How do heat pumps work?

A heat pump is a super-efficient, all-in-one electric heating and cooling system. Sometimes called mini-splits, they soak up heat from one place, then move it to another: Into your home in the winter, out of your home in the summer.

But what exactly makes a heat pump so efficient compared to a typical heating system? And if they work by soaking up the heat, how can heat pumps keep your house warm when it's cold outside? Is this too good to be true?

As millions of people who already own heat pumps can tell you, they're legit. Here's how the technology works.

You could think of a heat pump like a combined furnace and air conditioner, though that's not quite right. It's more like an air conditioner that can also run in reverse.

In cooling mode, a heat pump runs exactly like a traditional AC. It uses all the same components, including indoor and outdoor coils and a refrigerant line. It also relies on all the same tricks of physics to suck up heat from inside your house and move it outdoors while tamping down the humidity, too.

Central ACs and heat pumps often look nearly identical, and most AC manufacturers also make heat pumps. If you take a heat pump and a traditional AC with the same cooling-efficiency rating (measured in SEER), they'll cost the same amount of money to run per summer. Again, it's the same tech, so you can expect the same results.

In heating mode, the process runs backwards. The heat pump soaks up heat from outside your home, then moves it indoors.

The upshot is that a heat pump cuts your energy use for heating by 60 to 70% compared to a traditional heating system. It delivers the same amount of heat while using much less energy. That's true even after you account for inefficiencies with the electrical grid.

It's a totally different process than traditional heating systems.

A furnace or boiler converts fuel into heat: Burn stuff, make fire—the same basic tech we've used since the Stone Age. To be a little more scientific about it: Burning fuel (usually gas, oil, or propane) releases the chemical energy stored in its molecular bonds, and it comes out as heat. It's the same deal with wood stoves.

"Regular" electric heat, or electric-resistance heat, is another variation of the same idea: It turns one form of energy (electricity) into another (heat). A heating element resists the flow of electricity, and it harnesses the resulting heat into something useful—the same way that a toaster or blower dryer works.

At best, these systems take one unit of fuel or electricity and turn it into exactly one unit of heat—that is, they're 100% efficient, like all electric radiators. Fuel-burning systems always waste some energy, so they're usually closer to 80% or 90% efficient, depending on the system.

With heat pumps, the principles are totally different. They don't convert energy directly into heat, like traditional heaters.

Instead, heat pumps use electricity to power a system that attracts ambient heat and moves it. So essentially, you can use a little bit of electricity to harness a whole bunch of free heat that comes from the sun.

Remember those 100% efficient electric radiators? A well-installed heat pump blows that away: It can be more than 300% efficient over the course of a heating season. That is, for every one unit of electricity you put into the system, you get three units' worth of heating out of it. Even less-than-perfect installations still tend to be about 250% efficient.

(For what it's worth, these raw energy savings don't translate directly to lower utility bills or reduced carbon emissions. Natural gas is often much cheaper than electricity, for example. And a lot of the electricity in the US still comes from fossil fuels. Even so, many, if not most, people will save money by running a heat pump, and it's almost always better for the environment, even by the most conservative credible estimates.)

This seems to be the hardest concept for most people to wrap their heads around: If a heat pump works by sucking up heat, how can it keep your house warm when it's freezing outside— and it seems like there's no heat to suck up?

Here's the simplest explanation we've come across:

Heat pumps work because they can make themselves even colder than the air outdoors.

Even when the air feels painfully cold to human skin, it still carries some heat energy. It's warmer in Antarctica than in outer space, for example.

Another fact: Hot flows toward cold. This is a fundamental law of physics, and it's why opening the window on a really hot summer day just makes your house warmer.

So as long as the refrigerant inside the heat pump is less hot than the air, the heat energy in the air will flow into the heat pump.

Some heat pumps are better than others in the cold

Not every heat pump can pull this off. Basic heat pumps don't work well in cold weather because they can't make themselves cold enough. Starting around 35 F, they produce less and less warm air for your home—just as your home needs more of it, ironically. Most of these heat pumps come equipped with backup heating strips—essentially giant electric radiators—that kick on when the temperature drops low enough.

But high-performance mini-split heat pumps can make themselves colder as needed to perform well in lower temps. Plenty of models work perfectly well down to 5 F, and some even keep cranking at their full potential down to -5 F. (They all work with or without ducts.)

As the temperature drops, even these cold-climate heat pumps become less efficient. That is, they need to use more electricity to absorb enough heat. Even then, heat pumps still use much less energy than traditional heaters, particularly when you average out the energy usage over an entire heating season, from fall through spring.

Every heat pump's heating ability eventually tapers off below a certain temperature—though it has to be really cold for them to stop working entirely. The details vary from model to model.

In case you're worried: Any decent HVAC contractor will take all of this into account when they're designing a heat pump system for your home—it's basic HVAC stuff. Or, with a ground-source heat pump, you can sidestep the issue with cold weather entirely. It's always between 50 F and 60 F below the frost line.

If you can wrap your head around these facts, you'll be most of the way toward understanding the science and engineering behind heat pumps and why they're special.

