HVAC · Lesson 6 of 13Intermediate10 min read

Heat Pumps

A heat pump is an air conditioner that can run backward. In winter it pulls heat out of cold outdoor air, or out of the ground, and pumps it inside. Because it moves heat instead of making it, it can deliver several times more heat than the electricity it uses.

Running the loop backward

A heat pump has the same four parts as an air conditioner (Lesson 4), plus a reversing valve near the compressor. The valve changes the direction the refrigerant flows through the rest of the loop, so the two coils trade jobs.

Cooling mode: the indoor coil is the evaporator, absorbing heat from the house; the outdoor coil is the condenser, dumping it outside.
Heating mode: liquid refrigerant in the outdoor coil absorbs heat from the outdoor air and evaporates, even when that air feels cold. The compressor squeezes the vapor, and the indoor coil becomes the condenser, releasing the heat into the air the blower sends through the house.

Cold air still holds plenty of heat. Air at freezing is about 273 kelvins above absolute zero; the refrigerant only has to be colder still to soak some of that heat up.

This copy starts in heating mode. Flip to Cool to watch the reversing valve swap the coils’ jobs. The Carnot COP is the ideal limit from OpenStax for the temperatures you set; the coil temperatures use the gap you choose.

Why the COP can be more than 1

For heating, a heat pump’s coefficient of performance is the heat delivered divided by the electricity used. An electric furnace or baseboard heater turns each unit of electricity into one unit of heat: a COP of 1. A heat pump delivers the electricity’s energy plus the heat it gathered outdoors, so its COP is always more than 1. OpenStax notes that real heat pumps have COPs of about 2 to 4.

The Department of Energy says an air-source heat pump can deliver up to three times more heat energy to a home than the electrical energy it uses, and can cut the electricity used for heating by about 50 percent compared with electric furnaces and baseboard heaters.

The Carnot limit shows why the outdoor temperature matters. OpenStax works an example: pumping heat from air at −15°C (258 K) into a hot coil at 45°C (318 K), the best possible COP is 318 ÷ 60 = 5.30. Each joule of electricity could then bring in 4.3 joules from the cold air. The colder it gets outside, the bigger the lift and the lower the limit.

COP_heating = Q_h ÷ W = 1 ÷ Eff COP_heating = COP_cooling + 1
Carnot limit: COP_heating ≤ T_h ÷ (T_h − T_c) (kelvins)
OpenStax example: 318 K ÷ (318 K − 258 K) = 5.30

One unit of electricity in, one or three units of heat out. DOE says an air-source heat pump can deliver up to three times more heat energy than the electrical energy it uses. Sources: OpenStax College Physics 2e, 15.5; U.S. Department of Energy, Energy Saver: Air-Source Heat Pumps.

Cold weather, backup heat and defrost

As the outdoor temperature drops, a heat pump has to lift heat farther, so its efficiency falls, and it runs longer to keep up. Heat pumps were long limited to places without extended spells below freezing, but the Department of Energy says newer designs are a real heating option in colder regions, and that cold-climate heat pumps are designed to work in temperatures as low as 5°F. DOE advises that buyers in cold climates look for the highest heating rating they can get.

Most ducted heat pumps have backup heat, usually electric resistance elements in the air handler, for the coldest hours, for defrost, and for emergencies. Many thermostats show it as “aux” or “emergency” heat. Backup heat works at a COP of 1, so a heat pump that leans on it too much costs more to run.

In cold, damp weather, frost builds up on the outdoor coil and blocks the airflow. The heat pump melts it with a defrost cycle: a defrost control can flip the reversing valve for a few minutes so hot refrigerant flows through the outdoor coil. DOE recommends models with demand-defrost controls, which defrost only when needed and so use less energy.

Rating a heat pump: HSPF2 and SEER2

A heat pump gets two seasonal ratings. SEER2 rates its cooling, as for an air conditioner. HSPF2, the heating seasonal performance factor, rates its heating: the heat it delivers over a typical heating season divided by the electricity it uses, in BTU per watt-hour. Since January 1, 2023, new split-system heat pumps must reach at least 14.3 SEER2 and 7.5 HSPF2 nationwide. DOE suggests putting more weight on SEER2 in hot climates and on HSPF2 in cold ones.

Ductless mini-splits

A ductless mini-split is a heat pump (or air conditioner) with no ducts. An outdoor unit connects to one or more indoor units, usually mounted high on a wall, through a bundle of refrigerant lines, wiring and a condensate drain. DOE notes that installing one takes only about a three-inch hole through the wall. Multi-zone systems can serve up to eight rooms from one outdoor unit, each with its own setting.

