Heat pump outdoor unit

For homeowners

A heat pump outdoor unit looks similar to a straight-cool air conditioner condenser but does double duty — it cools your home in summer and heats it in winter. Same compressor, same coils. The difference is a few extra parts that let refrigerant flow in either direction.

The reversing valve is the key addition. When it’s in one position, the outdoor coil rejects heat (cooling mode). When it flips, the outdoor coil absorbs heat from outside air and the indoor coil rejects it inside (heating mode).

The accumulator is a safety device on the suction line that catches any liquid refrigerant before it reaches the compressor. The defrost control board monitors the outdoor coil temperature in winter — when frost builds up, it briefly runs the system backwards to melt the ice.

In heating mode, a heat pump moves heat from cold outside air into your home. That sounds backwards but it’s how all refrigeration works: refrigerant boiling at temperatures colder than the outside air can still absorb heat from that air.

For technicians

Four components distinguish a heat pump outdoor unit from a straight-cool condenser, in rough order of importance.

The reversing valve is the soul of the machine. Brass body, slide mechanism inside, solenoid coil on top. When energized — typically in cooling mode on residential equipment, though some manufacturers wire it the opposite way — the slide shifts and the four refrigerant ports re-route. The top port is always discharge from the compressor; the slide decides whether that hot gas heads to the outdoor coil (cooling) or the indoor coil (heating). Same physical compressor, same coils, opposite direction of heat transfer.

The accumulator is the suction-line vessel between the reversing valve and the compressor inlet. In heating mode, when the outdoor coil is acting as the evaporator and it’s cold outside, not all the refrigerant fully evaporates. Liquid slugs heading back to the compressor are a death sentence — compressors compress gas, not liquid. The accumulator is a trap that lets liquid pool at the bottom and gas pass through the top. Oil bleeds back to the compressor through a small port at the bottom.

The crankcase heater is shown as the dashed band around the lower compressor in the diagram. In cold weather, when the compressor sits idle, refrigerant migrates to the coldest part of the system and condenses into the compressor oil. On startup, that liquid refrigerant flashes off and washes the oil out. The crankcase heater keeps the compressor warm enough during the off cycle to prevent the migration. On modern variable-speed units it’s a stick heater inserted into a well; older units use a band heater wrapped around the outside.

The defrost board and defrost sensor work together. In heating mode the outdoor coil runs below freezing and accumulates frost from ambient humidity. Frost blocks airflow and tanks performance. The board watches the sensor clipped to the coil — usually a thermistor — and on a timer-plus-temperature logic, briefly reverses the unit into cooling mode while leaving the indoor fan off. The hot gas blasts the outdoor coil clear. You’ll see steam billowing off the unit. Then it flips back.

A few things not shown in the diagram:

• Bi-flow filter drier on the liquid line — filters debris regardless of flow direction, which heat pumps need because refrigerant flows both ways depending on mode.
• Check valves that some systems use around the metering devices.
• Discharge muffler on bigger units to quiet compressor pulsation.

Common heat-pump-specific failures:

• Reversing valve stuck mid-stroke. Gauges show pressures that don’t make sense for either mode because the valve is partially routing flow both ways. Performance is terrible in both heating and cooling.
• Reversing valve solenoid that doesn’t energize. Unit acts like a single-mode AC — works in whichever mode is the de-energized state, fails in the other.
• Defrost board failure. Outdoor coil ices up solid in winter, performance drops to nothing, may eventually damage the coil.
• Crankcase heater failure. Compressor floods on first cold-weather startup, scroll plates scuff, eventual compressor failure.
• Accumulator filling with oil. If the oil return path through the small port at the bottom clogs, oil pools in the accumulator and the compressor sump starves. Less common, harder to diagnose.