Here are cold, hard facts about your thermostat
Q: How do home thermostats work? If you keep your thermostat set at 68 degrees, does the heat come on at 68 degrees and go off at 69 degrees, or is there a bigger spread in temperature before the heat goes off? Or does it work the opposite? If you keep the heat at 68 degrees, does the heat come on at 67 degrees and go off at 68 degrees? There must be some built-in temperature spread between calling for heat and no longer calling for heat. – Janice B., Spokane
A: Interesting questions! Whether you have a round thermostat with a dial, square with temperature levers, or electric with display, the basic principal involves a temperature sensor to determine the room temperature.
First, the room’s desired temperature is set within the range of the thermostat’s scale. When the room temperature drops below this setting and the temperature sensor compares it to the thermostat setpoint (the setting), the furnace will kick on one or two degrees below this setting. The furnace will come on but the temperature normally continues to drop until the heat exchanger gets hot enough to start the fan, and then the heat anticipator will shut off the burner one or two degrees below this setting. The heat anticipator range (often adjustable) helps keep the furnace from cycling on/off too frequently. Essentially, if you set it at 68 degrees, it will shut off at 68 degrees.
To put it simply, the heat anticipator shuts off the burner prior to the air temperature reaching the thermostat’s set temperature. Depending on the design of the furnace, heat is then pulled from the heat exchanger to make the final increase until you reach the setpoint for the thermostat (each furnace operates according to its own design elements).
This type of thermostat is known as a non-programmable thermostat because the desired room temperature is set by hand. A programmable thermostat can be set to change a room’s temperature at specific times to match your lifestyle, so it can help save energy.
It is commonly believed that if you raise the thermostat higher, more heat will come from the furnace to heat the home faster. That is incorrect. A furnace puts out heat at the same rate no matter how high the thermostat is set. The real question is, “How long will the furnace stay on to reach the set temperature?” The answer to this question depends on the setting of the heat anticipator within the thermostat. The heat anticipator is supposed to be calibrated with the gas valve for a gas furnace and with an oil burner for an oil furnace.