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Here are the cold, hard facts about your thermostat

QuestionHow 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.

AnswerInteresting 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 that works with all types of furnaces.

Essentially, if you set the thermostat at 68 degrees, it will shut off at 68 degrees, but it is possible that a thermostat will exhibit a 1- or 2-degree difference, plus or minus, that will affect how it controls the furnace.

First, the user sets the room's desired temperature 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.

With the exception of newer solid state models, thermostats all contain some form of an anticipator. They have a factory setting for either a fuel-fired or an electric resistance heat furnace. The fuel-fired unit setting allows the burner to fire for a set time before the fan comes on. The electric resistance heat unit has the fan come on at the same time the heating elements are energized. The thermostat also has a factory setting, which can be changed by the installer, for the number of times the furnace will come on in an hour.

The heat anticipator fine-tunes the thermostat's upper temperature shutoff point and 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 it reaches the thermostat's setpoint (each furnace operates according to its own design elements).

Thermostats come in two basic styles: non-programmable, which allows the user to set the desired room temperature by hand, and programmable, which can help save energy because it can be set to change a room's temperature at specific times to match your lifestyle.

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. It should be calibrated with the gas valve for a gas furnace and with the control relay for an oil furnace; but for an electric forced-air furnace, it depends on the unit and what is outlined in the manufacturer's installation manual.

Finally, remember that the thermostat comprises only one component of your total heating system. Hopefully, a "heat load" calculation (heat loss and heat gain) was done room by room for your home before the heating system was installed to help determine the correct size of ductwork and registers to match the airflow output of the furnace. The supply air ductwork and registers need to equal the return air ductwork and registers in total. The heat loss calculation helps to achieve an efficient balanced system for maximum comfort.

Copyright © 2005, 2006, & 2007 LAF/C.R.S., Inc. All rights reserved.
Question answered by Leon A. Frechette.

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