SOMETIMES DOESN'T HEAT – Most of the time the system heats correctly, but sometimes it blows cool air and the temperature in the space drops below the heating setpoint.
If you have a new electronic thermostat it may be set to run a schedule and maintain different temperatures at different times. The thermostat should show the current temperature setpoint as well as the current space temperature.
When the systems is not heating, check the thermostat and the current heating setpoint.
This problem may show itself only during extremely cold outdoor air conditions when all stages of heat are required.
If the return duct system has openings or has come loose (especially in an unheated attic or crawlspace), it will draw-in cold air and, even if the heat pump is operating normally, blow cool air from the supply registers and may not heat the space.
This condition is easiest identified by checking the return air temperature entering the air handler. An extremely cool return air temperature would indicate ductwork problems.
This problem may show itself only during extremely cold outdoor air conditions.
A less likely, but possible, cause is a bad thermostat. We have seen instances on newer electronic thermostats where the relay outputs fail to energize the auxiliary heaters even though the thermostat display shows that "Heat" is activated.
Some York heat pumps incorporate a YorkGuard module which monitors equipment operation. If the Yorkguard senses an abnormal condition which could possibly harm the system it will enter a lock-out mode which disables the heat pump.
If wired-in, the Yorkguard should give an alert indication at the thermostat.
A lock-out can be reset by turning the thermostat to the "Off" position and then back to "Heat" or "Auto".