More insight offered on thermostats

Q: In your piece “Here are cold, hard facts about your thermostats” (2/11/05), you described how a gas/oil furnace thermostat works. The Northwest probably has more electric heat than anywhere in the country, but you never mentioned how electric furnace thermostats work.

I have my thermostat set to 70. When the temperature drops to 69, it activates the electric furnace. First the fan comes on. Then, and only if, the fan gets up to speed do the heating elements kick in. When the temperature reaches 71, the thermostat shuts off and the heating elements in the furnace shut down. The fan continues to run until the furnace temperature drops to a predetermined safe level.

I may be wrong, but I thought that only the old mercury analog thermostats had anticipators.

You state that how long a furnace stays on to reach a desired temperature depends on the setting of the heat anticipator. Funny, I thought it depended on the BTU output of the furnace, the cubic feet of space being heated, and the heat loss rate of the building. – Larry B., Spokane

A: Larry, are you talking about electric baseboard or electric furnaces? If it’s the latter, then basically all types of thermostats work with such units, and those units require you to consult the installation manual for specific instructions to correctly set the heat anticipator.

When I wrote, “Essentially, if you set it at 68 degrees, it will shut off at 68 degrees,” I was making a general statement. 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.

Thermostats all contain some form of an anticipator, with the exception of the newer solid state models. 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 it will come on in an hour.

I do want to respond to your last paragraph. The heat anticipator fine-tunes the thermostat’s upper temperature shutoff point, and that’s one thing. Sizing for the furnace—and its BTU output—is a separate issue from heat anticipators.

Basically you are describing a “heat load” calculation (heat loss and heat gain), generally done room by room before the heating system is installed. Calculations are made 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 a balanced system that works efficiently for maximum comfort.