NATE Air Conditioning and Heat Pumps Practice Exam

What occurs when a heat pump enters a defrost cycle?

1. The auxiliary heat comes on, the outdoor fan stops running, and the reversing valve switches to cooling mode

2. The outdoor fan stops running and the reversing valve switches to cooling mode

3. The reversing valve switches to cooling mode and the auxiliary heat comes on

4. The reversing valve switches to heating mode, the outdoor fan stops running, and the auxiliary heat comes on

The correct answer is: The auxiliary heat comes on, the outdoor fan stops running, and the reversing valve switches to cooling mode

When a heat pump enters a defrost cycle, the primary goal is to eliminate frost or ice that has accumulated on the outdoor coil. During this process, the system alters its operation to facilitate melting the ice. The correct choice highlights that the auxiliary heat activates, which helps to provide additional warmth to the indoor environment while the heat pump is not able to efficiently heat during the defrost cycle. The outdoor fan stopping is crucial during this cycle as it prevents outside air from being blown across the cold coil, thereby ensuring that the heat pump can redirect heat effectively from the indoor space to the outdoor unit. Additionally, the reversing valve switching to cooling mode is essential because it changes the flow of refrigerant; rather than transferring heat to the indoor space, the heat pump will now absorb heat from the indoor air and redirect it to the outdoor coil, helping to melt the frost or ice buildup. This combination of functions allows the heat pump to defrost effectively while maintaining some indoor comfort, making the correct choice an accurate reflection of the heat pump's operation during a defrost cycle.