How Does a Heat Pump Work? A Plain-English Guide for New England
How does a heat pump work? It moves heat with refrigerant instead of burning fuel. See the heating and cooling cycle step by step, the main parts, defrost mode, COP and backup heat, explained for North Shore homes.
A heat pump is a heating and cooling system that moves heat instead of making it, using refrigerant, a compressor and two coils. So how does a heat pump work in winter? It pulls heat out of the outdoor air, concentrates it and releases it inside your home. In summer it runs the same cycle in reverse to cool. This deep dive builds on our heat pumps in Massachusetts guide.
Key Takeaways
- It moves heat, not fuel. A heat pump carries heat from one place to another, like a refrigerator does.
- Refrigerant does the work. It boils at very low temperatures, so it can soak up heat even from freezing outdoor air.
- One valve flips the season. A reversing valve switches the system between heating and cooling.
- Efficiency comes from moving heat. A heat pump can deliver more heat energy than the electricity it uses.
- Defrost is normal. In winter the outdoor unit melts its own frost every so often. Steam from the unit is part of the cycle.
How Does a Heat Pump Work, Step by Step?
A heat pump works by pumping refrigerant around a closed loop. The refrigerant changes from liquid to gas and back again. Each change either soaks up heat or gives it off. Here is the heating cycle you get on a cold North Shore night.
- Absorb heat outside. Cold, low-pressure liquid refrigerant flows through the outdoor coil. The fan pulls outdoor air across the coil. The refrigerant is colder than the air, so it soaks up heat and boils into a gas.
- Compress it. The compressor squeezes that gas. Squeezing a gas makes it much hotter. The refrigerant leaves the compressor hot enough to warm your home.
- Release heat inside. The hot gas travels through copper line sets to the indoor coil. A blower moves room air across the coil. The air warms up, and the refrigerant cools and turns back into a liquid.
- Drop the pressure. The liquid passes through an expansion valve. Its pressure and temperature drop sharply. Now it is cold again and ready to pick up more heat outside.
That loop repeats over and over. Notice that nothing burns. Electricity runs the compressor and fans, but the heat itself comes from outdoors. DOE's overview of heat pump systems describes the same basic cycle.
Not sure your current system is sized for our winters? Request a heat pump sizing check and we will run the numbers on your home before you spend anything.
How Can a Heat Pump Get Heat From Freezing Air?
Cold air still holds heat, and refrigerant is far colder than winter air. Heat always flows from warmer to colder. As long as the refrigerant in the outdoor coil is colder than the outside air, heat moves into it.
Refrigerants are chosen because they boil at very low temperatures. R-410A, the refrigerant in most systems installed over the last 15 years, boils at about -55°F at normal air pressure. So a 10°F January night in Peabody is still "warm" compared with the refrigerant in the coil.
Heat output does drop as it gets colder, because there is less heat in the air to collect. That is why sizing matters. To earn the ENERGY STAR cold-climate label, a heat pump must still deliver a coefficient of performance (COP) of at least 1.75 at 5°F. Our guide to how heat pumps work in cold weather goes deeper on low-temperature performance and backup heat.
What Are the Main Parts of a Heat Pump?
Every air-source heat pump has the same core parts, whether it is a ductless mini-split or a ducted system. Knowing them helps you understand repair quotes and spec sheets.
| Part | Where it is | What it does |
|---|---|---|
| Compressor | Outdoor unit | Pressurizes refrigerant gas and raises its temperature |
| Outdoor coil | Outdoor unit | Absorbs heat in winter, rejects heat in summer |
| Outdoor fan | Outdoor unit | Pulls outdoor air across the coil |
| Reversing valve | Outdoor unit | Switches refrigerant flow between heating and cooling |
| Expansion valve | Outdoor or indoor unit | Drops refrigerant pressure so it gets cold |
| Indoor coil | Wall head or air handler | Releases heat in winter, absorbs it in summer |
| Blower | Wall head or air handler | Moves room air across the indoor coil |
| Line set | Through the wall | Copper pipes that carry refrigerant between units |
What Is a Reversing Valve?
A reversing valve is the part that lets one machine both heat and cool. It changes which coil gets the hot refrigerant from the compressor. In heating mode, the indoor coil gets the heat. In cooling mode, the outdoor coil does. When a heat pump blows cold air in heat mode, a stuck reversing valve is one possible cause.
How Does a Heat Pump Cool in Summer?
In summer the reversing valve flips the flow, and the indoor coil becomes the cold side. Warm, humid room air passes over the cold coil. The coil absorbs heat, and moisture condenses and drains away. The outdoor unit then dumps that heat outside.
