5-2 Day Programmable Honeywell Thermostat Upgrade

Factory Coleman-Mach thermostatMy 2014 Flagstaff 27RLWS travel trailer was outfitted from the factory with an analog Coleman-Mach thermostat. We found that it did not control the temperature swings above and below the set-point to our satisfaction. The RV warmed up too much before the AC kicked in and in the cooler weather, the temps would drop too low before the furnace came on. This means that a thermostat upgrade was in my future.

Research suggests that most people doing this mod install the Hunter 42999 Just Right Digital Non-Programmable Thermostat. I had no problem with having a non-programmable thermostat but I saw the Honeywell RTHL2310B for about the same price so I figured I'd get the extra capability for the same price.

In this write-up, click on any image to enlarge.

Installing the Fan Hi-Lo Switch

Since the replacement thermostats in these RV mods typically are installed in a house they have no switch to control the 2-speed fans in RV AC units. That means the first modification is to the thermostat itself. The most typical switch used is a SPDT (single pole double throw) switch from Radio Shack or wherever you prefer to shop.

The question became where to put this switch. Mods I have seen put it underneath the unit which I thought was a good place since it did not stick out beyond the face of the thermostat. Opening the back of the thermostat showed the area that I'd be working with at the bottom outer corners.

Long story short, after boring 2 holes (2 attempts) I decided the switch could not go at the bottom on this thermostat so it had to go on the front face, which means I had to orient the switch connectors so that they are vertically above one another - the only way it will fit in the narrow space at the side of the front face. For me, up would mean high speed while down would mean low speed.

I also took that opportunity to solder 3 wires to the 3 connectors on the switch since I was not messing with the switch after it was installed and the thermostat was put back together. Next stop - the RV.

Installing the Thermostat

First I turned off the breakers at the fuse box and made sure that the AC and furnace circuits were dead just to be safe.

Then I removed the front of the existing thermostat to reveal the backing plate which is attached to the wall by 3 screws. The Honeywell only uses 2 screws, which luckily align perfectly with the 2 bottom screws shown in the pictures - no drilling new holes!

With the screws removed I let the unit hang down and then undid the electrical tape that bound all the small wire nuts together. I then considered labelling which wires from the wall are connected to which wires on the existing thermostat - the Honeywell came with little labels for that purpose. However I noted that the colours for all the wires from the wall matched the colours of the wires on the thermostat EXCEPT for the brown wire from the wall which was connected to the grey wire on the thermostat. I just made a note of that on my instruction manual, skipped labelling any of the wall wires, and undid the wire nuts.

Now comes the tricky part - connecting the wires! Please note that the wires from the wall will likely vary from manufacturer to manufacturer, although from what I have read, the red, white and yellow wires seem to be standardized to the functions shown in the table below.

First you will need to cut the exposed wires that were in the wire nuts so that maybe only 3/16" (~5 mm) of wire is exposed. Then you insert the exposed wires into the holes in the bus after backing out the screws at each location. Insert the wire then tighten the screw. Do this for the Red, White and Yellow wires.

Next you need to connect the Green and Brown wires to the Top and Bottom wires on the switch - remember what I said above about the vertical orientation of the connectors? Anyway, you could swap them if that suits you better.

Wall Wire Function Bus Connector
Red +12VDC Red-RC
(leave jumper in)
*Thermostat wall plate BUS connectors
White Furnace / Heat White
Yellow AC Compressor Yellow
Wall Wire Function Switch Wire Connector
Green Fan - High Top wire Re-use the
small wire nuts
Brown Fan - Low Bottom wire
Blue -12VDC Unused Tape off
Switch Wire Function Bus Connector
Centre -12VDC Green *BUS

Last, I pushed the wires and connectors through the hole in the wall plate, installed 2 AAA alkaline batteries in the thermostat and then re-installed the front face of the thermostat taking care to align the two parts properly so that the pins from the circuit board go into the holes in the BUS.

Finished connections looked like in the picture on the left. Sorry I did not get a better composed picture but after messing around with this installation on a pretty hot trailer on a brilliant Texas July day, I was ready to call it quits - this was no 15 minute installation since there was some trial and error involved for the connections. If you do this installation after reading this you should get the switch installation and the RV part down to 30 minutes or less, each.

I made sure that the mode switch (Heat / Off / Cool) on the thermostat was set to "Off" before I flipped the breakers back to the ON position and tested the functions of the AC and furnace. I had to press the right hand UP and DOWN buttons to force the AC and furnace come on since the programming was not yet done. Everything seemed to work fine.

Programming

This is straightforward and full instructions are in the manual. This thermostat has separate programs for the weekdays and weekends with 4 program periods per day, for both heating and cooling modes. So far I only programmed the cooling function and will leave the programming of the heating function to another time.

The 4 periods per day typically correspond to wake / leave / return / sleep, so I set the following program periods for both the 5 week days and the 2 weekend days: 6:00 AM - 75°F / 8:00 AM - 75°F / 6:00 PM - 75°F / 11:00 PM - 78°F. If I know I am going to be away, I can use the Up / Down buttons to override the program if I wanted to. According to the manual, most settings are stored in non-volatile (permanent) memory. Only the clock and the day must be set after you replace the batteries.

In my house I have a more programmable auto-changeover thermostat which works perfectly - so much so that I have not fiddled with it in 12 years and have even forgotten how to program it! It does its thing and I am comfortable. I hope that the same is (at least approximately) true with this thermostat!