A heat anticipator is an electrical resistor device mounted in the center of many older mechanical thermostats, including the familiar Honeywell dial-type thermostat. The function of the heat anticipator is to fine-tune the point at which the thermostat turns off the furnace burners. Ideally, it turns the furnace's burners off a short time before the room reaches the desired temperature. This is to compensate for the fact that furnaces continue to produce and distribute heat for a short while until the heat exchanger cools down. If the shut-off is properly timed, the room temperature edges up to reach the precise desired temperature a short while after the burners shut off.
How the Heat Anticipator Works
The heat anticipator essentially consists of a small disc attached to the bimetallic coil of the thermostat. The disc has a thin wire on its surface and an adjustment arm that touches the wire. The position of the arm determines the electrical resistance of the wire, which in turn affects how hot the wire gets. As the wire heats up, it warms the bimetallic coil, which in turn shuts down the gas burners early, according to the heat anticipator's setting.
When the heat anticipator in your thermostat is out of adjustment, it can cause the furnace to "short cycle" (turn on and off frequently) or to exceed or never reach the desired thermostat heat setting. Fortunately, this problem is often easy to correct by adjusting the heat anticipator.
Equipment / Tools
- Torpedo level
Examine the Thermostat
Remove the thermostat cover. Using a torpedo level, make sure the thermostat body is level on the wall. If not, the thermostat's mercury switch will not work properly. Adjustments can be made fairly easily by loosening the mounting screws, rotating the thermostat body to the desired position, and then retightening the mounting screws.
Locate the Heat Anticipator
Look for the small disc with calibration marks located at the center of the thermostat. It will probably have the word "LONGER" or "LONGER CYCLES" printed on it. It will also have a lever arm and indicator relating to the calibration marks. This is the heat anticipator adjustment lever arm. (With some thermostats, the printing is found on a bracket adjacent to the adjustment arm.)
Adjust the Heat Anticipator
If the furnace is cycling on and off too frequently, move the heat anticipator adjustment lever closer to the "LONGER" setting by one calibration mark.
If the furnace is exceeding or never reaching the desired set temperature, then move the adjustment lever away from the "LONGER" setting by one calibration mark.
Test the Thermostat
Once the appropriate adjustment is made, let the furnace run and the temperature stabilize for a period of two to three hours. If necessary, repeat the above procedure until the thermostat works properly.
If the problem persists and you cannot resolve the problem with these steps, you may need to replace the thermostat.
If you have trouble getting your mechanical thermostat to properly time the shut-off, consider installing a new electronic thermostat. Electronic thermostats are so precise that there is no need for a heat anticipator. Best of all, most electronic thermostats have programmable controls that allow you to preset room temperatures for convenient, energy-saving operation.