Nothing can be more annoying than a heat pump that is not dissipating heat when you need it to. If your heat pump is not doing its job, the reason is likely related to a mechanical fault. But you shouldn't be concerned much about a heat pump that is blowing cold air for a few minutes; it is probably the defroster of the system doing its work. The defroster is set to turn on after every one hour so that it can prevent the buildup of ice. Hence, for some few minutes when the heat pump is defrosting, it will blow cold air rather than hot. If, however, it is taking too long to come back to normal, there could be a mechanical fault. The following are some of the common reasons why your heat pump may fail. 

A Faulty Thermostat

Diagnosing a thermostat isn't hard at all, especially if it is a digital thermostat. Make sure the system is set to heat mode and set the thermostat to 2 degrees above room temperature.  The thermostat should click within seconds and the heat pump is supposed to start running after detecting the click. So if the system fails to run, it implies that the thermostat also failed.

Note that some systems are built with a time delay program as a safety precaution. It means that the system might delay for five minutes after getting the click from the thermostat so give your heat pump an allowance of five minutes first.

A Faulty Fan

A faulty fan will find it hard to rotate and if so, it cannot blow air. Check to see if the belts attached to the fans have slipped. If they have slipped, the fan is the problem. The fan may also fail because of the fuses or breakers. Locate the fuses and check if they have been tripped. If so, it implies that there could be a bad connection to the fan. If the thermostat or the fan is the problem, the best remedy may be to contact a professional for assistance.

Sometimes, though, the problem might be related to the cold temperature outside rather than the parts of your heat pump.  If it is too cold, your outdoor unit may freeze and the heat pump may stop running. For this problem, you can rectify it yourself. Take some few minutes and check for ice in the outdoor unit. If it is indeed frozen, clear the ice from the system and start your heat pump again to see if the problem is solved.