• Heat always moves toward cold (bringing it back from the last section). Heat pumps absorb heat by making themselves colder than their surroundings—even when those surroundings are super, super chilly.

• When you increase the pressure of a substance, you also increase the temperature. (This is why you can cook a frozen chicken in an Instant Pot in like 12 minutes.) The opposite is also true: Depressurizing a substance makes it colder. Heat pumps use a bunch of pressure valves to manipulate the temperature of their refrigerant in the right places at the right times.

• Pressure also affects the "phase" of a substance. (That is, whether it's gas or liquid, in the context of refrigerants). Some quirky temperature-related stuff called superheating and subcooling happens during phase changes, and heat pumps leverage that, too.

• Heat pumps usually have one outdoor unit and one or more indoor units.

• Each outdoor and indoor unit is connected by a refrigerant line—essentially a long metal tube set up in a loop.

• The refrigerant line, of course, is filled with a refrigerant. A refrigerant is a substance that's excellent at storing heat and that will switch between a liquid and a gas at a practical temperature and pressure. (There are hundreds of useful refrigerants; only two are in common use in brand-new heat pumps.)

• Each indoor and outdoor unit contains a coil—sometimes called (more accurately) a heat exchanger. The coil is a contraption where the refrigerant tube is coiled up behind metal fins such that it can both absorb and release heat efficiently. Coils can also be called evaporators or condensers, and that's not wrong—it's just confusing in the context of a heat pump because the correct name changes when you switch from heating to cooling. So to avoid confusion, trained techs just call them the indoor coil or outdoor coil.

• Every indoor unit and outdoor unit also has a fan to blow air across the coils.

• Several valves sit along the refrigerant loop. Some are there to tinker with the refrigerant pressure (expansion valves), others to regulate what direction the refrigerant is flowing depending on heating or cooling mode (reversing valves and check valves).

• The compressor is the heart of the whole machine—the component that actually pumps the refrigerant through the system. It doubles as a pressurizing valve, too. Basic heat pumps use single-stage compressors that move the refrigerant at a consistent rate and pressure. Higher-performance heat pumps use variable-speed compressors that can adjust the refrigerant's flow rate and pressure level, sometimes down to a very fine level.

• And loads more, including filters, defrosting components, and electronics.

This is not an intuitive phenomenon, but in brief: The refrigeration cycle is a process that manipulates the pressure, temperature, and phase of a refrigerant to help move heat around quickly and efficiently.

Let's take a trip through the refrigerant loop in cooling mode.

• The heat pump turns on and starts drawing electricity. The compressor starts pumping refrigerant.

• Refrigerant is drawn into the compressor as a gas at a low temperature and a low pressure.

• The compressor squeezes the refrigerant, and it leaves the compressor as a high-pressure—and now high-temperature—gas.

• The refrigerant flows through a reversing valve, which is the part that can change the direction of the refrigerant's flow depending on the operating mode. Since we're in cooling mode, the reversing valve points the refrigerant toward the outdoor coil.

• The refrigerant—which is still a gas and very hot, much hotter than the air on an oppressive summer afternoon—flows into the outdoor coil. The heat in the refrigerant begins flowing to the less-hot air with the help of the fan and the coil's metal fins.

• The refrigerant cools off—so much so that it condenses into liquid by the time it leaves the outdoor coil. It's still highly pressurized.

• Further down the line, the refrigerant passes through an expansion valve, which de-pressurizes the refrigerant. This cools it off and also turns it into a gas.

• The cold, depressurized refrigerant passes through the indoor coil as a fan blows air across that coil. The coil will be noticeably colder than the air temperature inside your home. So the heat in your home's air absorbs into the cold coil, and the refrigerant begins to warm up once again.

• Moisture in the air also condenses on the cold surface of the indoor coil and drips down into a tray. (Cold air can't hold as much water vapor as warm air, so the moisture turns into liquid water—it's the same reason the glass holding a cold drink "sweats" on a hot day.)

• So, the air before it's blown across the indoor coil = hot and muggy. Air after it's blown across the indoor coil = cooler and drier. Over time, the temperature and humidity in your home drop, and you're a happy camper.

• The refrigerant exits the indoor coil and moves toward the compressor, where the cycle begins again. It repeats until the temperature indoors falls enough to match the thermostat's set point.

• Once the target temperature is reached: A basic single-stage heat pump will shut itself down until the indoor temperature rises again. A higher-performance, variable-speed heat pump will adjust its power level (and electricity consumption) downward so that it cools more slowly—attempting to maintain a steady temperature by running at a consistent (and energy-efficient) low power setting.

In heating mode, it's essentially the same process. The indoor and outdoor coils serve opposite purposes, and the refrigerant moves through a slightly different set of valves. It also doesn't really dehumidify your home in heating mode. But it's the same basic idea.

Still, have questions? We've got loads more to help you figure out whether a heat pump can be a good fit for your home.

And when you're ready to start getting quotes, our heat pump marketplace can help connect you with installers in select states.

Have we mentioned that heat pumps—efficient and electric as they are—pair very well with solar power? Learn more about all your solar options here.