A thermostat operates much like the wall switch that controls your lights. Obviously there are differences, but the main one is that the thermostat opens and closes the circuit all by itself. You don't have to do a thing except, of course, set the temperature.
A two-wire thermostat is the most basic type. It has only one internal pathway, so it can handle only a single appliance. Most people use a two-wire thermostat to control their furnace or a room heater, but you can also use one to control an air conditioner. Thermostat technology is constantly advancing, and digital battery-powered models are becoming more common than mechanical models, many of which contain mercury. When you replace a mercury thermostat with a digital one, remember that mercury is toxic. Handle the old thermostat with care and dispose of it as hazardous waste.
How a Two-Wire Thermostat Works
Most household thermostats operate at 24 volts, which is supplied by a transformer connected to the heating or cooling appliance that the thermostat controls. Inside the thermostat is some type of heat-sensing device. In electronic models, this is often a thermistor, a device that responds electrically to changes in temperature. The sensing device in mechanical models is usually a bi-metal strip that responds mechanically to temperature changes. Some models contain a small vial of mercury that tips one way or the other in response to the moving coil. Mercury conducts electricity, and when the vial tips far enough, the electrical circuit closes and the appliance goes on.
That's basically all there is to a two-wire thermostat. There are no wires to control fans, compressors or heat pumps, which are the usual components of sophisticated heating and cooling systems. You can use a thermostat with more than two wire terminals as a two-wire thermostat, but the reverse isn't true. If your furnace has an auxiliary fan, you need a thermostat capable of accepting more than two wires. Check your furnace wiring diagram if you aren't sure.
How to Wire It
The terminals on a two-wire thermostat are labeled R and W. If you use a thermostat with more terminals, but you only have two wires, you can ignore all the other terminals except for these two. The colors associated with these terminals are red and white, so if the wires coming out of your wall are these colors, simply connect the red one to the R terminal and the white one to the W terminal. If the wires are different colors, you may have to check the furnace control panel to find which wire is connected to which terminal. The terminals on the control panel have the same letter designations as the thermostat.
Be sure to turn off the power to the furnace before wiring the thermostat. You won't get a serious shock from handling 24-volt wires, but you'll still get a shock, and getting no shock is better than even a mild one.
Locating the Thermostat
Location matters when it comes to thermostat installation. If you're installing one for the first time, avoid putting it in a sunny location or above a heat register. In either place, it will sense a temperature higher than that in the room and won't come on when the room is cold. Conversely, it shouldn't be in a drafty place, or it will come on too often, and the room will be too warm. The best place for it is usually at shoulder height next to an interior passage door and out of direct line of the main entry door.
Chris Deziel is a contractor, builder and general fix-it pro who has been active in the construction trades for 40 years. He has degrees in science and humanities and years of teaching experience. An avid craftsman and musician, Deziel began writing on home improvement topics in 2010. He worked as an expert consultant with eHow Now and Pro Referral -- a Home Depot site. A DIYer by nature, Deziel regularly shares tips and tricks for a better home and garden at Hunker.com.