If you’re even thinking about energy efficiency, or if you’ve been considering HVAC (heating, ventilation and air conditioning system) improvements or building renovations of any kind, you’ve likely heard many mentions in recent communications and news about heat pumps. What are heat pumps, and why are they suddenly everywhere? The short answer is that heat pumps can do the same work as fossil fuel powered equipment when it comes to heating, cooling and related equipment and processes with a lot less energy, while improving air quality and comfort and reducing environmental impact. 

Modern heat pumps are more advanced, not surprisingly, but much of the essential technology has been around for quite some time.  First invented in the 1850s, heat pumps have been available for residential HVAC use since the 1960s.  Heat pumps work by leveraging the properties of refrigerants, a class of chemicals which change from lower-pressure liquids to higher-pressure gas and back at different temperatures.  By compressing and expanding refrigerants within closed loop systems, heat can be removed (or essentially added by removing in reverse) as the refrigerant expands and contracts with energy efficiency that almost seems unreal.  

There are three types of heat pumps, all named for or really related to the energy source heat will be extracted from: air sourced, ground sourced, also known as geothermal, and water sourced.  Air source heat pumps extract heat from outside air, even in cold weather, and can also use the air as a heat sink as needed in warmer weather.  They work well in a variety of climates and generally save space over traditional HVAC systems.  Ground source heat pumps leverage the difference in temperature between the relatively stable ground and outside air, drawing from the ground in warmer weather, and sinking into the ground in warmer weather.  They work best in more moderate climates where the ground temperature remains more reliable.  The stability of ground temperatures makes ground source systems even more energy efficient, but they are less common as the up front installation costs and amount of work are significantly higher, requiring wells or long loops in the ground for the heat exchange.  The space required for these systems can also pose a challenge.  Water source heat pumps are less common in residential applications due to the requirement of a water body like a pond or lake, more expensive installation costs and considerations, and the requirement of land for the systems, similar to ground source heat pumps. 

Here are some essential concepts for understanding heat pumps:

• Refrigerant is the key driver in how heat pumps work 
• Heat pumps move energy, they don’t generate it
• Heat pumps generally transfer energy in two directions
• Heat pumps really work 
• Heat pumps are in more equipment than you might think 
• The time for heat pumps is NOW 
   

Image: iea


Refrigerant is the key 

Heat pumps work by expanding and contracting refrigerant, a class of substances that change from liquid to gas and back by expanding and contracting at specific temperatures that vary by refrigerant.  Refrigerant is a highly efficient substance for transferring energy.  When air or water flows over copper wiring called an evaporator coil, the refrigerant inside the wires absorbs the heat in the air as it expands and turns into a gas.  This process explains how refrigeration is used to pull warm air out of refrigerators to cool the food inside. The air isn’t being cooled, as much as the heat is being removed from inside the insulated box.  When the refrigerant then flows through copper wires to the compressor, the refrigerant is literally compressed back to a liquid, releasing the heat outside the insulated box.  The transfer of heat at both ends is enabled by a conductive metal coil, tube or plate known as a heat exchanger, and the heat is then passed on to what is known as a heat sink, the space or material that is to be warmed or where waste heat can be exhausted if it is not to be used.  

Heat pumps transfer energy, they don’t generate it
(So they are very energy efficient, and reduce emissions and energy bills.)

Heat pumps work by moving heat through heat exchange using refrigerant instead of converting electricity or another fuel source into heat.  Heat pumps are becoming increasingly popular as this technology is highly energy efficient, literally up to 300%-400%, meaning that heat pumps yield 3-4 times as much energy as they draw in electricity.  Today’s more efficient gas boilers are 95% energy efficient, impressive compared to past technologies but nowhere near as efficient as heat pumps.  According to the Sierra Club, the average US home could reduce emissions more than 45% and energy use more than 50% over the next decade by installing heat pumps and a heat pump water heater in place of or instead  of a gas furnace and water heater.  

Heat pump equipment can cost more up front for retrofit projects, but these costs are also offset by energy savings over time.  The Sierra Club estimates that switching to heat pumps for heating, cooling, and water heating would result in an immediate savings of $37 billion annually in the US alone.   Payback times vary by cost differences and equipment efficiency, but building new homes with electric heating, cooling and water heating is consistently cost effective over the life of the equipment. There are several scenarios where retrofit payback is also shorter.  According to the Sierra Club and Rocky Mountain Institute (RMI), these are: 1) when switching from propane or oil heating, 2) replacing both a gas furnace and air conditioning units, and 3) when also incorporating rooftop solar.  Tax credits and other financial incentives from the Inflation Reduction Act and other federal and state programs can greatly shorten the payback period.  