With no ducts, there are no duct leaks, which makes mini-splits popular for additions, older homes with radiators, and rooms the central system can’t reach. Each indoor unit has its own filter to clean and its own drain line.

Geothermal (ground-source) heat pumps

A few feet below the surface, the ground stays at a fairly steady temperature all year, about 40°F to 70°F depending on where you live, says the Department of Energy. A geothermal heat pump trades heat with the ground through a loop of buried pipe, so in winter it lifts heat from ground that’s warmer than the air, and in summer it dumps heat into ground that’s cooler than the air. Smaller lifts mean higher COPs.

DOE describes four loop types: three closed loops (horizontal trenches, vertical boreholes, and pond or lake loops) and an open loop that uses well or surface water. Compared with air-source heat pumps, DOE says geothermal systems are quieter, more efficient and longer-lasting: up to 24 years for the indoor parts and 50 years or more for the ground loop. They cost several times more to install, which DOE says energy savings can repay in 5 to 10 years.

Four ways to connect a geothermal heat pump to the earth: three closed loops and an open loop. Schematic, not to scale. Source: U.S. Department of Energy, Energy Saver: Geothermal Heat Pumps.

Heat pump parts and what they do
Part	What it does	Signs of trouble
Outdoor unit	Holds the compressor, the outdoor coil and fan, and the reversing valve.	Iced over for hours; humming; a fan that won’t spin.
Reversing valve	Switches the refrigerant’s direction so the coils trade jobs.	Stuck in one mode: cools when you want heat, or the reverse.
Outdoor coil	Evaporator in heating mode, condenser in cooling mode.	Frost that never clears; blocked by snow or debris.
Indoor coil and air handler	Condenser in heating mode, evaporator in cooling; the blower moves the air.	Weak airflow; water leaks in cooling.
Backup heat	Electric resistance elements for the coldest hours and for defrost.	High winter bills; “aux heat” showing most of the time.
Defrost control	Decides when to run a defrost cycle and ends it.	Heavy ice that stays; endless defrosting.
Thermostat	A heat pump thermostat controls the compressor, the reversing valve and the backup heat.	Wrong mode; backup heat running when it shouldn’t.

NoteThermostat setback and heat pumps

The Department of Energy says ordinary programmable setback is generally not recommended for heat pumps in heating mode: a deep setback can call for backup heat when the house recovers, canceling the savings. Moderate settings, or a thermostat designed for heat pumps, work better (Lesson 9).

StopLeave these to a technician

Refrigerant, the reversing valve and anything inside the outdoor unit: refrigerant work requires EPA Section 608 certification, and the cabinet holds 240-volt circuits.
An outdoor coil that stays solid with ice for hours, or a system running on backup heat all the time.
Drilling, trenching and connecting geothermal loops.

Check yourself

Tap an answer to check it, or open “Show the answer.”

Which part lets a heat pump switch between heating and cooling?
Show the answer
The reversing valve. The reversing valve changes the direction of refrigerant flow, so the indoor and outdoor coils trade jobs.
In heating mode, which coil is the evaporator?
Show the answer
The outdoor coil. The outdoor coil absorbs heat from the outdoor air as the refrigerant evaporates.
A heat pump running with a COP of 3 delivers:
Show the answer
Three units of heat for every unit of electricity. COP = heat delivered ÷ electricity used. The extra comes from the outdoor air or ground.
Why does a geothermal heat pump usually beat an air-source heat pump in winter?
Show the answer
The ground is warmer than winter air, so the lift is smaller. A smaller temperature lift means a higher possible COP. DOE puts the shallow ground at about 40°F to 70°F.
What does a defrost cycle do?
Show the answer
Melts frost off the outdoor coil so air can flow through it. Frost blocks airflow through the outdoor coil; the defrost cycle clears it, often by briefly reversing the valve.

Go deeper

U.S. Department of Energy, Energy Saver: Air-Source Heat Pumps DOE on air-source heat pumps, including ductless systems.
U.S. Department of Energy, Energy Saver: Geothermal Heat Pumps DOE on geothermal heat pumps.
U.S. Department of Energy: Pump Up Your Savings with Heat Pumps DOE’s overview of heat pump savings and tax credits.
Science Lab: Engines, Refrigerators and Heat Pumps Heat pumps and the Carnot limit in the Science Lab.

Sources

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A free course from Teddy Rook Book. It explains how HVAC systems work in general and follows U.S. sources; your equipment’s manuals and the local code are the final word, along with your gas utility for anything involving gas.