Heat Pumps

Written by Tobias Chan, 2030 District Performance and Outreach Coordinator

Unlike air conditioning, space heating is traditionally supplied by the burning of fossil fuels, most commonly natural gas.

The solution to this problem lies in heat pumps. Unlike furnaces and boilers, heat pumps do not require a fuel source, rather, much like air conditioners and refrigerators, they are designed to move heat from one space to another, generally from outside air to the inside of a building.

Heat pumps are not a new technology; they have been around for over 150 years². Heat pumps in the US gained popularity during the oil crisis of the 1970s², and while the concept remains the same, the technology has improved markedly since then, making heat pumps a viable electric space heating option³.  

Heat pumps consist of four main parts: two heat exchangers, a compressor, and an expansion valve. They operate as follows: 

1. A refrigerant absorbs heat from outside air blown over a heat exchanger. 

2. The refrigerant, which evaporates from the heat gain, is compressed to further increase the temperature.

3. Heat is transferred from the refrigerant to the inside air via another heat exchanger. 

4. The refrigerant cools and condenses back into a liquid.

5. The refrigerant passes through an expansion valve, decreasing the temperature, and exits the building to repeat the cycle.

Source: Modernize Home Services 

There are two main types of heat pumps: air source and ground source.
  

Air Source Heat Pumps 

Air source heat pumps (ASHP) utilize outdoor air as the heat source or heat sink. They generally have a COP (coefficient of performance) of 2-4, meaning that for every unit of electricity supplied to the unit, 2-4 units of heat are emitted⁴. The downside of ASHPs is that their efficiency is tied to the outdoor temperature. As the temperature drops, so does the unit’s efficiency. Most ASHPs experience this around 32°F⁴, but there are cold weather models that can operate efficiently in temperatures down to -10°F⁵. ASHPs have around a 20-year life expectancy, depending on the model⁵.  

There are two subtypes of ASHPs:  

1. Air-to-air heat pumps supply heat directly to indoor air via forced-air ducted system, or from individual ductless units (mini-splits).  

2. Air-to-water heat pumps supply heat to an indoor water system and deliver it via radiators or underfloor heating. If radiators are used, they generally must be larger, since heat pumps supply lower water temperatures than furnaces and boilers. For this reason, underfloor heating works well with heat pumps. Air-to-water heat pumps can also provide domestic hot water. 
   
   

Ground Source (Geothermal) Heat Pumps 

Ground source (geothermal) heat pumps (GSHP/GHP) utilize the ground as the heat source or heat sink. Ground temperatures fluctuate less than air temperatures, so GSHPs are more efficient in colder weather than ASHPs. However, there are concerns about long-term thermal degradation of geothermal systems in very cold climates. In places that have long winters and short summers, the ground cannot recharge the temperature during the summer⁶. This causes ground temperature to decline over the lifespan of the system, reducing its efficiency. Ground temperature can be remedied by pumping heated water from solar thermal arrays during the summer⁶.  

GSHPs work very similarly to ASHPs, with the addition of an underground loop containing a water/antifreeze mixture to provide heat to the system. GSHPs traditionally use underfloor heating but can also provide heat to forced air systems.

GSHPs generally have a COP of 3-5 and can maintain these high efficiencies in colder temperatures⁷. The heat pump has a 20-year lifespan, much like an ASHP, and the underground piping can last 50-80 years before replacement⁷. The underground piping increases the cost of the system, making GSHPs more expensive than ASHPs.

Source: Greenmatch

There are four subtypes of GSHPs: 

Horizontal Closed Loop

Pipes are buried in ~4ft deep trenches in looped or straight alignment. They require a large amount of land. 

Vertical Closed Loop

Small boreholes are drilled up to 150m and contain two pipes. Installation requires a special drilling machine. This system is more space efficient and doesn’t need as much piping since temperatures deeper underground are warmer and more stable.   

Pond Closed Loop (Water Source Heat Pump) 

A looped collector is placed underneath a water surface. The water body must be deep enough to avoid freezing during the winter.

Open Loop 

This system circulates well or surface water instead of water/antifreeze mixture. These systems are less common than closed loop systems and must comply with more environmental regulations since system water discharges into local water sources. 

Source: Carolina Country

Heat Pump Pros and Cons

Pros

• Can operate in reverse and provide cooling during warmer months 

• Lower operating costs than traditional HVAC systems⁸

• Do not require gas lines or fuel storage, making them safer than furnaces and boilers 

• Very efficient, with average COP around 3 

• Will continue to be the cleanest way of providing heating & cooling, especially as grid generation becomes cleaner and more efficient⁹ 

Cons

• Higher upfront installations costs, especially for GSHPs, than traditional HVAC systems⁸

• Operating efficiency is tied to outdoor temperature, may require backup heat source for extremely cold days 

• Carbon emissions and sustainability are tied to electricity generation source.

  • In Pennsylvania, over 60% of the state’s electricity is supplied from fossil fuels, compared to 4% from renewables¹⁰ 

Sources

1. U.S. Energy Information Administration. “2012 CBECS Survey Data: Table E1. Major fuel consumption (Btu) by end use, 2012” (PDF). Released May 17, 2016. https://www.eia.gov/consumption/commercial/data/2012/c&e/pdf/e1.pdf  

2. Zogg, Martin. “History of heat pumps: Swiss contributions and international milestones” (PDF). May 2008. https://www.ehpa.org/fileadmin/red/03._Media/03.02_Studies_and_reports/‌History_of_Heat_Pumps.pdf  

3. AC Heating and Air Conditioning Services. “How new heat pumps are more efficient than ever.” September 27, 2017. https://www.achvac.com/article/new-heat-pumps-efficient-ever  

4. EDF Energy. “A complete guide to air source heat pumps.” Accessed December 14, 2021. https://www.edfenergy.com/heating/advice/air-source-heat-pump-guide 

5. Renewable Energy Hub. “Heat pump efficiency vs. temperature.” Accessed December 14, 2021. https://www.renewableenergyhub.co.uk/main/heat-pumps-information/heat-pump-efficiency-vs-temperature/  

6. Gartman, Michael, and Amara Shah. “Heat pumps: A practical solution for cold climates.” December 10, 2020. https://rmi.org/heat-pumps-a-practical-solution-for-cold-climates/  

7. Gao, Yan, Zhi Sun, Xinxing Lin, Chuang Wang, Zongyu Sun, and Yanhong Chen. “Designing and optimizing heat storage of a solar-assisted ground source heat pump system in China.” International Journal of Photoenergy 2020, no. 1: 1-18. https://doi.org/10.1155/2020/4102350 

8. Vourvoulias, Aris. “Ground source heat pumps: A complete guide.” Last modified September 8, 2021. https://www.greenmatch.co.uk/heat-pump/ground-source-heat-pumps-in-the-uk  

9. Vekony, Attila Tamas. “Heat pumps: 7 advantages and disadvantages.” Last modified November 15, 2021. https://www.greenmatch.co.uk/blog/2014/08/heat-pumps-7-advantages-and-disadvantages  

10. Cho, Renee. “Heating buildings leaves a huge carbon footprint, but there’s a fix for it.” January 15, 2019. https://news.climate.columbia.edu/2019/01/15/heat-pumps-home-heating/  

11. U.S. Energy Information Administration. “Pennsylvania profile analysis.” Last modified October 21, 2021. https://www.eia.gov/state/analysis.php?sid=PA#102  

Top banner photo: Bas van Breukelen (Unsplash)