Heat Pump
A heat pump uses refrigerant to move heat from a low-temperature source to a higher-temperature destination. This allows air-source heat pumps, one of the most common types, to do two things:
- Move heat in cold weather from outdoors into a building to provide heat for the building
- Move heat in hot weather from the building to the outdoors to provide cooling for the building
The ability to provide both heating and cooling functions only using electricity is one of the most important aspects of a heat pump. This makes heat pumps the best choice for many buildings with electrification or decarbonization as a design or compliance goal. There are many types of heat pumps and for many building performance software applications, every kind of heat pump may have different inputs and resulting reports.
How It Works
Heat pumps use the same vapor compression refrigeration cycle as most air conditioning systems. The big difference is being able to reverse the refrigeration cycle so that the heat can flow either into or out of the building. The vapor compression cycle shown below consists of a refrigerant that circulates between two heat exchangers using a compressor and an expansion valve.
Air source heat pumps extract or dump heat outdoors often need to have two additional functions that consume energy:
- Since the capacity of the heat pump cycle decreases with lower outdoor temperatures, it is common for some type of supplemental heat to be used.
- The surface of the outdoor heat exchanger coil is colder than the air temperature when extracting heat from outdoors when heating the building. Due to this, moisture in the air condenses on the outdoor coil and freezes, which reduces its effectiveness. To remove this ice build-up, some method of defrosting the coil is used.
The control for these functions is described more below under Common Control Measures.
There are other configurations of heat pumps, including:
- water-source heat pumps
- ground-source heat pumps, also commonly called geothermal heat pumps
These are covered on this page, plus other configurations that are not covered on this page:
- heat pump water heaters
- chillers that can act as heat pumps
Information Needed for the Model
Since heat pumps describe a range of physical equipment, the most important information is understanding the exact type of equipment being used in the building and the capacity and efficiency of the equipment over a range of cooling and heating operating conditions. In addition, details such as controls are often needed. Variable refrigerant flow units (VRF) often have heat pump modes, and they are covered separately.
Air Source Heat Pumps
Air source heat pumps are probably the most common type of heat pump and are widely supported in building energy modeling software. In the U.S., for the smallest unit, under 65,000 Btu/h the minimum efficiency requirements are set by US DOE and use SEER (Seasonal Energy Efficiency Ratio) or SEER2 metrics for cooling efficiency and HSPF (Heating Seasonal Performance Factor) or HSPF2 metrics for heating efficiency which are determined by AHRI 210/240. Larger sizes of air source heat pumps are tested according to AHRI 340/360 and use metrics like IEER for cooling and COPH for heating. The heating COP values often come in at two different outdoor temperatures, 17F, and 47F. With all of these rating standards, one would think that it is easy to find the correct parameters to put into the BEM software, but often that is not the case. It is usually necessary to download detailed performance data from the specific manufacturer in order to get the inputs needed for the software. Also needed for air source heat pumps are the capacity, operating temperatures, and control sequencing for supplemental heat and defrost.
Ground Source Heat Pumps
Ground-source heat pumps, also known as geothermal heat pumps, reject or absorb the heat from the building from the ground or sometimes through groundwater. There are a large number of different configurations, but again capacity and efficiency are the most important parameters. They are often rated in terms of EER or COPH based on ISO 13256-1 or AHRI 1230. The ground heat exchanger, whether it is horizontal or vertical, open or closed loop, is often the most expensive component of the system. So sizing of the heat exchanger becomes often a driving factor for the design, with supplemental heating provided by electric resistance for residential and small commercial buildings or boilers for larger buildings and cooling towers used for supplemental cooling in larger buildings. Often software tools related to just the sizing of ground heat exchangers are used by the design team and when these are used, the inputs needed for simulating the ground source heat pump can often be taken from the inputs and outputs from these sizing tools. The energy modeler should work closely with the mechanical engineer and may provide added value by helping size the components based on simulation results. Of course, site soil conditions play a big part in how well the ground heat exchanger works, so make sure you understand the heat capacity and heat transmission characteristics of the soil. Local soil conditions can be analyzed by a local drilling contractor by creating test bores. Depending on the BEM tool, multiple-year simulations may be necessary before the temperature swings in the ground stabilize.
