Mod Description:

The standard analog thermostat found in most RVs doesn’t control the A/C and furnace with much accuracy. The “set” temperature is not always the temperature the thermostat cycles, which results in as much as a +/- 5°F temperature change before the A/C or furnace cycles on or off. This is known as the hysteresis of the thermostat. For example, if you have the furnace set at 72°F, this could mean the temperature would fall to 67°F before cycling on and rise to 77°F before cycling off! These temperature swings aren’t very comfortable so what to do? The digital thermostat mod of course.

Mod Difficulty:

Hunter Digital Thermostat With A/C Switch

Hunter Digital Thermostat With A/C Switch Wiring

Hunter Digital Thermostat Back Cover

Duotherm Analog to Hunter Digital Thermostat Wiring Digram

Duotherm Analog to Hunter Digital Thermostat Wiring Digram cont.

Analog thermostats have what’s called a mechanical heat anticipator, which essentially tells the thermostat when to cycle. Some anticipators are more accurate than others but for the most part are not very good at controlling temperatures accurately. The digital thermostat is much better at temperature control since it controls its hysteresis using electronics to within +/- 1°F degree of accuracy.

This mod is commonly performed using the Hunter brand residential thermostat. It’s cheap and very straightforward to install. Other residential thermostats can be used but this one seems to be the most popular. Just be sure the thermostat you select is battery powered. Some residential thermostats use 24 VAC to power them. But if they have the battery backup feature, they can be used for RV 12-volt systems.

The wiring diagram above assumes you have a Dometic A/C unit and an Atwood furnace. Other combinations such as a Coleman Mach A/C unit and a Suburban furnace are very similar so be sure to consult your A/C and furnace manuals to ensure correct wiring. See the resources below for some really nice write-ups by fellow RVers who have performed installations with various A/C-furnace combinations.

ModMyRV recommends these parts for this mod:
Hunter 42996 Just Right Digital Thermostat with Remote
SPST Sub-Mini Toggle Switch
Grainger Universal Thermostat Guard Universal Thermostat Lock Box

General Reference
Hunter Digital Thermostat Installation
Coleman Mach to Hunter Digital Thermostat Instructions

43 Comments For This Mod

1. cccale (February 23rd, 2009 at 8:45 pm)

   This is a great mod! i only did one thing differently. i spent a couple bucks more and got a programmable thermostat. i have it programmed to turn on the heater in the early morning so we can wake up to a toasty coach!
2. ModMyRV (February 24th, 2009 at 9:35 am)

   Yeah, the programmable t-stats that manage both heating and cooling timing are pretty nice, but a little expensive. The one in this mod is about $25 but having the automation is cool (or hot?).
3. cccale (February 24th, 2009 at 11:24 am)

   they’re not too bad. Lowes has one for $29
   http://www.lowes.com/lowes/lkn?action=productDetail&productId=126338-79-44110

   The one i bought was $40. I spent the extra $10 because it had a slightly larger display.
4. Bob Vaughn (February 28th, 2009 at 9:16 am)

   I am so happy with the control I now have of my temps…..my mechanical thermostat was awful In order to get the heat to come on I had to slide the leaver to over 90 then play with it to get it to where we wanted it….in the AC mode I had to take it down to 50 to get it to come on and then play with it..The swing was about 10 degrees……so out with the old and in with the new….as my ac has a high and low fan switch I had to rewire a toggle switch into the bottom of the unit….the instructions on how to do this was on one of the rv forums…..
5. Bob Vaughn (March 4th, 2009 at 5:48 pm)

   From what I learned doing this mod I was able to figure out what was wrong with the thermostat in my daughters house….So now I will be doing a mod at her house…
6. Bob Vaughn (March 9th, 2009 at 10:55 am)

   Here is my Hunter Thermostat along side my outside/inside thermometer
   http://community.webshots.com/album/570449799ViDYKl
7. tvman44 (April 1st, 2009 at 10:12 am)

   One of the best mods I have done, sure beats that cheap analog stat by keeping the temp constant. Very easy mod.
8. Kurt (May 31st, 2009 at 3:09 pm)

   My thermostat had two stage cooling. So I hooked up the high fan wire to the second stage cooling contact. The A/C now runs on low, but if the temperature difference is two great it then switches automatically to high fan. You can only one the fan on low setting thought if your running fan only.
9. ModMyRV (May 31st, 2009 at 6:25 pm)

   @Kurt: Hey that’s an idea I haven’t heard of before. Good call. I may have to try that. What brand and model of t-stat are you using?
10. rvoutreach (June 1st, 2009 at 2:15 pm)

    Great mod. Completed mine today and it was very easy to do. I was even able to use the existing mounting holes of the OEM thermostat.
11. JayAre62 (July 27th, 2009 at 8:13 am)

