Heat pumps are amazing in that they can cool your home in one mode, and heat it in another. But what if you flip the switch to turn the heat pump to heating mode, and cold air keeps coming out? Clearly, this won't keep you warm in the winter -- so you'll want to try these methods to fix the problem.
Check your thermostat.
If your heat pump is connected to an external thermostat, you must make sure that thermostat is also set to heating mode. If it is still set to cool, the heat pump may not release heat, even if the heat pump itself is set to heating mode. If your thermostat setting was wrong, fixing it should solve your problem.
Also check the thermostat batteries. If they are dead or about to die, this may affect the communication between your thermostat and heat pump, resulting in a lack of heat. Change the batteries, and see if that helps.
Look for refrigerant leaks.
Many homeowners assume that heat pumps only use their refrigerant during air conditioning season. But beside the fact that it is called refrigerant, this component of your heat pump actually comes into use during heating season, too. It absorbs heat from the outdoors, expands, and then transfers that heat to your home interior. If there is a refrigerant leak, your heat pump may not be able to heat effectively. The cold air blowing in may just be chilly outdoor air -- not air that the heat pump has cooled.
To detect a refrigerant leak, place a white sheet of printer paper beneath the heat pump. Come back and check it a day later. If there are pink or green drips on it, you have a refrigerant leak. Have an HVAC contractor come fix the leak and add refrigerant to the system.
Consider the Valve
If your thermostat seems to be functioning well and there is not a refrigerant leak, then the problem may lie in the valve that switches the pump from heating to cooling mode. This valve may become stuck, or it may detach from the solenoid coil that it moves to change the heat pump's mode. This is not a difficult problem for an HVAC contractor to fix, although they may need to take apart your heat pump in order to do so.
If you cannot figure out why your heat pump won't work, talk to your HVAC contractor ASAP. Cold weather is coming!