On some heat pumps, an air flow sensor detects air flow pressure change across the coil in order to detect icing or frosting of the coil (on some units).Some heat pumps use only a timer others use a timer combined with a temperature sensor or thermostat that monitors temperature at the outdoor coil
On many heat pumps, a defrost cycle control board in the outdoor unit uses a timer and other information to initiate a defrost cycle. Outdoor humidity (more humidity, faster ice formation, more frequent defrost needed). At about 30☏ you will usually begin to see ice forming on the outdoor coil or heat exchanger Outdoor temperature (closer to freezing, more frequent defrosting needed). The compressor and control design (timer versus ice sensors). The heating load - amount of heat being delivered indoors. Factors that affect the frequency of a heat pump (in heating mode) outdoor defrost cycle include: Fewer defrost cycles - eliminating unnecessary defrost cycles - means a more efficient heat pump system and some energy savings. Other more sophisticated defrost control circuit boards monitor other operating details such as refrigerant pressure in order to avoid wasting the energy of a defrost cycle if it's not actually necessary. The defrost cycle ends when the temperature sensor at the outdoor coil senses the temperature of the coil at about 75 ☏ or when the circuit board's timer overrides the sensor. The defrost cycle is initiated at the end of the defrost timer cycle. The indoor room thermostat is calling for heat. The outdoor temperature has fallen to about 31☏ as reported by a temperature sensor in the outdoor unit (OD unit). The timer starts when two events have occurred: Typically on the defrost cycle control circuit board, the installer can set a jumper to control the defrost cycle at 30, 60, or 90 minute intervals once the defrost temperature sensor on the outdoor unit senses that outdoor temperature has fallen close to 31☏. Just how often a defrost cycle will occur is determined either by a timer on the outdoor unit’s control board (when in heating mode) or by sensors that some heat pumps may use to detect the presence of ice on the coil.ĭepending on your heat pump brand and model - thus the heat pump's design - the outdoor unit in heating mode will run a defrost cycle as often as once every 30-35 minutes OR (on some more sophisticated designs), the heat pump may only run a defrost cycle if sensors tell it that the outdoor coil is iced over. A control board in the outdoor unit is the control over when a defrost cycle occurs. So periodically the outdoor unit will go through a defrost cycle. Ice on the outdoor coil blocks air flow across the coil with no air flow, no heat can be extracted from the outdoor air and worse, the compressor motor itself could be damaged. Then moisture in outdoor air freezes on the coil surface. In heating mode, as heat is extracted from the outdoor coil, its surface becomes colder than the outdoor air temperature and can drop below freezing. The outdoor unit fan runs, air moves across the outdoor coil, heat is absorbed from outdoor air into the coil and into the refrigerant inside the coil where ultimately that heat is delivered indoors through the indoor air handler. When your heat pump is in heat mode, the outdoor unit (compressor condenser) is extracting heat from (even chilly) outdoor air. It is normal for the outdoor fan to run while the outdoor unit compressor is running AND the system is in heating mode, not in defrost mode. 
Why and when a heat pump goes into a defrost cycle when in heating mode In both of our outdoor compressor/condenser unit photos shown here, the snow cover itself is not a functional issue for the equipment, but the placement of the condenser unit just a few inches from the building wall may be an operating issue by restricting air flow around and through the unit. Note: this heat pump defrost cycle Q&A were posted originally at FAN, COMPRESSOR / CONDENSER UNIT - be sure to see the repair advice at that page. HVAC Compressor/Condenser Unit Defrost Cycle We also provide an ARTICLE INDEX for this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need. This article also discusses the diagnosis and repair of problems with the outdoor compressor / condenser fan and fan motor, including fans that won't run, fans that run at slow speed, and fans that won't stop running. This article discuss the reasons for, duration of, and control of the defrost cycle at the outdoor compressor/condenser/fan unit of a heat pump when in heating mode. We have no relationship with advertisers, products, or services discussed at this website. InspectAPedia tolerates no conflicts of interest.
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