Where is the defrost control board?

The defrost control board on a residential air-source heat pump is typically located inside the electrical control compartment of the outdoor condensing unit housing. To access the board, you must first turn off the electrical power to the heat pump at your home's main breaker panel for safety, then remove the exterior service access panel on the outdoor unit. Inside, you will find a weather-resistant metal or plastic control box that houses the defrost board, contactor, and wiring terminals. The board is easily identified by its low-voltage terminal strip (with labels like R, C, O, Y, W2), sensor connections, and timing pins. You can verify its location by following the manufacturer wiring diagram affixed to the inside of the service panel. Never touch internal wiring without turning off power.