    Easy to do upgrade. I used the Hunter 42999 and added a SPST mini slide switch for Hi/Lo fan speed control. I kind of rushed thru my initial troubleshooting so… after I got the thermostat installed, I discovered my Hi speed relay was also faulty and I ended up replacing the PC Board/Junction Box (as an entire assembly) in my ceiling plenum. The end result is fully functional a/c.
12. Jason (July 30th, 2009 at 8:11 am)

    I performed this MOD - great AC improvement. I used the Hunter t-stat in the article. I wired up the ground connection and incorporated a small 3.3V voltage regulator to eliminate the need for AA batteries.
13. Lloyd BC (September 2nd, 2009 at 7:01 am)

    I have an old Duo-therm 58915 (wall mount thermostat only), that I’m trying to get operational for a friend. Our problem is that his trailer was never wired for wall mount. Do you think this mod will work if we mount the thermostat on the ceiling next to the return air grill, and use it to control the air only (not heating)?
14. Portagie68 (September 2nd, 2009 at 9:00 am)

    Kurt

    What Model T-Stat did you use. I have the same type of system and need to change my T-Stat
15. ModMyRV (September 4th, 2009 at 12:34 pm)

    @Portagie68: Here is a link to a 2-stage thermostat that may help you with you mod. It is similar to the one that @Kurt is referring to:

    http://www.klenger.net/RV_General/HVAC_Honeywell_TH5220D_T-Stat/index.html
16. ModMyRV (September 4th, 2009 at 12:38 pm)

    @Lloyd BC: Well, it would work but would be controlling the temperature based on the air temperature near the ceiling. If the ceiling gets hot (from the sun beating down on the roof), the A/C would run longer thinking it needs to cool more. You would have to compensate for that when setting your on/off temperature.

    Is it possible for you to mount the t-stat on the wall near the ceiling? That would help with false temperature readings…
17. Lloyd BC (September 7th, 2009 at 8:39 pm)

    We could possibly wall mount it, but it would probably be on an outside wall, with about the same insulation value as the ceiling, so I don’t see what difference there would be in sensing. Most non-ducted units sense in the return plenum, and they have little or nothing in the way of insulation there. Consider the ‘Sunchaser’ I’m using, no insulation of any sort on the plenum (I think maybe I’ll buy a closed-cell (ensolite) sleeping pad and cut and glue it to the ouside of the plenum when I’m up there doing the start capacitor mod). This is just another way they went Cheap-O on this unit (Single control, single speed, no start capacitor).
18. stewartfan (September 9th, 2009 at 4:05 pm)

    I Like this but I have 120 volts AC at my yellow wire at the unit mounted thermostat.
    Can a thermostat like above handle and switch 120 volts AC?

    Reading some more, I see that this is a “replacement” for a wall mounted thermostat.
    I do not have a wall mounted thermostat to control my AC unit, the controls are mounted directly on the unit.

    I think I will take this Idea and switch a 12 volt relay and wire the the normally open contacts to the existing thermostat wires.

    Do the older thermostats switch 120 volts?
19. Lloyd BC (September 9th, 2009 at 9:56 pm)

    I think what you’re dealing with is a typical non-ducted, integral control A/C, like my old ‘SunChaser’, with the sensing unit (thermostat) mounted in the return plenum. Yes, they do switch 120V. They’re basically just an automatic, temperature controlled, on-off switch for the compressor. I think you will find that these digital units will not handle much more than 24V, if even that. So, if your unit works as is, I would leave it alone.
    Like Mike says, sensing at the ceiling isn’t too accurate, especially if there’s no insulation on the plenum. But it’s better than no cooling at all, in 100+f temps.
20. Portagie68 (September 10th, 2009 at 11:20 am)

    ModMyRv. Making procress on my conversation using the 2 stage T-Sat you suggested. One part I am having a small problem with is the relay that He used to control the high/low fan speed. Have you got any suggestions?
21. routemaster (September 22nd, 2009 at 12:44 pm)

    I have no field wires in my Everest bedroom were we had a Coleman Mach instaled.Bought the hunter stat dont know how to wire it into the AC,Do you use spade conectors.Any pics of how you do this in the AC,what guage wire do toy use, please.
22. Portagie68 (September 26th, 2009 at 7:33 am)

    Got the relay numbers from Kurt. He used 5Z486 and purchased it at Grainger. Grainge also says the relay 1EHE3 maybe a good sub.
23. Portagie68 (September 26th, 2009 at 7:35 am)

    My bag. The relay number is for the Hi/Lo Ac fan only.
24. mickdund (October 4th, 2009 at 9:41 am)

    I have the same set-up in my forest river wildcat,dometic therm.I replaced it with a Hunter model # 44110 works great.I used the same wiring diagram printed from #37 mod.I found a good deal on a Honeywell RTH 7500 therm,it has the automatic heat to cool change over which I think would work great during the fall during temp changes.I tried installing it with same wiring diagram from dometic to hunter,everything works great on it except for the a/c and the heat come on at the same time,any suggestion on what could be the problem.
25. Terry Kennedy (November 11th, 2009 at 11:29 am)

    I am looking for a 12 volt digital readout thermostat to control my 12 volt refridgeration aboard my boat.

