Learn everything about heat pump defrost with this video lesson and unboxing video from Bryan and Bert. This video features the Emerson White-Rodgers Universal Heat Pump Defrost Control, and you can learn more about that defrost control at http://www.hvacrschool.com/wrdefrost. Heat pumps have to operate in cooling mode to melt the ice; the reversing valve switches to send the discharge gas to the outdoor unit (as it would in cooling mode). The coil has to be colder than the ambient temperature to absorb heat from the outdoors, and there is often moisture on the coil; under those conditions, frost accumulates on the coil. A little bit of frost is to be expected, but excessive frost indicates that a heat pump's defrost cycle isn't working as it should. Bryan and Bert force the unit into defrost, and they start off by running it in heating mode. The outdoor unit blows cool air, not hot air, in heating mode because it is absorbing heat; the reversing valve is also de-energized, and you would not pick up voltage between O and Common on the defrost board. Some units have thermostats that monitor the coil temperature; other units may use thermistors. Regardless of operating mode, the contactor must pull in, so there should be around 24v between Y and Common. In this particular unit, a bimetallic disk will snap shut at 32 degrees Fahrenheit (0 Celsius) to initiate defrost. You can check the defrost board to see if the disk is opened or closed by checking for 24v power; if it's missing, then the disk has closed. You can jumper out the pins on the board and speed up the unit's operation to test demand defrost, but that method doesn't allow you to troubleshoot the thermostat. The board in this video (White-Rodgers Universal Defrost Control) can be programmed to go into defrost for a maximum period of time (10 minutes, in this case) or come out of defrost at 65 degrees Fahrenheit. You can also use the board to set defrost at timed intervals (in this case, 30, 60, or 90 minutes). The speedup function on the White-Rodgers universal defrost control allows you to bypass the short-cycle delay or initiate defrost to test the board. To initiate defrost, the board energizes the O terminal, which allows hot discharge gas to flow through the coil with frost buildup without the fans running. When the target temperature is hit, the defrost will end. While the unit is running in the equivalent of cooling mode in defrost, the defrost board also brings on the auxiliary heat strips to keep the occupants from getting cold. Some boards also have a quiet mode to make the defrost process quieter. The Emerson White-Rodgers Universal Defrost Control replaces most single-stage defrost controls and comes with a comprehensive guide for user-friendliness and education. It also comes with two thermistors for demand defrost and a chart with the corresponding temperature and resistance readings; you can test thermistors by ohming them out and comparing them to that chart. This control also has time delays to prevent short cycling, a user-friendly and interactive display, and brownout protection. Learn how a reversing valve works at https://www.youtube.com/watch?v=lFV3xT5HCH0. Read all the tech tips, take the quizzes, and find our handy calculators at https://www.hvacrschool.com/.