This is exactly how a central air conditioner works. The only difference is the reversing valve. That is why a heat pump replaces both a furnace or boiler and an AC unit. For triple-deckers in Lynn or Chelsea that never had central air, this is often the biggest selling point.
What Is Defrost Mode?
Defrost mode is a short cycle that melts frost off the outdoor coil in winter. When the coil is colder than freezing and the air is damp, frost builds up. Frost blocks airflow and cuts heat output.
To clear it, the heat pump briefly runs in cooling mode. Hot refrigerant flows through the outdoor coil and melts the frost. You may see steam rising from the unit and hear a "whoosh" when the valve switches. A defrost cycle usually lasts a few minutes. This is why outdoor units need a raised stand and a clear path for meltwater, so ice does not build up under them in a nor'easter.
Frost that never clears, or a unit that turns into a block of ice, is not normal. That calls for service.
Why Is a Heat Pump So Efficient?
A heat pump is efficient because moving heat takes less energy than making it. An electric baseboard heater turns one unit of electricity into one unit of heat. A heat pump uses that same electricity to move several units of heat from outdoors.
What Is COP?
COP, or coefficient of performance, is heat delivered divided by electricity used. A COP of 3 means the heat pump delivers 3 units of heat for every 1 unit of electricity. Electric resistance heat always has a COP of 1. According to air-source heat pump basics from the Department of Energy, a heat pump can reduce electricity use for heating by about 50% compared with baseboard heaters or electric furnaces.
COP changes with outdoor temperature. It is highest on mild days and lower on the coldest nights. Seasonal ratings like HSPF2 average that out over a whole winter. Since 2023, U.S. DOE rules require new split-system heat pumps to meet at least 7.5 HSPF2 for heating and 14.3 SEER2 for cooling.
What Is an Inverter Compressor?
An inverter compressor is a variable-speed compressor that ramps up and down to match the heat your home needs. Older heat pumps ran at full blast, shut off, then started again. Inverter models run longer at lower speed. That gives steadier temperatures, quieter operation and better efficiency. Most cold-climate heat pumps sold today use inverter compressors, and it is a big reason they now handle Massachusetts winters.
How Does a Heat Pump Compare to a Furnace or Boiler?
A furnace or boiler makes heat by burning gas, oil or propane. A heat pump moves heat with electricity. A furnace's efficiency is capped below 100% (its AFUE rating). A heat pump routinely delivers more heat than the energy it uses.
- Furnace: burns fuel, heats air, sends it through ducts. Heating only.
- Boiler: burns fuel, heats water or steam for radiators and baseboard. Heating only.
- Heat pump: moves heat with refrigerant. Heats and cools with one system.
Running cost depends on your electric and fuel rates, your home and how the system is set up. Many North Shore homes keep a gas or oil system as backup. We compare the two in detail in heat pump vs furnace: which is better for your home.
What Is Auxiliary and Emergency Heat?
Auxiliary heat is backup heat that kicks in when the heat pump alone cannot keep up. In ducted systems, this is often an electric heat strip in the air handler. In some homes, it is the existing furnace or boiler, controlled to run only on the coldest nights.
Emergency heat is a thermostat setting that shuts off the heat pump and runs only the backup. It is meant for when the heat pump is broken. Leaving it on by mistake can drive up your electric bill fast. If your thermostat shows "AUX" often on mild days, have the system checked.
What Changed With Heat Pump Refrigerants?
As of 2026, new heat pumps use lower-impact refrigerants like R-454B and R-32 instead of R-410A. EPA rules under the AIM Act are phasing down high-global-warming refrigerants in new home systems. The cycle works the same way. Installers just need the right tools and training for these mildly flammable refrigerants.
Refrigerant work is never a DIY job. It requires an EPA-certified technician, a deep vacuum and a leak test. HomePros handles R-410A, R-32 and R-454B systems.
Which Heat Pump Brands Should You Consider?
Most major brands use the same basic parts. The differences show up in cold-climate performance, controls, parts availability and local support. A great brand installed badly will still underperform.
Look for models on the ENERGY STAR cold-climate list, check the AHRI certificate for real ratings, and ask who services the brand locally. Our guide to heat pump brands to avoid and what to look for walks through the red flags.
See How a Heat Pump Would Work in Your Home
Now you know how a heat pump works: refrigerant soaks up outdoor heat, the compressor concentrates it and the indoor coil delivers it. The same loop runs backward for summer cooling. The real question is how it fits your house, your ducts or radiators, and your winter heating load.
Jose has worked in HVAC since 2015 and installs cold-climate Mitsubishi, Daikin, Bosch and LG systems across the North Shore. HomePros starts every job with a load calculation, not a guess. Get a written heat pump estimate for your home, or call (781) 426-8428.