My mechanical Duo Therm thermostat crapped out, so I googled around for a replacement and came across a posting (http://home.roadrunner.com/~morodat/hunter_install.html) that describes how to convert a Hunter Model 42999B digital thermostat for RV use. The Hunter thermostat is available at WalMart for less than $20. It will work as-is, but it does not have a switch to toggle between high and low fan speeds or a switch to toggle between a gas furnace and a electric heat strip. The web posting describes how to add a switch for high and low fan speeds. I did this, but I also added a sub-miniature SPDT switch to toggle between gas and electric heat. The "W" post of the Hunter thermostat is wired to the center terminal of the SPDT switch, and the other terminals are connected to the gas furnace and electric strip leads (white and orange wires, in my camper). There is a catch, however. The Hunter thermostat is designed to be connected to a gas furnace or an electric heater, but not both. When connected to a gas furnace, there is a slide switch on the back of the thermostat that must be set to "HG." In this mode, the thermostat does not turn the overhead AC/heat strip fan on when the heat comes on. When connected to an electric heater, the switch must be set to "HE." In this mode, the thermostat does turn the overhead fan on when the heat comes on. So, to toggle between gas and electric heat with the modified thermostat, you actually have to move two switches. I very rarely use my electric heat strip, so this was OK with me. Note that the packaging of the Hunter thermostat says that it is not compatible with heat pumps or electric baseboard heaters, but not to worry, it works just fine with a heat strip. The posting on the Roadrunner web site cautions against lifting up the printed circuit board too far to avoid damaging the spring wire battery contacts and to prevent the switch sliders from falling out of their slots. I went ahead and removed the circuit board completely, as it is much easier to install the switches with it out of the way, and it is not difficult to put the unit back together. You have to use a small screwdriver to prise the battery spring contacts out, but they go back in easily. This whole project may belong in the category of being more work than it is worth, because it does require some painstaking drilling, filing, and soldering, but I did end up with a nice digital thermostat for a total cost of $23.

OK, I'll take a couple of pictures. No, the thermostat does not work with two-stage furnaces, but if each stage has it's own turn-on wire, it would simply require installing another SPDT switch to toggle between stages.

Jerry how about showing a couple pics as to what the setup looks like. My two thermostat setup is not too pretty but I like the way it works. Does your thermostat work with the two stage furnace? I assume it does, the two stage is two heat outputs of the furnace not just fan speeds. If you have pics it would be great.

Thanks, Dave Mc

I haven't actually used the new thermostat with the a/c while camping, but I tested it by changing the set point until it was one degree above or below ambient, to make sure it would turn the a/c on and off, which it does, so I assume it's going to work just fine. One nice feature about the Hunter digital thermostat is that you have the option of selecting one, two, or three degrees as the difference between the turn-on and turn-off points (hysteresis). So, if it's cycling too quickly, you just increase the hysteresis.

A thought I had after my initial post is that it would be a much easier installation to put the two extra switches on the wall panel next to the thermostat, rather than squeezing the switches into the thermostat itself, where there is very little room. Soldering inside the thermostat also is a challenge, as it is easy to de-solder existing connections when you make the new ones. Might not be quite as sanitary an installation, but I think it would look OK.

I like your idea, how does it handle the AC. I did not like how the old thermostat handled AC so i put the Ac on a separate thermostat. Works great. The old thermostat was slow to start and cycled before the unit reached set point. Let me know how it handle the AC.

Thanks, Dave Mc