When I say "Heat Pump," what come to mind for you? For some, it evokes a mini-split, for others a fancy upgrade to their central air. But Heat Pumps are deployed in many more ways and environments than just those two, and Heat Pump Review intends on covering all of it (over time). Below, we try and set the stage for the multitude of ways heat pumps are used, as well as introductory notes on why each matters.

Applications

How We Use Heat Pumps: Ambient Heating & Cooling (aka Home, Office, Auto HVAC), Water Heating, and Clothes Drying.

Ambient Heating & Cooling (HVAC)

This is probably what you think about first when someone says "Heat Pump," and this rightfully will dominate the discussion and content of Heat Pump Review given nearly 10% of US greenhouse emissions come from heating our homes and when combined with cooling with inefficient radiant heat, cooling with inefficient AC units, this number is likely a lot higher.

It should be noted – of course – that everyone's existing air conditioner is already a heat pump (pumps inside heat to the outside), so we'll mostly be talking about using heat pumps to heat the home / office.

Water Heating

After ambient space heating and cooling (HVAC), the second most the second most energy-hungry / carbon emitting machine in your home is the giver of long hot showers: the hot water heater. Today, most water gets heated by burning a fossil fuel (usually natural gas) or by using an inefficient electric filament inside the water tank (aka "resistance heat").

The future of heating hot water, however, also turns out to be with heat pumps. Not only are they more energy efficient, but they also eliminate yet another source of toxic fumes in the home (e.g., carbon monoxide). Heat pump-based water heaters have gotten incredible good and inexpensive, so expect significant coverage of this space in the Heat Pump Review .

Clothes Dryers

Perhaps the least well known home heat pump use-case is the clothes dryer. While these are already popular in places like South Korea, they have been dragged down in the US by user reviews that claim they take too long to dry and so in the US dryers are approximately 25% fired by natural gas and 75% fired by conventional electrical (read: power hungry resistance heat). But, in addition to energy efficiency, there are some extra benefits for consumers who switch to heat pump powered dryers, such as the ability to operate without an external dryer vent. With all the innovation happening to make heat pumps more efficient, we expect this space to see innovation in the next few years.

Other

The most commonly found heat pump is your refrigerator/freezer! We likely won't cover that use-case much in Heat Pump Review , but it's a good reminder that heat pumps can be found all around us; and inspires the thought that we might find novel uses for them in machines we haven't considered of as of yet.

Heat Source or Dump

When we move heat, where does it come from and where does it go? The three main categories of heat source are air, ground, and water:

Air Source

The most common type of heat pump is an air source heat pump. With an air source heat pump, heat taken out of the air and then dumped where it's needed – often air in another space. Take your refrigerator for example, heat inside of the refrigerator is taken from the air and dumped in your living space's air. Your conventional air conditioner takes the heat found in your indoor space and dumps it outside; conversely, when we use conventional heat pumps to heat our homes, heat found in the outside air is moved inside.

Ground Source

While air source is the most common and simple, it's not always the most efficient course of action to go find heat in super cold outdoor air in the winter or to try and dump indoor heat outside when it's already very hot. Ground Source heat pump systems try and overcome these inefficiencies by using the earth's natural ability maintain a more temperate ~55F – making it easier to extract heat if you're trying to heat your home or dump heat if you're trying to cool it.

Water Source

Dusting off our high school chemistry books, we'll recall that water has a high heat capacity , making it a perfect substance to use in order to store heat in and extract heat out of. In fact, it should be noted that many ground source heat pumps systems (discussed above) are actually water source systems where the ground is used to condition that water to the ~55F, making it easier to store or remove the targeted heat.

Scale: Machine, Room, Whole Home, Block, or District

Another vector around which we can think about heat pumps are the scale they operate on.

Machine

As already covered, a lot of our everyday use-cases for heat pumps are at the self-contained machine-level: think your refrigerator, a hot water heater, or a dryer. At the machine scale, the heat is made for or removed from one unit specifically, without regard to other appliances or applications for heat nearby.

Room/Zone

The next unit higher is the room or zone-level. Window AC units or mini-splits are perhaps the most common version of this, where a machine is a self-contained heat pump designed to condition just one area (e.g., there is one compressor per room or zone conditioned).

Whole Home/Office

Up from this are whole home systems commonly found with today's new construction homes, also known as multi-zone systems. Whole home systems can be ductless - with mini-splits installed in each room or zone - or ducted / forced air systems with simple vents in each room connected via ducts behind the walls or ceilings. Whether ductless or ducted, a large heat pump (or series of large heat pumps) is found in a "central" location outside the house with the refrigerant running inside the house either to a central air handler in the basement or attic (in the ducted use-case) or to mini-split units.

Block & District Heating/Cooling

Just like whole home systems provides energy efficiency benefits and economies of scale over room-by-room systems, one of the most interesting areas of innovation in home heating and cooling is using infrastructure at a block or "district" level. In these models, individual buildings (homes/offices) can tap into common air and increasingly ground-source heat pump systems.

While this sounds new and novel, district level energy systems have been a big part of how school and office campuses have operated for over a century (think about the big boiler systems at the local university carrying steam heat to radiators throughout campus). What is truly novel in the 2020s is how these principles of economies of scale are being reimagined for residential communities. We expect to stay on top of this trend with Heat Pump Review .

Jobs to be Done: Single Direction or Bi-Directional

While we've already hinted at it, throughout the above, it's worth also noting that you can organize heat pump systems into whether they are designed to move heat in a single direction or in two directions. Most applications only need to move heat in a single direction: inside the refrigerator to the kitchen living space, inside the utility room to inside the water tank, or - in the case of a conventional air conditioner - inside the house to the outside. Single direction heat pumps need to do one thing, and do it super efficiently.

But with the most important next phase of heat pump deployment being the use of heat pump technology to both heat and cool our living spaces, bi-directional heat pumps are going going get a lion's share of innovation going forward. Watch this space.

Refrigerants

One of the most technical but overlooked aspects of heat pump technology is the refrigerant coursing through the pipes and pumps, doing the hard work to move heat. While refrigerants have gotten way less harmful in the past 50 years, most systems still rely on gases that are extremely harmful if they accidentally get released. Much innovation in the heat pump industry is on transitioning to less harmful refrigerants (ironically, CO2 is one of the most benign ones already being tested in production use-cases), as not only would less harmful refrigerants be better for the environment, they would also require less specialty and expense in handling and disposing of them, ultimately saving money and accelerating adoption of heat pump technology.