A heat pump works by removing heat from one area and releasing it in another. To do this the heat pump requires refrigerant. Freon, a brand of refrigerant, is a chemical that can easily absorb heat and transport it to another area. If the heat pump system becomes low on refrigerant it will no longer work properly. Only a qualified HVAC technician should test the Freon levels in your system, but there are a few common things you can look for that will clue you in to when it is time to call a technician.
Listen to the compressor in the outside unit. Compressors that run too often or for longer and longer periods of time can be a signal that Freon levels are low. When Freon levels are low, the compressor must work harder to maintain performance.
Check the evaporator coil. When the system is low on Freon, condensation can form on the evaporator coil. When the temperature drop occurs, this condensation freezes. Once the coil is encased in ice, it no longer has the ability to remove heat from the air.
Check for leaks in your system. Use a spray bottle filled with soapy water. Spray the refrigerant lines with the soapy water, concentrating on joints and fittings. If a joint or seam begins to bubble, this is a sign of a refrigerant leak.
Call an HVAC repair technician. Technicians are better qualified and have proper equipment to accurately test the Freon levels in your heat pump.