If you are used to simple, straight-cool split systems, you know that the low voltage to the outdoor unit is usually VERY simple. In many cases, there's just a Y (contactor power) and a C (common) connected to the outdoor unit. When the condensing unit controls are strictly two-wire low voltage, there is no continuous low voltage power. So, there are also no timers or other logic in the condensing unit. Usually, in these cases, the LV wires connect directly to the contactor coil.

However, a heat pump needs to be able to switch between heat, cool, and defrost. So, they are a bit more complex and need more control conductors.

What is a defrost board? What does it do?

A heat pump defrost board, like most modern controls, contains both loads and switches to control different functions. Because it has timers and some basic “logic,” the board requires a power supply, and for most residential split system boards, this power comes from the C (common) and R (hot) terminals from the indoor 24v transformer.

The defrost board also utilizes the constant power on the defrost board R terminal to back feed voltage through the W2 wire back to the secondary heat inside, whether it be heat strips, furnace, or hydronic secondary heat.

That helps to counteract the cooling effect that occurs when the heat pump shifts from heat to cool mode for defrosting. It's critical to test this function on heat pumps to reduce cold draft complaints during the winter.

Force the board into a defrost and check for 24v between W2 and C at the outside board to confirm proper operation, or check the secondary heat via an ammeter or visual confirmation during the defrost cycle.

—Bryan

P.S. – Jesse Claerbout made a video showing how to test defrost on a heat pump and how defrost works. You can watch that video HERE.