Water Source Heat Pumps
Water-source heat pumps are often used in a water-loop configuration where heat is rejected or absorbed from a hydronic loop that is also served by a cooling tower or other heat rejection device, sometimes a chiller, and a boiler or some other heating device. Often there are individual heat pumps serving each zone and sometimes each room of the building. The capacity and efficiency of water loop heat pumps is the main parameter, and the test standards include AHRI 1230, ISO 13256-1, or ISO 13256-2 for EER or COPH. While as a category, this type of equipment is called “water-source,” sometimes the fluid is actually brine. Brine, usually antifreeze mixed with water, is used in applications where the fluid may get close to the freezing point of water to protect the system although often the use of brine decreases the overall performance some compared to the use of just water. The rated entering water temperature for the rating conditions vary by application from 32F to 86F depending on the heating or cooling mode and the equipment type. One of the strengths of water source heat pumps is when used in a building where some zones are being heated and others are cooled; this will result in heat being added and removed from the water loop without activating the cooling towers or boilers, saving a lot of energy.
Understanding “Capacity”
Another item to be aware of concerning using manufacturers’ data, is that capacities are often expressed as “net capacities” that already account for the heat from the fan, either reducing the cooling capacity or increasing the heating capacity. Since most simulation programs have a separate model for the fan, these net capacities can be misleading and gross capacities may be more appropriate. Unfortunately, each software and each manufacturer expresses these values a little differently, so care must be taken. The number of compressor and fan speeds may impact which model to select in the BEM software, which is usually available from the manufacturer’s data. Also, the test conditions for heat pumps from the test standards may not match the design conditions for the building, so adjustments may need to be made to the capacity and efficiency if that is required by your BEM software.
Performance Curves
To improve the accuracy of the model, performance curves or performance data that is input should reflect the actual data provided by the manufacturer. This can be a laborious process, but hopefully, efforts by ASHRAE Standard 205 will help once manufacturers start providing data in this format and the format is adopted by common modeling software.
Common Measures
Some common energy-efficiency measures to consider are shown below:
Air-source heat pumps
- Higher nominal efficiency
- Better low-temperature performance - this will require altering the heating capacity and efficiency performance curves as well as altering the backup heat operation
- Substitution of ground-source heat pumps
- Multiple-speed or variable-speed fans
- Multiple-stage or variable-speed compressor
- Better defrost operation
- Evaporatively cooled outdoor coil
- Increased heating capacity - often air source heat pumps are sized for cooling
- Combine with the use of a dedicated outdoor air system (DOAS)
- Heat exchanger assisted dehumidification
- Desuperheaters to capture waste heat for water heating
Ground source heat pumps
- Higher nominal efficiency
- Multiple-speed or variable-speed fans
- Multiple-stage or variable-speed compressor
- Larger ground heat exchanger
- Different ground heat exchanger configurations
- Use of groundwater source (pond or lake)
- Adding cooling tower to water loop for cooling
- Adding solar thermal to water loop for heating
Water source heat pumps
- Higher nominal efficiency
- Multiple-speed or variable-speed fans
- Multiple-stage or variable-speed compressor
- Variable-speed pumping
- More advanced controls on water loop temperature setpoints to optimize performance for each operating mode (cooling, heating, and mixed)
- Higher-efficiency central pumps
- Thermal water storage
- Adding or optimizing the cooling tower
- Addition of ground heat exchanger
- Water loop bypass for inactive units
Common Control Options
For air-source heat pumps, the most common control options include the following:
- Control of defrost cycle for outdoor coil, including whether the length of the cycle is fixed or automatic, the temperature above and below when it occurs, the humidity range that it occurs, the performance impact of the use of defrost, and if reverse cycle or separate heater.
- Control of the backup heat, including the type (usually electric resistance), the maximum temperature it can occur, and the lowest temperature that the heat pump cycle can be used.
- Control of the crankcase heater by specifying the maximum temperature for operation.
For ground source heat pumps, the most common control options include:
- Control of backup heating based on loop temperature
- Control of cooling tower (if present) based on loop temperature
For water source heat pumps, the most common control options include:
- Loop temperature setpoints (which affect heating and cooling)
- Condensate removal time
- Bypass inactive units
Common Applications
Air source heat pumps come in packaged rooftop, packaged terminal, and split configurations. In a packaged rooftop configuration, both coils and fans are located in a single rooftop unit that feeds conditioned air into the zone below. This is the most common configuration for both small and large buildings with just a few floors, including:
- Retail
- Office buildings
- Hotels
- Restaurants
A packaged terminal unit is often a “through the wall unit” typically seen below windows are common in:
- Motels and hotels
- Schools
- Cooled portions of warehouses
For split applications, where the outdoor unit is separated from the indoor unit, and refrigerant lines run between them, the most common applications are:
- Residential
- Multifamily
- Office buildings
- Nursing homes
Water source heat pumps are often used in larger building applications, including:
- Office buildings
- Shopping centers
- Nursing homes
- Hotels
- Multifamily
- Any building with significant hours of both heating and cooling at the same time
Ground source heat pumps can be applied to any application that split air source heat pumps may be used, and when using, a larger central hydronic loop can be applied to any application water source heat pumps are used.
