The Fundamental Principle: Moving Heat Instead of Generating It

A heat pump does not generate heat — it moves it. In heating mode, it extracts heat energy from a source (outdoor air, ground, or water) and delivers it to the building interior at a higher temperature. In cooling mode, it reverses direction, extracting heat from the interior and rejecting it to the source. This is the same refrigeration cycle used in air conditioners, but with a reversing valve that allows the cycle to run in either direction.

Because a heat pump moves heat rather than generating it by combustion, it can deliver more energy to the building than it consumes electrically. A heat pump with a coefficient of performance (COP) of 3.0 delivers 3 units of heat energy for every 1 unit of electrical energy consumed. No combustion appliance — gas furnace, boiler, electric resistance heater — can exceed a COP of 1.0 because they are limited by the energy content of the fuel. This thermodynamic advantage makes heat pumps the most efficient heating technology available.

The Refrigeration Cycle in Both Modes

Heating mode: Refrigerant in the outdoor coil (now functioning as the evaporator) absorbs heat from the outdoor air, ground, or water source — even at temperatures well below freezing, because any substance above absolute zero contains heat energy. The refrigerant evaporates and is compressed, raising its temperature significantly. The hot compressed refrigerant flows to the indoor coil (now the condenser), where it releases heat to the indoor air or water distribution system. The refrigerant then expands through the expansion valve, cooling down, and returns to the outdoor coil to repeat the cycle.

Cooling mode: The reversing valve switches direction. The indoor coil becomes the evaporator (absorbing heat from indoor air, providing cooling), and the outdoor coil becomes the condenser (rejecting heat to the outdoor environment). This is identical to standard air conditioner operation.

Air-Source Heat Pumps (ASHP)

Air-source heat pumps exchange heat with outdoor air. They are the most common and least expensive heat pump type. The outdoor unit looks like an air conditioner condenser — a cabinet with a coil and fan — and connects to an indoor air handler or fan coil unit via refrigerant lines.

Performance and COP: ASHP efficiency varies significantly with outdoor temperature. At mild temperatures (40–55°F), a modern ASHP may achieve a heating COP of 3.0–4.0. As outdoor temperature drops, the temperature difference between source and delivered heat increases, requiring more compressor work and reducing COP. At 0°F, an older ASHP might deliver a COP of only 1.5–2.0. Modern cold-climate heat pumps (CC-ASHPs) — using variable-speed inverter-driven compressors and optimized refrigerant circuits — maintain useful heating capacity and COPs of 2.0 or above at temperatures as low as -13°F to -22°F.

Defrost cycles: When outdoor temperature is below approximately 40°F with high humidity, frost accumulates on the outdoor coil, reducing heat transfer efficiency. The heat pump periodically reverses to cooling mode briefly to melt the frost (defrost cycle). During defrost, auxiliary electric heat (strip heat) is often activated to prevent cold air from blowing into the space. Frequent defrost cycles at low outdoor temperatures reduce overall heating efficiency and are a key reason older heat pumps had poor reputations in cold climates.

Mini-split and multi-split systems: Ductless mini-split systems are a variant of ASHP where the refrigerant lines connect directly to wall- or ceiling-mounted fan coil units rather than a ducted air handler. Multi-split systems connect one outdoor unit to multiple indoor units with individual zone control. Mini-splits eliminate duct losses and allow room-by-room control, making them efficient solutions for additions, retrofits, and buildings without existing ductwork.

Ground-Source Heat Pumps (GSHP)

Ground-source heat pumps exchange heat with the ground or groundwater rather than outdoor air. Because ground temperature below the frost line remains relatively constant year-round (50–60°F in most of the continental U.S.), GSHP performance is far more consistent than ASHP and does not degrade in cold weather.

Closed-loop systems: A loop of polyethylene pipe buried in the ground circulates a water-antifreeze solution. The loop extracts heat from the ground in winter and rejects heat to the ground in summer. Horizontal loops are buried 4–6 feet deep in trenches and require significant land area. Vertical loops are drilled boreholes 100–400 feet deep and require far less surface area — the preferred approach for commercial projects and sites with limited land. Pond/lake loops coil pipe on the bottom of an adjacent water body where permitted.

Open-loop systems: Groundwater is pumped from a well, heat is extracted or rejected, and water is returned to a discharge well or surface water. Very efficient where groundwater is abundant and permits allow, but subject to water rights, mineral scaling, and regulatory restrictions.

Performance: GSHP systems typically achieve heating COPs of 3.0–5.0 year-round due to the stable source temperature. EER values in cooling mode are also higher than ASHP because the ground is cooler than outdoor air in summer. The higher efficiency comes at significantly higher installation cost — primarily the ground loop installation (drilling or excavation).

Water-Source Heat Pumps (WSHP)

Water-source heat pumps are common in large commercial buildings as part of a two-pipe water loop heat pump system. Multiple water-source heat pump units throughout the building connect to a common building water loop maintained at 60–90°F. Units in cooling mode reject heat to the loop; units in heating mode extract heat from the loop. When cooling and heating loads are balanced across the building, the system operates very efficiently with minimal boiler or cooling tower operation. A boiler adds heat to the loop when the loop temperature drops below 60°F; a cooling tower rejects heat when the loop exceeds 90°F.

WSHP systems are well-suited to buildings with simultaneous heating and cooling needs — interior zones that require cooling while perimeter zones require heating. The loop acts as an internal heat transfer medium, moving heat from where it is unwanted to where it is needed.

Sizing Heat Pumps

Heat pump sizing follows the same Manual J load calculation process used for conventional HVAC systems — calculate the peak heating and cooling loads for each zone, then select equipment to meet those loads. However, several heat-pump-specific considerations apply:

Design temperature selection for ASHP: Because ASHP capacity decreases with outdoor temperature, the equipment must be sized based on its capacity at the design heating temperature, not at standard rating conditions (47°F). Always obtain heating capacity data at the actual design outdoor temperature for your location (ASHRAE 99% design temperature).

Dual-fuel systems: In cold climates where ASHP efficiency falls at low temperatures, a dual-fuel system pairs a heat pump with a gas furnace backup. The heat pump handles heating above a balance point temperature (typically 30–35°F); the gas furnace takes over below. This captures most of the efficiency benefits of heat pumps while avoiding the COP penalty at extreme low temperatures.

Oversizing penalty: Oversized heat pumps short-cycle — they satisfy the thermostat setpoint quickly, shut off, and restart frequently. Short-cycling reduces dehumidification in cooling mode (the coil doesn't stay cold long enough to condense adequate moisture) and increases wear on compressor components. Proper Manual J sizing is more important for heat pumps than for gas systems.