An AC heat pump works by using electricity to move heat from one place to another rather than generating it, and a reversing valve lets it provide both cooling and heating from the same system.
If you have ever wondered how one appliance handles both sweltering summer days and chilly winter nights, you are looking at an AC heat pump. It is a standard air conditioner with a clever upgrade — a reversing valve that flips the flow of refrigerant. In cooling mode, it pulls heat from your indoor air and dumps it outside; in heating mode, it reverses to capture ambient heat from the outdoor air and bring it inside. Because it moves heat rather than making it, it can be dramatically more efficient than a furnace or baseboard heater.
The Basic Refrigeration Cycle Behind Every Heat Pump
An AC heat pump relies on the same vapor-compression cycle used by refrigerators and standard air conditioners, running through four steps in a closed loop.
- Evaporation: Refrigerant passes through the outdoor coil, where it absorbs ambient heat from the surrounding air and changes from a liquid to a low-pressure gas.
- Compression: The compressor pressurizes the refrigerant vapor, which raises its temperature significantly — often to 100°F or higher.
- Condensation: Hot, high-pressure refrigerant flows to the indoor coil, where it releases its stored heat into your home and condenses back into a liquid.
- Expansion: The liquid passes through an expansion valve, dropping its pressure and temperature sharply, ready to absorb heat from outside air again. The cycle repeats until the thermostat says to stop.
This sequence works in both directions. During summer, the reversing valve simply swaps the roles of the indoor and outdoor coils — the indoor coil becomes the evaporator (absorbing heat from your rooms) and the outdoor coil becomes the condenser (dumping it outside).
What the Key Components Actually Do
Several parts work together inside that metal box outside your wall, and knowing what each does helps demystify the whole system. The reversing valve is the defining difference between a heat pump and a standard AC unit — it is the part that redirects the refrigerant flow to change between cooling and heating. The compressor is the main electricity consumer; it pushes refrigerant through the loop and raises its temperature. The two heat exchanger coils (indoor and outdoor) are where the actual heat transfer happens, aided by fans that move air across their surfaces. The expansion valve controls how much the refrigerant pressure drops, which determines how cold the evaporator coil gets.
If you are shopping for a system that handles both seasons well, our roundup of the best AC units with heat pumps compares current models by efficiency, reliability, and features. That page is a direct next step for anyone ready to buy.
Common Misconceptions About Heat Pumps
Several misunderstandings trip people up. First, a heat pump does not create heat — it transfers it. This is why it can deliver three or four times more heating energy than the electricity it consumes. Second, cold outdoor air does not mean there is no heat to capture. Air molecules contain heat energy down to nearly absolute zero, and heat pumps can extract usable warmth from outside air well below freezing, though efficiency drops as temperatures fall. Third, a standard air conditioner and a heat pump are not the same thing — a standard AC has no reversing valve and cannot provide heat. If you see a unit described as a “heat pump,” it can do both jobs; if it is labeled simply an “air conditioner,” it only cools.
Efficiency, Performance, and What Affects Them
Heat pump efficiency is typically measured by HSPF (Heating Seasonal Performance Factor) for heating and SEER (Seasonal Energy Efficiency Ratio) for cooling. Higher numbers mean better efficiency. Performance in heating mode depends on outdoor temperature and the system’s design — some units include backup electric resistance heat that kicks in during extreme cold. A well-sized and properly installed heat pump handles most climates, but if you live somewhere with sustained sub-zero winters, you may want a cold-climate-rated model or a dual-fuel setup that pairs the heat pump with a gas furnace.
References & Sources
- National Grid. “How Do Heat Pumps Work?” Provides the plain-language step sequence of the refrigeration cycle.
- Energy Saving Trust. “In-Depth Guide to Heat Pumps.” Covers efficiency, system types, and operational principles.
- Goodman Manufacturing. “How a Heat Pump Works.” Explains the reversing valve and seasonal reversal of the refrigeration cycle.