RV Repair Manual
for the RV Do-It-Yourselfer
-article by Les Adams
There was a thread on another BBS by Alan Paskey about his installation of a Hunter Digital Thermostat for his A/C and heat... I have been wanting to do this for some time now and finally got around to going over to "Wally World" (Wal Mart) and buying the thermostat and "getting off my duff" and installing it!!!
I installed it yesterday... Unfortunately, I have a Dometic (Duo Therm) ducted A/C unit and Atwood Furnace and Alan Paskey's unit was a Coleman... His wiring documentation DID NOT work for me... I spent the better part of the day figuring out how the Dometic contact closures worked and then another few hours doing the same for the digital thermostat and marrying the two together...<BR>
The digital thermostat is a Hunter - Part #42995 and is $19.95 at "Wally World" (Wal Mart)... The advantage to the digital thermostat is tighter hysterisis (+/- 1 degree of temperature change yields "turn on" or "turn off"), a digital temperature readout, and no guesswork about where temperature is set for your particular "comfort zone"...
The Hunter Thermostat runs on 2 AA pencells which are NOT supplied... I would suggest you carry spares as well, for if the batteries go dead, so does your thermostat (AND A/C and Heat)!!!
The following documentation will apply ONLY to my particular Dometic type A/C unit and Atwood furnace although it should work on other similar Dometic systems and possibly other manufacturers as well... As previously mentioned, Alan's wiring documentation (his was a Coleman) DID NOT work on my unit, so bare in mind that some wiring changes may be necessary to make the Hunter Digital thermostat function properly with your particular system if they are different than the two referenced within this article...
Duo Therm OEM Wire Color Hunter stat terminal
(Original) used in trailer
Ground Black or green "RC"
Fan Orange or Tan "G"
Cool Yellow "Y"
Ground Black or Green "RH"
Furnace White "W"
High Fan Blue Thru SPST switch
to "G" (Tan)
Red +7.5 volts DC not used
Hunter thermostat Heating System Selector switch set to "HG"... "F/C" selector switch to "F" ...
Note that both the Hunter "RC" and "RH" terminals have the ground lead connected to them... I fashioned a jumper wire from one terminal to another... BOTH these terminals are grounded from the same point in my installation...
This setup worked very well for me last night... I will continue to test but I believe this wiring documentation is correct for Dometic/Atwood...
The Hunter Thermostat has two fan positions - "Auto" and "ON" which enables you to run the fan only, if desired, in the A/C mode only...
I used a Radio Shack subminiature DPDT slide switch for the "High Fan" function... I cut out a little notch in the upper right hand corner of the back of the Hunter thermostat and mounted the switch there with the supplied "very tiny" screws... The Radio Shack part number for this switch is 275-407 ... I had this switch in my "junque box" of electronic components for quite a few years so I don't know if they still make it, but any switch will work... Actually you only need a SPST but this is what I had so I used it...
Additionally, I kept all of the standard thermostat leads (OEM red wire is not used with Hunter) just in case the Hunter fails and I have to switch back to the Duo Therm... The Duo Therm thermostat is in the TT and can be reinstalled in just minutes if necessary...<BR>
For those with a Coleman setup, I also have the original article for the Coleman written by Alan Paskey and the wiring directions associated with it... The text of Alan's article is shown below and was used for Alan's Coleman setup...
Good Luck and Best Regards,
The Hunter Digital Thermostat was installed in my 27' Nomad travel trailer in the Spring of this year(2001)... We used the TT all year long utilizing both the heating mode and cooling mode... The Hunter performed quite well coming "on" at 1 degree above set point and going "off" at 1 degree below set point...
Although this is actually a 3 degree range as opposed to the advertised +/- 1 degree, theoretically, it is within the spec of +/- 1 degree (of set point) and the Hunter operates much better than the standard Duo Therm analog thermostat...
This is a great modification to make if your dissatisfied with the hysterisis (on/off ratio) of your current thermostat... I find the digital much easier to use because you simply set the desired temperature... With the OEM Duo Therm analog thermostat that came with the TT, you had to play with the "temp set arm" until you found the most comfortable setting while trying to "interpolate" the temperature marks on the thermostat... The hysterisis of the Duo Therm thermostat seemed to be larger than the Hunter...
It was not uncommon to be either too hot or too cold while waiting for the old thermostat to kick on or off... The temperature range within the trailer is much more comfortable using the Hunter digital thermostat...
TEXT OF ALAN PASKEY'S ARTICLE as FOLLOWS
(FOR COLEMAN UNITS)
The wires coming out of the a/c are as follows:
Red = +12vdc
Blue = -12vdc
Green = Fan High
Gray = Fan Low
White = Furnace
The wire terminals on the new stat are as follows:
RC = cooling relay
RH = heating relay
G = Fan
W = Heat
Y = a/c compressor
I hooked up everything as follows:
RC = Red wire
RH = Red wire
G = see notes below
W = White wire
Y = Yellow wire
1. I did not use the Blue wire (-12vdc)... The book says it goes to all relay coils but as far as I can tell in the stat, it is used for a delay circuit for the compressor... I MAY BE COMPLETELY WRONG!!! I have the circuit drawn out if there is an EE or someone smarter than I am that can figure it out... It looks like a simple circuit with 2 caps and a transistor... If someone knows what it is really for, please let me know...
2. I took a wire from "G" on the stat to the common on a single pole double throw (SPDT) switch... I wired the fan high and fan low to the switch... This allows me to still control the high and low of the fan just like the old stat...
3. This has worked for me but I can't make any promises for anyone else... A little research should be able to determine if it will work for you or not...
4. Total cost: $19.99 +tax for the stat from Wal Mart and $2.99 +tax for the switch from Radio Shack...
Hope this helps someone else out