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Tuesday, October 20, 2009

Aftermarket Thermostat

I have a 2003 motorhome. Can I replace the existing furnace thermostat control with a better type like a digital home model?
Jack, (Shippensburg, PA)
Jack, indeed you can install a digital thermostat designed for home use to control the heat in your coach. (Note: the one pictured is an analog type thermostat). You can also use a combination heating/cooling thermostat to control both the heating and AC in your coach from one wall thermostat, but that’s a discussion for another column. The home models usually reference 24-volt and/or 48-volt systems, typically what you’d find in a house, but you can ignore this, as you will not be using the logic portions of the digital thermostat. The only function of the thermostat you will be using is the electronic thermostat switching, which is controlled by the batteries in the thermostat. Ensure you buy a 24/48V model and not one for gas systems only. If you open up your existing analog thermostat, you will find two wires connected to it. The thermostat is basically a switch that closes the circuit across the two wires when the unit calls for heat. These are the only two wires needed on the digital thermostat for your furnace to operate. Digital thermostats are designed to perform on a variety of home systems, from heat pumps to gas furnaces among several others. Therefore, they usually have micro-switches mounted to the control board that need to be set to the correct position. Fortunately, you can ignore most of these since your furnace will not be utilizing these functions. There may be one micro-switch that you might want to set which I’ll discuss later.

Whether you purchase a heating-only thermostat or a heating/cooling model, they all will have labeled wiring terminals. Disconnect all power to the furnace and remove the existing thermostat. Mount the new one with the two thermostat wires coming from the furnace. To control the furnace only, you will be using the terminals labeled “RH” and “W.” If you’ve purchased a heating-only thermostat, they just may be the only terminals on the new one. If you have a heating and cooling thermostat combination there will be several others terminals. Connect the two furnace wires to these two terminals. It doesn’t matter which wire goes to what terminal. Your furnace should now perform as before.

There are two major benefits to digital thermostats and one drawback. The benefits are that as you select a specific temperature, the hysterisis is much tighter. Hysterisis means that the thermostat will maintain much tighter control over the temperature than the analog, stock thermostat you have now. The thermostat will keep the temperature within 1 or 2 degrees of the set temperature, although this may translate to more frequent furnace cycling. There may be a micro-switch on the thermostat that enables the user to select the actual amount of hysterisis. But it’s usually 1 or 2 degrees. If you set it to 2, it is a good trade-off between heating comfort and furnace cycling. The drawback is that you need to keep spare batteries for the thermostat on hand at all times. If the batteries die, so does the furnace.


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