred to me to use my endoscope. I attached it to the pushrod with the nut still in place with the endoscope I could see what was happening inside the skins and was able to hook the nut and string.
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2. Mount the controller. The Coleman controller is set up to install inside a Coleman AC. I had to drill holes in the intake side of the AC for mounting.
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3. The thermostat was installed on the outside of the wardrobe, so I put the inverter inside the wardrobe. I pulled 12 volt power from the wires for the wardrobe light.
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4. Install the thermostat, hook everything up and test.
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