Heat pumps generally transfer energy in two directions - and they unfortunately have a confusing name

Do heat pumps always work in two directions? That depends on who you ask, but the general answer is yes, because they can work in two directions and often do.  And many say they always do, as a requirement to be called a heat pump.  The name causes a challenge, as some people hear the word “heat” and don’t realize at first that heat pumps working in two directions are cool as well. And some people will call a device that only cools a heat pump as well.  

Further confusion results from the fact that some equipment is literally called a heat pump, while other equipment is said to contain heat pump technology.  People often call equipment that contains heat pump technology a heat pump, while there also is a type of HVAC equipment which is actually simply known as a heat pump. (In other words, not all technology that contains heat pumps is called a heat pump!)

Even though they are working similarly, refrigerators and air conditioners remove heat in one direction, providing cooling as a result, where there’s general agreement that a heat pump means heat can be removed in two directions  Most heat pumps or equipment containing heat pump technology have a reversing valve that literally changes the direction of the refrigeration and therefore literally switches the functions of the evaporator and condenser.  Some argue that a heat pump must only work in one direction, so be careful to ensure the equipment you research and purchase serves all the functions you intend (one author of this piece bought a “heat pump” that turned out to only cool and not heat, while some manufacturers suggest that heating is essential for a heat pump but that cooling is the added functionality.  This feels counterintuitive in that a heat pump always works by removing heat through the refrigeration phase change, which is technically a cooling function. 

Key is that we’re always talking about refrigerants expanding and contracting to move heat.  The question is where the heat is moving from, and where the system can “sink” the heat absorbed.  

 

Heat pumps work! 

Heat pumps work efficiently because it’s far easier to move energy than to generate it.  Heat pumps can generate 3 to 4 times the energy required to power the heat pump, and even in colder weather a heat pump can still be 100% efficient.    

Given the popularity and ever growing demand for air conditioning equipment and keeping in mind that heat pump technology in certain forms is actually quite common, it’s understandable to wonder why (two-directional) heat pumps weren’t already more popular for HVAC previously.  Keep in mind that heat pumps suffer from some common misunderstandings or misperceptions, and they’re still unfortunately not widely understood.  As mentioned, even the name suggesting that heat pumps only heat is one cause for confusion.  Perhaps the name problem started with air conditioning and refrigerators standing only for cooling?  

A significant challenge is that decades ago heat pump technology was not as advanced as it is today. In particular, heat pumps did not work well in colder temperatures, and some building professionals recall earlier efforts in colder temperatures that were just not successful.  One reason was also that electricity prices were higher at times compared to fossil fuels, and this impacted the perceived advantage of heat pumps.  This is counterintuitive in some ways though, as fossil fuel price volatility works in two directions.   

Some people also find it hard to believe that even cold outside air can generate heat, but it is the case, as all air above -273 degrees Celsius (known as absolute zero) holds some energy that is available to transfer.  Most air source heat pumps are highly efficient above 30 or 40 degrees fahrenheit, and operate at 100% efficiency even at 20 degrees, and there are cold weather heat pumps that heat effectively at temperatures up to -5 degrees, but can work well as low as -13 and -22 degrees fahrenheit.  This said, heat pumps are not as efficient at colder temperatures.  This is one reason some people keep or still install fossil-fuel systems as back up energy sources in colder climates, or why some households are adding heat pumps for cooling instead of just changing to heat pumps for both heating and cooling.  Selecting a heat pump with electric heat strips to add heating capacity is another option, but this adds to electricity consumption.  In a colder climate, or in a colder season, it is currently more expensive and may require larger and therefore more expensive equipment to heat solely with electricity.  Most homeowners will see savings from cooling with heat pumps, as they are so much more efficient than traditional cooling equipment. 

 