Model Output Checks
The following output checks should be considered when modeling heat pumps:
- Find a heating time and cooling time that the operating conditions, primarily the outside temperature, are similar to rated conditions and make sure the output for the overall efficiency and capacity is as expected. For air source heat pumps, in heating, you may want to check at both 17F and 47F operating temperatures.
- At different outdoor temperatures and loads, check if the capacity and efficiency are as expected.
- Check that the backup heat only activates below the temperature that is specified.
- Check that just above the activation temperature for the backup that the heating load is satisfied.
- For water source heat pumps, identify hours with no cooling, no heating, and mixed cooling and heating, and check if the boiler and cooling tower are operating properly in those modes.
- Check that water loop temperature is being maintained between the expected setpoints.
- For air source heat pumps, during the defrost cycle, make sure the performance is as expected.
- For ground source heat pumps, make sure the ground temperatures have stabilized when simulating multiple years. If simulating a single year, make sure the final temperatures of the ground are similar to the initial temperatures.
Related Energy Code Requirements
ASHRAE 90.1-2019 includes a requirement for heat pumps with electric resistance heat, Section 6.4.3.5 that applies to heat pumps other than very small residential sized units:
“Heat pumps equipped with internal electric resistance heaters shall have controls that prevent supplemental heater operation when the heating load can be met by the heat pump alone during both steady-state operation and setback recovery. Supplemental heater operation is permitted during outdoor coil defrost cycles.”
This standard also regulates water loop heat pumps in Section 6.5.2.2.3:
“Hydronic heat pumps connected to a common heat pump water loop with central devices for heat rejection (e.g., cooling tower) and heat addition (e.g., boiler) shall have the following:
a. Controls that are capable of and configured to provide a heat pump water supply temperature dead band of at least 20°F between initiation of heat rejection and heat addition by the central devices (e.g., tower and boiler).
b. For Climate Zones 3 through 8, if a closed-circuit cooling tower (fluid cooler) is used, either an automatic valve shall be installed to bypass all but a minimal flow of water around the tower (for freeze protection) or low-leakage positive closure dampers shall be provided. If an open-circuit cooling tower is used directly in the heat pump loop, an automatic valve shall be installed to bypass all heat pump water flow around the tower. If an open-circuit cooling tower is used in conjunction with a separate heat exchanger to isolate the tower from the heat pump loop then heat loss shall be controlled by shutting down the circulation pump on the cooling tower loop.
Exception to 6.5.2.2.3: Where a system loop temperature optimization controller is used to determine the most efficient operating temperature based on real-time conditions of demand and capacity, dead bands of less than 20°F shall be allowed.”
And Section 6.5.4.5:
“6.5.4.5.1 Each hydronic heat pump and water-cooled unitary air conditioner shall have a two-position automatic valve interlocked to shut off water flow when the compressor is off. Exception to 6.5.4.5.1: Units employing a fluid economizer. 6.5.4.5.2 Hydronic heat pumps and water-cooled unitary air conditioners having a total pump system power exceeding 5 hp shall have controls and/or devices (such as variable-speed control) that will result in pump motor demand of no more than 30% of design wattage at 50% of design water flow.”
Similar or Related Systems
Lots of different configurations using heat pumps have not been covered on this page, including:
- Some chillers can be operated in a heat pump mode to provide heating.
- Variable refrigerant flow units (VRF) often have heat pump modes
- Heat pump water heaters
Additional Resources
- 2020 ASHRAE Handbook - Heating, Ventilating, and Air Conditioning Systems and Equipment. Chapter 48 - “Unitary Air Conditioning and Heat Pumps,” Chapter 9 “Applied Heat Pumps and Heat Recovery Systems,” and Chapter 2 “Decentralized Cooling and Heating part 4 “Water-Source Heat Pumps”
- 2019 ASHRAE Handbook - Heating, Ventilating, and Air Conditioning Applications. Chapter 35 - “Geothermal Energy”
- “Geothermal Heating and Cooling: Design of Ground-Source Heat Pump Systems” Steve Kavanaugh and Kevin Rafferty, ASHRAE, 2014.
- “Improved Modeling of Residential Air Conditioning and Heat Pumps for Energy Calculations” D. Cutler, J. Winkler, N. Kruis, C. Christensen, M. Brandemuehl. NREL/TP-550-56354. January 2013.
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References
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