Hunter Digital Thermostat Installation

 

This page describes a project to replace the mechanical thermostats in my motorhome with digital ones to provide better temperature control. The motorhome has two thermostats; one that controls a DuoTherm roof air conditioner and an Atwood gas furnace in the front and another that controls just an air conditioner in the back.

I used the Hunter 42999B, available at WalMart for about $20.


The Hunter mounts on a base plate that attaches to the wall. The plate includes a terminal strip for attaching the wires from the heater and air conditioner.

The housing containing the controls, display and batteries is detachable. It has five gold-plated pins that mate with sockets on the base plate to provide the electrical connections.

An added switch is needed to control the two-speed fan in the air conditioner. I installed one on the bottom edge of the housing. When the switch is open the fan runs at low speed. When closed it runs at high speed.

The switch is a SPST sub-miniature bat-handle SPST switch from Radio Shack. I chose it because it was the smallest I could find and because it is easy to install. Solder two small-gage wires about a foot long to the switch terminals before mounting it.

It was necessary to remove seven screws (the ones with the arrowheads printed next to them) from the circuit board and lift it up a little to get the switch into its mounting hole. Lift the bottom edge of the board only and be careful not to damage the spring wire battery contacts at the top. Keep the housing face down and as flat as possible to prevent the switch sliders from falling out of their slots.

 

This diagram shows the wiring for the standard mechanical thermostat and for the Hunter. Wire colors may not be the same in all RVs.

The +7.5 volt power wire is not used by the Hunter. Leave it disconnected and insulate the end with electrical tape.

One switch wire is soldered to the base of terminal G on the back of the circuit board. The other connects to the Fan High wire. I used an automotive spade-type connector. When soldering to the circuit board pin be carefull. If you apply too much heat the solder that attaches the pin to the board will melt. If that happens hold the pin in position until the solder cools.

Leave the factory installed jumper between the Rc and Rh terminals in, and set the Fan Option switch on the back of the circuit board to HG.

Wiring for the unit that controls the rear air conditioner is the same except that there is no furnace wire.

 

This is an easy project that almost anyone can do. Turn off the DC power before starting to avoid accidentally shorting the wires while moving them and carefully mark each of the wires as you remove them from the old thermostat. Some nice little adhesive labels for that are provided with the thermostat.