Heat Pumps: Types, Sizing, Efficiency Ratings, and Cold-Climate Performance

How Heat Pumps Work

A heat pump uses a refrigerant cycle to absorb heat from one place and release it in another. In heating mode, it extracts heat from outdoor air (or the ground) and moves it inside. In cooling mode, it extracts heat from indoor air and dumps it outside. The compressor does the heavy lifting, pressurizing the refrigerant to raise its temperature on the discharge side.

This is the same technology as your refrigerator or air conditioner — the only difference is that a heat pump can run the cycle in both directions. A conventional AC unit is technically a one-way heat pump that only cools.

Efficiency comes from the fact that moving heat requires less energy than creating it. A 96% efficient gas furnace converts 96 cents of every dollar of gas into heat. A heat pump with a coefficient of performance (COP) of 3 delivers $3 of heat for every $1 of electricity. Even accounting for the higher cost of electricity per BTU, the heat pump usually wins on operating cost.

Air-Source Heat Pumps

Central air-source heat pumps look and install like traditional AC condenser units — an outdoor unit connected to indoor ductwork via a line set. If you're replacing an existing central AC and furnace, a ducted air-source heat pump uses the same ductwork and thermostat location. The outdoor unit is slightly larger than a conventional AC condenser.

Performance drops as outdoor temperature drops because there's less heat in the air to extract. Standard air-source heat pumps become inefficient below about 25 to 30 degrees Fahrenheit and need supplemental heat (electric resistance strips or a backup furnace) for the coldest days. Cold-climate models (Mitsubishi Hyper-Heat, Fujitsu XLTH, Bosch IDS, Daikin Fit) maintain rated capacity down to 5 degrees and produce usable heat well below zero.

A dual-fuel system pairs a heat pump with a gas furnace. The heat pump handles heating above a balance point (typically 25 to 35 degrees) and the furnace takes over below it. This captures the heat pump's efficiency for most of the heating season while keeping the furnace as backup for extreme cold. Dual-fuel makes particular sense in cold climates with cheap natural gas.

Mini-Split (Ductless) Heat Pumps

Mini-splits consist of an outdoor compressor unit connected to one or more indoor wall-mounted heads by refrigerant lines. Each head heats and cools a single zone independently. No ductwork needed — which makes them the go-to solution for additions, converted garages, older homes without ducts, and rooms that the central system doesn't reach.

A single outdoor unit can serve multiple indoor heads (multi-zone systems). A 3-zone system with one outdoor unit and three indoor heads is common. Each head has its own thermostat and operates independently, so you can heat the bedroom at 68 while leaving the guest room off.

Installation is less invasive than ducted systems. Each indoor head needs a 3-inch hole through the exterior wall for the refrigerant lines, electrical cable, and condensate drain. The outdoor unit sits on a pad or wall bracket. A competent installer can do a single-zone system in half a day.

The indoor heads aren't invisible — they mount high on the wall and are about 32 inches wide, 12 inches tall, and 8 inches deep. Some people find them aesthetically objectionable. Ceiling cassette models (flush-mounted in the ceiling) are less visible but require ceiling access for installation and maintenance.

Ground-Source (Geothermal) Heat Pumps

Ground-source heat pumps extract heat from the ground via buried loops of pipe filled with antifreeze solution. Below the frost line, ground temperature is a constant 45 to 55 degrees year-round, which gives the heat pump a much more stable heat source than outdoor air. This makes ground-source systems the most efficient option — COP of 4 to 5 is typical, compared to 2 to 3.5 for air-source.

The buried loop is the expensive part. Horizontal loops need 150 to 300 feet of trench per ton of capacity, 4 to 6 feet deep. Vertical loops use boreholes 150 to 300 feet deep. Horizontal is cheaper but requires a lot of yard space. Vertical works in smaller lots but requires a drilling rig.

Total installed cost for a residential ground-source system is $20,000 to $40,000, compared to $5,000 to $15,000 for a quality air-source system. The operating cost savings are real but the payback period is long — typically 10 to 15 years. Ground-source makes the most financial sense for new construction where the loop installation can be done before landscaping, and in climates with extreme temperatures where air-source efficiency drops.

Efficiency Ratings

SEER2 (Seasonal Energy Efficiency Ratio) measures cooling efficiency. Higher is better. Minimum federal standard is 14.3 SEER2 for split systems. Good heat pumps are 16 to 20 SEER2. Premium models reach 22+. Compare SEER2 values when shopping, but understand that the highest SEER2 doesn't always deliver the best value — the jump from 16 to 20 SEER2 costs more than the jump from 14 to 16, with diminishing returns.

HSPF2 (Heating Seasonal Performance Factor) measures heating efficiency. Minimum federal standard is 7.5 HSPF2 for split systems. Good cold-climate heat pumps are 10 to 12 HSPF2. This is the number that matters most if heating is your primary concern.

COP (Coefficient of Performance) is the simplest metric: output BTUs divided by input BTUs. A COP of 3 means you get 3 units of heat for every 1 unit of electricity consumed. COP varies with outdoor temperature — a heat pump might have a COP of 4 at 47 degrees and a COP of 2 at 5 degrees. Ask for COP at specific temperatures, not just the rated-condition value.

Sizing

Proper sizing requires a Manual J load calculation, which accounts for your home's square footage, insulation levels, window area and orientation, air leakage, and local climate data. Don't let an installer size the system based on square footage alone — that's a shortcut that routinely produces oversized systems.

Oversizing is a bigger problem than undersizing. An oversized heat pump short-cycles (turns on and off frequently), which reduces efficiency, increases wear on the compressor, and causes uncomfortable temperature swings. In cooling mode, short-cycling also prevents adequate dehumidification — the system cools the air before it can remove enough moisture.

Variable-speed (inverter-driven) heat pumps are more forgiving of sizing imprecision because they can modulate output from about 30% to 100% of rated capacity. A variable-speed system that's slightly oversized will simply run at a lower speed most of the time. A single-speed system that's oversized will short-cycle no matter what.

Frequently Asked Questions