The Great A/C Thermostat Mod - Fiberglass RV


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Old 03-21-2006, 11:10 PM   #1
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Trailer: 2007 Casita Liberty (Sold 2011)/ Honda Odyssey
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This modification makes RV-style roof air conditioners (that normally run continuously) turn on and off like a normal home air conditioner, and replaces the Suburban propane furnace thermostat with a modern digital one. It was recommended by Gene at the Casita Forum, and you can find his extensive writeup here: http://www.casitaforum.com/invboard/...?showtopic=379

An annoying problem with the Coleman R/V roof air conditioners that are installed by Scamp, Casita and others is that they run continuously, shutting the compressor off but not the fan when the selected temperature is reached. This modification adds a heat/cool thermostat and a relay that turns the A/C completely off, and provides a central control for both heating and cooling.

Gene has verified with Coleman that this is not harmful to the A/C, and I noticed in my testing that the Hunter brand thermostat even has a time delay for the A/C to avoid compressor problems from stopping and re-starting too close together.

Two main items are required for this mod:
1. Thermostat - Hunter Digital #42995B or #42999B - WalMart $20. See Photo #1
2. Relay - Omron #G8P-1A2T-F12VDC, (12 volt, switching 120 volt) - Mouser Electronics $4 + $6 shipping - http://www.mouser.com/index.cfm?&handler=d...*&N=0&crc=false. See Photo #2
3. Assorted wire, crimp-on connectors, screws and enclosure for the relay.

The simple explanation is you replace the existing furnace-only thermostat with the new one and install a relay controlled by the thermostat that acts as a switch on the A/C power line. To operate you leave the A/C turned on, with A/C thermostat turned up to maximum cooling. The new thermostat and relay then control it, and turn the A/C on and off using the setting on the new thermostat. As a bonus, the propane furnace is also now controlled by a digital thermostat. A wiring diagram is shown in Photo #3.

Some roof A/C's have heat strips and those can also be controlled by adding a switch and a jumper as mentioned in the instructions below. Gene covers this in detail as that is what he had.

This is a relatively simple process but involves some contorting in the 13' to mount the relay. Be sure to disconnect the power cable while doing the installation.

The process is described starting in the next post.
Attached Thumbnails
Thermo_Outside.jpg   Relay.jpg  

Schematic.jpg  
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Old 03-21-2006, 11:15 PM   #2
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Remove thermostat
Pop the lid off of the existing thermostat (if so equipted) and you'll see picture #1. The top left wire is the +12 volt coming into the thermostat. The bottom left is the feed going to the furnance. You may want to confirm yours is the same with a voltmeter. Mark the +12v line. Remove the screws for the wires, then the screws holding the thermostat. (Careful, the 12 volt is live!) Cut the connectors off to allow complete removal of the thermostat.

Run an 18 gauge (or larger) wire from the thermostat location using the existing hole to where the relay will be installed. This will usually be in the converter/breaker box electrical area. On the Scamp it's directly below the thermostat, so an easy wire run. Install a female wire connector on the end that will connect with the relay. See relay picture above for the connectors I used.

Locate and modify A/C cable
The cable on my Scamp runs down the wall under the wall covering and emerges into the electrical compartment connecting to the converter/circuit breaker. Identify which circuit breaker is for the A/C and which AC cable is connected to that breaker. The easy way to do this is to run the A/C and turn off breakers until the culprit is found.

While the cable could be cut and the relay installed using the two ends (assuming there is enough slack), it's better to remove the cable from the breaker and install a new short piece of cable to run from the breaker to the relay.

To do this, first make sure your AC is disconnected! Remove the cover from the converter and locate A/C breaker and the three wires to it. See picture #2. Loosen these wires, then go to the back side and loosen the cable clamp and pull the A/C cable out. See picture #3. Insert the pre-trimmed and stripped cable segment (mine is blue) and attach to the three screws on the breaker. Female wire connectors sized for the relay should be installed on the two black wires, while the white and ground wires will be connected with wire nuts.

More below
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Thermo_Inside1.jpg   Breakers.jpg  

Breakers_Back.jpg  
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Old 03-21-2006, 11:18 PM   #3
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Install the relay
Find a suitable location for the relay enclosure. I used a rather large electrical junction box I had, but a normal size one works too as seen in Gene's pictures. Install the relay in the j-box. Attach the box to the wall. Feed the A/C cable ends into the box and plug the black wire connectors into the relay (Make sure you are connecting to the 120v relay connections!). See picture #1 and relay picture in Post #1.

