In an air-to-air heat pump, defrost is accomplished by switching to which mode?

• A. Heating mode
• B. Auto mode
• C. Cooling mode
• D. Fan-only mode

Answer: (C)

When frost builds up on the outdoor coil of an air-to-air heat pump, the system reverses the refrigerant flow to heat the outdoor coil so it can melt the frost. This is done by switching to cooling mode. In cooling mode, the outdoor coil acts as the condenser and gets heated by the high-pressure refrigerant, which warms the coil and melts the ice. After the frost is cleared, the unit returns to heating mode to continue providing heat to the space.