When the heat anticipator is out of adjustment it can cause the furnace to exhibit symptoms of short cycling (turning on and off frequently) or to exceed or never reach the desired thermostat heat setting. To adjust the heat anticipator perform the following steps:
- Remove the thermostat cover
- Make sure the thermostat is level. If it is not level the mercury switch will not work properly.
- At the center of the thermostat find the small disc with calibration marks as shown in the photo above. It will probably have the word "longer" on it. The disc will have fastened to it a lever arm and indicator relating to the calibration marks. This is the heat anticipator adjustment lever arm.
- 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, move the adjustment lever away from the "longer" setting by one calibration mark.
- Once the appropriate adjustment is made let the furnace run and the temperature stabilize for a period of 2-3 hours.
If necessary, repeat the above procedure.
- If the problem persists and you cannot resolve the problem with these steps, you may need to replace the thermostat.