Plug the wire from the thermostat into one of the relay's 12v connectors. Run another wire from the other 12v connector to a -12v source, or ground. There are a number of 12v wire pairs available having wire nuts. Use a volt meter to locate a negative 12 volt line, and use that. Or, if available, use the grounding bar on the breaker box as seen in picture #2.

Install the new thermostat
Feed the two original wires through the back of thermostat along with the new 18 gauge wire. Make sure the thermostat is Off. Install to the connections as shown in picture #3. The line to the furnance goes to W, the line to the relay Y, and the +12v line to Rh.

As mentioned earlier, if the A/C has heater strips and you want to control them, then a toggle switch needs to be added in the line to the propane furnance to keep it from running, and a jumper added between connector points W and Y, as the relay then controls both heating and cooling.

More below
Attached Thumbnails
Relay_Installed.jpg   Breakers_Back.jpg  

Thermo_New_Inside.jpg  
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Old 03-21-2006, 11:19 PM   #4
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Finishing and testing

Reinstall the thermostat cover, plug in the trailer's 120v cable, turn the A/C on with the thermostat all the way up (but nothing should happen!), switch the new thermostat to Cool, lower the temperature setting, and the A/C should come on. Turn to Heat with the propane on, raise the setting, and the furnance should come on.

Good luck and enjoy the automation! Many thanks to Gene for the idea and inspiration!

Patrick

Disclamer
This is how I did it, and it worked the first time, and it worked for a number of people as documented on Gene's Casita website, but if your A/C, converter, furnance, or thermostat melts or explodes I disclaim any responsibility.
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Old 03-24-2006, 09:06 AM   #5
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Patrick,

The problem we have with the Coleman A/C that we had installed in our Trillium 5500 is that it darn near blows us out of the trailer. I can't put napkins on the dinette as they will blow away.

Do you think this would help with that? I feel like I am living in a wind machine.

Kathy
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Old 03-24-2006, 09:28 AM   #6
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These roof A/Cs are definitely overkill for our small trailers. A three speed, or even variable, fan speed control would be nice! But until that happens, this mod will help by turning the fan off so its not constantly running.

BTW, people with window A/Cs could possibly (if their A/C supports a cut-off then on of power) also use this technique to get better temperature control by having a more centrally located thermostat!
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Old 02-14-2011, 10:01 AM   #7
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To update this thread a bit:

> The latest thermostat to use has a backlight which is very handy: LUX DMH 110 from Wal-Mart.

> Adding
a second relay to an A/C outlet controlled by this thermostat to run a small cube heater some people use, gives better temperature control over these simple heaters.

The thread on Gene's forum is still the best place to go for up-to-date info on this valuable mod.
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Old 02-20-2011, 08:55 AM   #8
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Quote:
Originally Posted by Patrick M. View Post
BTW, people with window A/Cs could possibly (if their A/C supports a cut-off then on of power) also use this technique to get better temperature control by having a more centrally located thermostat!
I'm going to have to look into this. Our Scamp 13' came with a factory installed (AFAIK) window style A/C mounted on the wall at the bottom of the closet area. I'll have to check into how it's wired for power.

When it's turned on, it runs incessantly, with no temp control.

Having a thermostat would be SWEET!
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Old 02-25-2011, 05:05 PM   #9
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Electrical Upgrades - The Casita Club Forums
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Old 08-30-2017, 10:26 AM   #10
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Name: Kevin
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2017 UPDATE

I have made this modification and it works perfectly! However, there are some differences.

The parts listed are no longer available. The relay I used was the one shown available as a replacement from Mouser, 653-G4A-1AE-DC12. There was a significant difference though. The relay connections have to be soldered for the 12V side. This was a bit time consuming and cumbersom.

If you would like an easier and actually better relay, you can go solid state. it's a bit pricier but I plan on converting mine later. The unit is available from Marshall Wolf Automation (www.wolfautomation.com) for around $30. The model number is RSSDN-25A. Both 12v and 120v inputs have screw terminals. I have ordered it along with a fingersafe cover and thermal pads for mounting.

The Thermostat I used is a SUPCO 43054 for FiltersFast.com (https://www.filtersfast.com/P-Supco-...thermostat.asp).

Wiring is the same as shown in the original posts. I have been using this modification for several months now and it is working flawlessly. Thanks so much for posting this modification!
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