Although the term "heat pump" implies warmth, it actually serves dual duty. Heat pumps drive cooling systems using the same principles under which refrigerators work. Instead of generating heat, a heat pump displaces heat with the help of refrigerant. The successful movement of the refrigerant through coils is dependent on internal and external factors, many of which can cause the heat pump to freeze up.
In the summer, a heat pump uses its refrigeration cycle to move heat from the inside of your house to the outside, which keeps your home cool. In the winter, it reverses the refrigerant flow to keep your home warm. If the refrigerant level becomes low, the balance of temperatures is disrupted and ice forms on the heat pump's evaporator coils. Refrigerants circulate in a closed system without being consumed, so if the refrigerant level is low, suspect a leak somewhere. A certified HVAC technician can repair the leak and add refrigerant to charge the system fully.
External Temperature and Precipitation
In freezing weather, frost or ice may build up on a heat pump's coil because of precipitation. The ice insulates the fins of the coil and hinders heat transfer. Even though a built-in defroster switches on and off automatically to deice the outside unit, sometimes this precaution is overwhelmed by limiting factors such as a dirty coil or water that drips from the roof onto the outside coil. Repair leaky gutters or install a guttering system over the coil to keep excess water off it. Clean the coil by turning off the heat pump and dislodging debris with a jet of water from a garden hose.
Location, Location, Location
Proper placement of a heat pump helps mitigate ice forming on it. A suitable location is a sheltered area away from prevailing winds where air can flow over it unobstructed. Trying to hide the unit by placing it under a porch causes the exhaust air to be drawn back into the fan, which decreases the efficiency of the heat pump. As the heat pump cycles on and off through the defrosting cycle, the melted ice must drain freely and not back up into the unit. Elevate the heat pump so it is not sunk into the ground, which prevents proper drainage.
A heat pump's fan, and the duct system through which it moves conditioned air, must be sized correctly so the heat pump doesn't freeze up. Fans should be able to move 450 to 500 cubic feet per minute (CFM) of airflow per ton of the heat pump's capacity. Undersized fans that generate less than 350 CFM per ton may cause the coil to freeze. Ducts must be sized so they are able to deliver the proportionate air quantity for the heat pump's size. If ducts are not sealed properly, they cannot maintain airtight connections, and the resulting air leakage taxes the system unnecessarily, which can cause ice formation.
Clogged and Faulty Components
The airflow must remain unrestricted throughout a heat-pump system. When the filters become clogged with debris, the refrigerant freezes and stops circulating. This causes the heat pump evaporator coil to freeze. Some disposable filters need replacing as often as once a month; permanent filters may need cleaning that often. If any component of a heat pump fails, such as the defrost control or timer, thermostat or sensor, defrost relay, reversing valve or reversing valve solenoid coil, the integrity of the entire system is compromised, which can cause ice to form.