How Does a Heat Pump Work?
A heat pump moves heat from one place to another using a refrigeration cycle.
In summer, it moves heat from inside the home to the outdoors.
In winter, an air-source heat pump reverses direction and moves heat from outdoor air into the home.
DOE describes heat pumps as efficient because they transfer heat rather than generating all of their heating directly.
Source: DOE — Pump Up Your Savings with Heat Pumps
The Four Core Components
A basic vapor-compression heat pump relies on:
- Compressor
- Indoor heat exchanger
- Outdoor heat exchanger
- Expansion device
A reversing valve allows a conventional air-source heat pump to change which coil acts as the evaporator and which acts as the condenser.
How a Heat Pump Cools
In cooling mode:
- Indoor air passes over the cold indoor coil.
- Refrigerant absorbs heat from the indoor air.
- The compressor raises the refrigerant pressure and temperature.
- The outdoor coil rejects that heat to outdoor air.
- The refrigerant pressure is reduced.
- The cycle repeats.
That is fundamentally similar to how a central air conditioner works.
How a Heat Pump Heats
In heating mode, the refrigerant circuit is reversed.
- The outdoor coil absorbs heat from outdoor air.
- The compressor raises the refrigerant temperature.
- The indoor coil releases heat into the home.
- The refrigerant pressure drops.
- The cycle repeats.
Outdoor air can contain usable heat even when it feels cold.
What changes in cold weather is how difficult it becomes to extract and lift that heat to the indoor temperature.
What Does the Compressor Do?
The compressor is the mechanical heart of the refrigeration cycle.
It raises refrigerant pressure, which also raises refrigerant temperature.
Traditional single-stage compressors largely operate at one output.
Modern inverter-driven systems can vary compressor speed and therefore system capacity.
A DOE-funded heat-pump guide describes how variable-speed inverter compressors modulate output to meet changing loads and limit short cycling.
Source: DOE-funded Decarbonizing Building Thermal Systems guide, page 15
What Does the Reversing Valve Do?
The reversing valve changes refrigerant flow so the indoor and outdoor coils trade roles.
That is the basic feature that separates a conventional heat pump from cooling-only air-conditioning equipment.
What Is Defrost Mode?
In heating mode, the outdoor coil can become cold enough for frost to accumulate.
The heat pump periodically needs to remove that frost.
A defrost cycle temporarily changes system operation to warm the outdoor coil.
During defrost, indoor comfort may be supported with auxiliary heat depending on the system.
A brief defrost cycle is normal.
A unit permanently encased in ice is not.
Why Does Heat Pump Efficiency Change With Temperature?
Heat pumps work harder as the temperature difference between the heat source and delivered heat becomes more difficult.
For an air-source heat pump, colder outdoor temperatures normally reduce COP and can reduce heating capacity.
That is why low-temperature performance matters for northern installations.
Cold-weather heat pump efficiency
What Is COP?
COP compares delivered heat with electric input at a specific condition.
If a heat pump supplies 3 kW of heat while consuming 1 kW of electricity, COP is 3.
COP changes with operating conditions.
It should not be treated as a fixed number for the entire year.
How Can a Heat Pump Deliver More Heat Than the Electricity It Uses?
Because the electricity powers the movement of heat.
The heat delivered indoors includes:
- electrical input to the heat-pump process
- thermal energy moved from outdoors
The heat pump is not creating energy from nowhere.
It is transporting it.
How Does a Ground-Source Heat Pump Differ?
A geothermal heat pump uses the same general heat-pump principle but exchanges heat with the ground or groundwater rather than outdoor air.
The relatively stable underground temperature can make the refrigeration cycle easier across the year.
How Does a Ductless Heat Pump Differ?
The refrigeration cycle is still similar.
The difference is air distribution.
A ductless mini-split delivers conditioned air directly from indoor units rather than using central whole-house ductwork.
Ductless mini-split heat pumps
What Determines Real Heat Pump Performance?
The equipment design matters, but so do:
- sizing
- outdoor temperature
- indoor setpoint
- ductwork
- airflow
- refrigerant charge
- controls
- defrost strategy
- backup heat
- building envelope
That is why buying from a specification sheet alone is a weak decision process.
For seasonal efficiency, compare SEER2 for cooling and HSPF2 for heating.
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