A number of heat-pump malfunctions can be traced to the thermostat. Here are ways to troubleshoot some of the most common problems:

Heat pump turns on & off too often
If your heat pump cycles off and on too frequently, the problem is likely to be with the thermostat. See How to Test a Thermostat. Or, the problem may be that the unit is overheating because of a clogged filter or blower that is malfunctioning. Try cleaning or replacing the filter. If that doesn’t do the trick, call a heat pump repair technician.

Major room temperature swings
When room temperatures swing more than about 3 degrees between when the heat pump goes off and on again, it generally means that it isn’t cycling on often enough. See How to Adjust a Thermostat’s Heat Anticipator.

Room temperature goes either too high or too low
When room temperature rises higher or drops lower than the set temperature on the thermostat, it usually means that the thermostat is improperly calibrated or installed where it doesn’t sense a proper sampling of room air.

Blower runs continuously
This may be caused by two things: the thermostat mounted on the wall or the limit switch located on the heat pump just below the plenum (the box that distributes heated air to all of the ducts). The limit switch is designed to shut off the heat pump if the air in the plenum gets too hot.

Check the thermostat to see if the “Fan” switch has been turned on. If it has, turn it to “Off” or to “Auto.” If it is set to “Off” or “Auto” already, the heat pump’s limit switch must be adjusted. Call a heat pump repair technician to adjust the limit switch, or, if you are handy with this type of repair, follow the instructions in your owner’s manual to reset the pointers on the fan side of the limit control. The lower pointer should be set to about 90 degrees Fahrenheit and the upper one at about 115.