Heat Pumps 101

In past newsletters, we've gone deep into technical, economic, and policy aspects of heat pumps. As the audience has grown, I think it's time to step back and cover the fundamentals. Whether you're new to the heat pump space or a seasoned pro, this issue should get everyone on the same page.

How Heat Pumps Work

A heat pump moves heat from one place to another. It’s important to remember that a heat pump doesn’t generate heat—it simply transfers it, which is what makes it so efficient.

At its core, a heat pump uses refrigerant as the working fluid. The refrigerant absorbs heat at the evaporator and releases it at the condenser. Both the evaporator and condenser are heat exchangers, similar to a car radiator, consisting of tubes and fins.

In cooling mode, the heat pump pulls heat from inside your home and dumps it outside. When heating, it reverses the process, drawing heat from the outside and bringing it indoors. The system doesn't change—it just switches directions.

Heating and Cooling in One

One of the most common questions homeowners ask is whether heat pumps provide cooling. The answer? Absolutely. While a traditional air conditioner only moves heat from inside to outside of your home, heat pumps have a reversing valve which allow them to switch modes and move heat in either direction.

In cooling mode, the system operates just like an air conditioner. In heating mode, the heat pump pulls heat from the outside and transfers it inside, even when it's cold outside.

Heat Pump Applications Beyond HVAC

Heat pumps can be used anywhere heating or cooling is needed. We often focus on heat pumps for HVAC (heating, ventilation, and air conditioning), since home heating and cooling is the largest energy usage for most homeowners. However, heat pumps aren’t limited to HVAC. For instance, heat pump water heaters work by pulling heat from the surrounding air and transferring it to heat water, effectively replacing gas-powered water heaters. Similarly, heat pump pool heaters operate on the same principles, pulling heat from the air to warm large volumes of water.

There are also heat pump dryers, which use the surrounding air to dry clothes more efficiently than traditional resistive-electric or gas dryers. Even your refrigerator works using the same principles—it extracts heat from the interior and exhausts it into the surrounding room. Heat pumps are already a part of our daily lives, even outside the context of HVAC systems.

Beyond the home, heat pumps play a critical role in decarbonizing industries like pharmaceuticals, petroleum refining, paper manufacturing, and even beer brewing. Electric cars also use heat pumps in their HVAC systems, increasing their range and efficiency from early models that relied on resistive heating. The applications are broad, but the fundamental physics are identical.

Air Source vs. Ground Source Heat Pumps

Let’s refocus on home HVAC applications. Even within this area, there’s a huge variety of heat pump technologies and types.

There are two main heat pump architectures: air source and ground source (commonly referred to as geothermal). Air source heat pumps exchange heat between the air inside your home and the air outside. Ground source heat pumps, on the other hand, exchange heat with the ground.

The advantage of ground source systems is that they are more efficient, especially in very cold temperatures, because they rely on the relatively constant and warmer temperature of the earth. However, ground source systems are significantly more expensive to install because they require trenching and installing a series of pipes underground, which is labor-intensive and can involve complex permit approvals. Installing an air source heat pump on the other hand simply involves placing a small outdoor unit for heat transfer.

In the past, air source heat pumps struggled to operate in extremely cold temperatures, making geothermal systems the only viable option for some parts of the country. But with advancements in technology, many air source heat pumps can function effectively in temperatures lower than -15°F. For most American homes, air source heat pumps are not only viable but also the easiest and most cost-effective option, which is why we’ll focus primarily on those.

Air-to-Air and Air-to-Water Heat Pumps

Within air source heat pumps, there are two main types: air-to-water and air-to-air.

Air-to-water heat pumps transfer heat from the ambient air into refrigerant on one side and into water or another fluid on the other side. A common example is heat pump water heaters, which move heat into water for household use. These systems can be extended with secondary loops using fluids like water or glycol for space heating.

Air-to-water systems are more common in Europe, where radiators and radiant underfloor heating are widespread, making them a natural replacement for gas boilers.

In the US, air-to-air heat pumps are more common. These systems exchange heat between refrigerant and air, both inside and outside the home. Because furnaces are more common than boilers in the U.S., air-to-air systems are a more straightforward replacement. While there’s potential for air-to-water systems to gain traction, particularly in the northeast where boilers are more prevalent, air-to-air remains the norm for now.

Unitary and Split Systems

When it comes to packaging in air-to-air heat pumps, there are two main form factors: unitary and split systems.

Unitary heat pumps (often called packaged systems) contain all the components, including the heat exchangers, within a single box. A packaged HVAC unit is typically a large box that sits outside the home and connects directly into the ductwork. All the refrigerant connections are made inside the unit at the factory, and a large blower pushes air through the ducts into the home. Window air conditioners are another example of unitary systems, where all components are housed together.

Split systems, on the other hand, divide the equipment into an indoor unit and an outdoor unit. The outdoor unit exchanges heat with the outside air, while the indoor unit exchanges heat with the conditioned space inside the home. The most familiar example is a traditional central HVAC system, where an indoor air handler unit (which replaces a gas furnace) is installed in a closet, attic, or crawl space, and an outdoor unit handles the outside heat exchange.

Honey, I shrunk the split system!

Mini-split systems are a more compact version of traditional split systems. Fundamentally, they operate the same way as traditional split systems, but are smaller in size. They come in a range of configurations—like high-wall units, floor-mounted units, and ceiling cassettes that recess into the ceiling. There are even ducted units, which function similarly to central air handlers but are much smaller and only supply air to one or two rooms in a home.

The advantage of mini-splits is their efficiency. By delivering conditioned air directly to where it’s needed, they avoid the energy losses associated with ductwork in traditional systems. This often results in higher overall efficiency and greater control over temperature in specific areas of the home.

Heat Pumps are Everywhere

Hopefully, you now have a better understanding of heat pump lingo and can spot this technology in your daily life. Although there’s a dizzying array of configurations and applications, the bottom line is that we need to replace gas appliances with heat pumps as quickly as possible. Knowing how they work and where to use them is the first step. Now that you’ve got a handle on how heat pumps work and where to use them, you’re in a perfect position to make smarter energy choices—and help your friends and family do the same.

Before you go…

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