    Any ideas, and prices would be great… Terry Kennedy
26. wowlites (December 14th, 2009 at 11:53 pm)

    Wow…awesome mod. I used a basic Honeywell digital. Easy to install and works like a charm. No more wild temp swings in our 5ver.
27. mrseas (December 26th, 2009 at 9:08 pm)

    I think this is a great mod! I have a question concerning doing this mod on a Coleman brand A/C heat pump any ideas on how to wire the T/stat?

    Thanks
28. Dave Border (January 20th, 2010 at 3:06 am)

    Could you just hook up the high speed AC wire on the fan terminal, and have high speed only?
29. ModMyRV (January 20th, 2010 at 6:50 am)

    Sure can and that’s how I did mine in a previous TT. Worked just fine.
30. DaveB (January 21st, 2010 at 2:50 am)

    Thanks, I’ll switch the wires today and make sure high speed fan comes on.
31. original (January 21st, 2010 at 10:50 pm)

    Have you seen the Ecobee Internet Enabled programmable thermostat?

    If you haven’t it’s here: http://www.ecobee.com/

    It’s $469.00 though. I know, I know…excessive.

    But cool!!! (or hot?) >haha<@ModMyRV February 24th, 2009 at 9:35 am
32. DaveB (January 22nd, 2010 at 5:38 am)

    Fan did not come on either way. Did more looking on the web, found another fellow hooked fan and high speed wires to terminal G. Fan now works in high speed mode only, no need for the SPST switch.Fan only switch on thermostat also works correctly now.
33. jgmccoy (January 29th, 2010 at 10:24 pm)

    I have a 2009 jayco pop-up. would this mod work for me? I Do have roof top ac but I control it with the knobs. how ever its my heat I would like to operate with this digital thermostat. what do you think?
34. ModMyRV (January 30th, 2010 at 8:07 am)

    It should. Since your pop-up is new, it should have the “standard” thermostat most newer units come with. What brand is it? Have you removed it and looked at the wiring?
35. fireman_tom (February 8th, 2010 at 11:16 am)

    I just did this mod on 2009 keystone Cougar, works great.
    Vary easy to do to.
36. davidp (February 13th, 2010 at 1:18 pm)

    how would you install the digital thermostat if you have two airs and two thermostats?
37. RoadToad (February 14th, 2010 at 1:50 pm)

    One tstat per unit. Like it is now.
38. RoadToad (February 14th, 2010 at 2:01 pm)

    OR, if both stats are mounted at the same location next to each other; then, both Yel on Yel terminal; both Wh on Wh terminal; etc etc. The tstat is nothing but a switch that makes closures to close a connection or open a connection. The “digital” aspect determines “when” to do so. If you wish to route two sets of wires through one stat, you can; so long as the A/C units are electrically the same. (i.e. both switch the “hot” side of the circuit or both switch the “low” side of the circuit). If the A/Cs are the same Model they definitley qualify.
39. Rolling Condo (February 21st, 2010 at 5:20 pm)

    One of the best things you can do to improve the heating in your RV. No more wide swings in the heat cycle.
40. davidp (February 21st, 2010 at 5:50 pm)

    Did the mod with the hunter therm.easy to install by the instructions on mod #37…..Thanks modmyrv….now going to get one for the back air…
41. gafftaper (February 21st, 2010 at 11:54 pm)

    My TT’s thermostat just has two wires, they are not labled but are colored Red and White. I’m looking at buying the Honeywell RTH221B1000, a basic programmable with an on/off switch, for only $24 on Amazon. However I’m not sure how to match up the wires. The manual says it is fine for two wire operation but doesn’t say which two wires to use. Ideas?
42. RoadToad (February 22nd, 2010 at 8:47 am)

    Go here for the manual:
    http://customer.honeywell.com/techlit/pdf/69-0000s/69-2060EFS.pdf

    It looks like your stats wires are for heat only.
    Para; 2.3 shows the White to “W” and the Red to “Rc/Rh”
    Hope this helps.
43. gafftaper (March 10th, 2010 at 10:31 pm)

    Finished the mod tonight took less than 10 minutes and a screw driver. As noted above got a $24 thermostat on Amazon. VERY easy mod and it works great. I look forward to more efficient heating.

Let's hear your comments about this mod!