What it is

An air-source split heat pump is a straight-cool split system that can also run backwards. In summer it works exactly like an air conditioner — pulls heat out of the house, dumps it outside. In winter it reverses the refrigerant flow and pulls heat out of the cold outdoor air and dumps it into the house. The same compressor and coils do both jobs; a valve just decides which direction the refrigerant flows.

That "run it backwards" ability is the whole difference. To pull it off, the outdoor unit carries several parts a plain AC condenser doesn't have. If you understand a straight-cool condenser already, a heat pump is that plus a reversing valve, a defrost system, an accumulator, and a metering device that works in both directions.

How it's built — what's added over an AC

A heat pump outdoor unit has the compressor, condenser/outdoor coil, fan, contactor, and capacitor of any condensing unit, plus:

  • Reversing valve (4-way valve). The heart of the heat pump. A solenoid-piloted valve that swaps which coil is the condenser and which is the evaporator. Energizing its solenoid (the O wire) shifts it. In most residential units the valve is energized in COOL and de-energized in HEAT (O-energized cooling), but always confirm — some are reversed.
  • Defrost control board and sensors. In heat mode the outdoor coil runs below freezing and frosts up. The defrost board, watching a coil temperature sensor (and time), periodically reverses the unit to cool mode to melt the frost, runs the outdoor fan off, and brings on the indoor backup heat so the supply air doesn't blow cold.
  • Accumulator. A reservoir on the suction line ahead of the compressor that catches liquid refrigerant before it can slug the compressor. Heat-mode and defrost cause liquid flood-back, so the accumulator is protection.
  • Bi-flow metering / check-valve arrangement. Refrigerant has to be metered in both directions, so heat pumps use a bi-flow TXV (or a piston that meters one way and lets refrigerant bypass the other) at both the indoor and outdoor coils, often with check valves routing flow around the off-direction device.
  • Bi-flow filter drier. A drier that filters in both flow directions, since the liquid line carries flow both ways across the seasons.
  • Crankcase heater (common) to keep refrigerant from migrating into and condensing in the compressor oil during off cycles in cold weather.

The indoor side is an air handler or furnace coil, and it almost always carries electric backup/auxiliary heat strips (or a gas furnace in a dual-fuel setup) because the heat pump alone can't keep up in the coldest weather and needs help during defrost.

How it's wired and plumbed

Refrigerant: same two lines as an AC, but now both lines change roles by season. In heat mode the "liquid line" and "suction line" effectively swap pressure/temperature behavior because flow reverses. Control: adds the O (reversing valve) wire to the usual R/C/Y/G, plus W/Aux and E (emergency) for the backup heat, and a defrost interface. The thermostat is a heat-pump thermostat that knows about O/B, auxiliary, and emergency heat. Power: 240V single-phase through the disconnect to the contactor, same as any condenser.

In the field — sequence of operation

Cooling: identical to an AC — Y energizes the contactor, O energizes the reversing valve (typical), compressor and outdoor fan run, outdoor coil is the condenser, indoor coil is the evaporator.

Heating: Y energizes the compressor with the reversing valve de-energized (typical), so the outdoor coil becomes the evaporator (absorbing heat from cold air) and the indoor coil becomes the condenser (releasing heat into the house). The indoor blower runs; if the heat pump can't keep up, the thermostat calls auxiliary strips.

Defrost: the board sees frost conditions (coil temp + time/demand) → reverses to cooling momentarily, shuts the outdoor fan off, energizes backup heat → hot gas melts the outdoor coil frost → on termination (coil warms or time-out) it flips back to heat. You'll see steam off the outdoor unit; that's normal.

Normal values & targets

  • Charging in heat mode: subcooling and superheat targets shift with outdoor temperature; the most reliable method in cold weather is weigh-in to the factory charge plus line-set adjustment, or the manufacturer's heat-mode charging chart. Don't chase a single number.
  • Reversing valve performance: there should be a clear temperature difference across the valve's lines; a valve bleeding internally shows a too-warm suction and weak capacity.
  • Defrost interval: typically every 30–90 minutes of heat-mode run in frosting conditions, terminating on coil temp (or a time backup). Demand-defrost boards stretch the interval when frost is light.
  • Balance point: the outdoor temperature where the heat pump's output equals the building's heat loss — below it, the strips have to help. Often falls somewhere around 25–35°F depending on equipment and house.
  • Capacitor / amps / voltage: same targets as any condensing unit.

Common faults & what they mean

  • Heats fine, won't cool (or cools fine, won't heat). Reversing valve not shifting — stuck valve, bad solenoid coil, lost O signal, or a charge so low the valve can't shift. Check for 24V at the coil and listen/feel for the valve to shift.
  • Blows cold air in heat mode. Heat pump not actually heating (low charge, bad valve), stuck in defrost, or strips not coming on during defrost. Could also be a reversing-valve bleed-through robbing capacity.
  • Outdoor coil iced solid, supply air cold. Defrost not initiating — failed defrost board, bad coil sensor, or low charge preventing proper defrost. The unit can't shed frost so it freezes over.
  • Strips run constantly / high winter bills. Heat pump isn't carrying its share — low charge, bad valve, defrost stuck, or aux set to release too early. The strips mask the heat-pump fault.
  • Compressor slugging / liquid noise. Accumulator overwhelmed, overcharge, or flood-back from a metering problem in heat mode.
  • Reversing valve hisses/whistles constantly. Internal bleed (worn valve) — replace; it's bypassing hot gas and killing efficiency.

Tech tips & gotchas

  • Confirm the reversing valve's energized state for THIS unit. Most are O-energized in cooling, but assuming wrong sends your whole diagnosis sideways. Verify against the wiring and the thermostat's O/B setting.
  • A no-heat heat pump is often a charge problem, not a valve. Low charge mimics a bad valve (weak capacity, won't shift). Verify charge before condemning the reversing valve.
  • Steam off the outdoor unit in winter is defrost working — not a fire. Reassure the homeowner; it's the unit shedding frost.
  • Don't condemn defrost on a coil-iced call without checking the sensor and charge. The board only defrosts when the coil sensor says it's cold enough; a bad sensor or low charge defeats it.
  • Heat-mode charging is its own skill. Standard cooling-mode superheat/subcool targets don't apply in winter. Weigh-in or use the heat-mode chart.

Safety / code notes

  • Disconnect within sight per NEC §440; lock out and discharge capacitors before service.
  • Recover refrigerant per EPA Section 608 — never vent. Bi-flow systems still require certified recovery.
  • Backup electric heat conductors/overcurrent per the data-plate MCA/MOCP and NEC; the blower-strip interlock is a safety.
  • Heat pumps need clearance and elevation so the outdoor coil drains defrost meltwater and doesn't sit in ice; follow the install clearances so airflow and drainage aren't blocked.