Heat pumps are in more equipment than you might think 

Heat pumps are becoming most widely known for heating and cooling building spaces more efficiently, but heat pumps are also used in other equipment.  Within HVAC systems, there are also a number of types of equipment that use heat pumps, which can be confusing.  Heat pumps can exist as stand alone systems or split systems with outdoor compressors and indoor evaporators. They can also be contained in self-contained package units, which are smaller in residential applications and larger in commercial buildings.  Mini split units, which are often ductless, are perhaps the more known heat pump applications.  Ductless mini split systems consist of smaller outdoor units connected to indoor units. They can provide single-zone heating and cooling with a one-to-one connection, or several single indoor units can be connected to an outdoor unit.  The indoor units have commonly been installed on walls (wall-mounted units), but there are now more options available including in-ceiling cassettes which are much less visible, and concealed units where only the supply and return air vent grills are visible. There are also units which can be installed on floors, or suspended from ceilings (larger and typically commercial). Mini split systems can also be ducted, adding to some confusion.  Ducted mini split units can also be visible or concealed.  Heat pump systems can also be installed as part of hybrid or dual fuel systems where heat pumps and a backup heat source such as natural gas can work together, shifting the heat source according to outdoor temperature and demand.  Many heat pump systems are now VRF, or use “variable refrigerant flows.”  VRF systems allow greater zone control and also allow more flexibility than systems which only allow the motor and therefore system to be simply on or off.  This flexibility is important because it facilitates longer run times and therefore lower energy levels on average, allowing less consumption overall with better air quality results.  It may sound obvious, but HVAC systems only work and ventilate and filter when they are on and running.  Longer run times also mean more even air mixing and more even temperatures, as extremes due to system starting and stopping are reduced. Longer run times are similarly helpful for humidity management if humidity equipment is present. 

Heat pumps are also increasingly incorporated into water heaters, which are also much more efficient than fossil-fuel powered water heaters.  In this equipment, the heat sink is the water in the water heater that then circulates through the home.  There’s now even a plug-in-water heater, which makes water heating retrofitting easier.  Radiant floor heating systems can incorporate heat pumps to heat the water that circulates through the heating pipes.  Heat pumps are also used in dryers, which operate at about 28% less energy than traditional dryers. Heat pump dryers and the lower-voltage water heaters make installations possible in some situations where higher voltage outlets are not available.  Heat pump dryers are also ventless, which is essential for buildings where vented dryers are not allowed.  These dryers don’t draw and exhaust conditioned air from homes so they don’t cause negative pressure issues, and they also don’t exhaust the fine particles and other pollutants and health hazards produced by combustion processes.  The lack of venting also means that this equipment can fit in smaller spaces.  And finally, heat pumps can be used in swimming pool heaters.   

Heat pumps are also used in commercial and industrial HVAC systems and processes, where heating, cooling and drying are involved.  Waste heat and water can provide heat for larger-scale commercial and industrial heat pumps which may require higher input temperatures than residential buildings require.  

 

The time for heat pumps is now 

Heat pumps are an energy choice that can become even cleaner over time.  As the electric grid becomes cleaner, existing systems that are already electric continue to amplify these improvements, where fossil fuel systems will miss out, continuing to generate pollutants through emissions and worse, through lower efficiency.  In addition to wider environmental concerns, growing awareness and interest in better indoor air quality, or IAQ will also lead to increased electrification, or a move to electric equipment over fossil-fuel combustion inside buildings. 

Another reason to make a switch to heat pumps sooner is that opportunities to replace equipment are spaced apart, by years and even decades.  This means there’s a longer wait each time a decision is made that does not result in a more efficient equipment purchase.  It’s advisable to have an equipment plan in advance, before equipment fails, so that buyers aren’t pretty much forced to buy what happens to be available, or to replace equipment with what is already in place, just as there’s an emergency need. 

Demand is Only Going to Grow for Heat Pumps


The demand for heat pumps will continue to grow, as the comfort, health, safety, and energy efficiency benefits become more clear, and with available tax credits and other financial incentives.  This is a great time to start planning for heat pump equipment in place of fossil fuel burning systems so that you will be ready when it’s time to replace aging equipment, ideally before a crisis.  For new energy efficient construction, heat pumps are a clear choice for savings and lower emissions.  This is a great time to start a conversation with a trusted HVAC professional who can help you design the optimal system to meet your needs.  It’s worth noting that this is also a great time to consider an HVAC career - as a considerable-size workforce will be needed for retrofitting existing buildings, in addition to new construction.  

Have you installed heat pump equipment?  Do you have performance data you can share with others from your projects or from your personal experience? What other top information do you think is essential for understanding heat pumps? Let us know, and add your voice to the conversation!  

To Learn more:

Heat Pump Systems - US Department of Energy
Heat Pump 101: The Lowdown on the Hottest (And Coolest) Appliance You’ve Never Heard Of - Sierra Club
Clean Energy 101: Heat Pumps - RMI (Rocky Mountain Institute)
6 Advantages and Disadvantages of Heat Pumps - Triple T Heating and Cooling
Heat Pump Primer: Understanding the Technology That Will Drive Decades of Residential Decarbonization -  Pecan Street: 

To learn about federal and state financial incentives and tax credits available in your area (US only) see the DSIRE database (Database of State Incentives for Renewables & Efficiency): https://www.dsireusa.org/ 

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