Residential heat pumps offer a more energy-efficient solution for both space conditioning and water heating compared to conventional electric resistance or natural gas systems. However, homeowners retrofitting existing systems may face the added challenge of electrical service upgrades, which can increase costs and delay installation.
Multi-Function Heat Pumps (MFHPs) address this barrier by combining space conditioning and domestic water heating into a single system using one efficient compressor and outdoor heat exchanger coil. By reducing peak power demand relative to separate heat pump systems, MFHPs are more likely to operate within the limits of existing electrical panels on one circuit, minimizing the need for costly upgrades.
Some MFHP models can also capture waste heat from space cooling to heat water, improving overall energy efficiency. Additional gains in efficiency and thermal comfort may come from MFHPs equipped with defrost modes that draw heat from the water storage tank to defrost the outdoor coil during winter.
Despite these potential advantages, air-to-air MFHPs have not been studied in detail to evaluate opportunities for improvement or their suitability for the California and broader U.S. market.
Current Work
2025 Laboratory Testing
We evaluated the performance of a residential single-speed air-to-air MFHP through controlled laboratory testing. The testing assessed the unit’s capacity and energy consumption across multiple operating modes, including space cooling, space heating, water heating, simultaneous space and water heating, and defrost.
By testing the MFHP under outdoor conditions representative of different climate zones, the project aimed to develop accurate performance curves for each mode of operation during both winter and summer. These curves will support future studies in estimating the potential energy savings and grid impacts of MFHP technology in residential applications.
Results
The project successfully tested the residential single-speed MFHP in WCEC’s environmental chambers. Testing covered a range of outdoor conditions, including those specified by AHRI 210/240-2023 and ASHRAE 206-2024 standards, as well as additional conditions tailored to California climate zones.
Under standard test conditions, the MFHP achieved a space cooling capacity of 45.3 kBTU/h at a coefficient of performance (COP) of 3.57, and a space heating capacity of 49.9 kBTU/h at a COP of 3.48.
Water heating performance followed expected trends, with capacity and COP decreasing as tank setpoint temperatures increased. In dedicated water heating mode, the unit achieved a first-hour rating of 82 gallons and demonstrated higher efficiency than electric resistance heating, except under extreme conditions.
Testing also revealed that the refrigerant-to-water heat exchanger was undersized relative to the compressor. Improving the water heating tank design is expected to enhance performance without increasing cost.
In simultaneous space cooling and water heating mode, the MFHP achieved notable efficiency gains—saving an average of 38% in electrical energy compared to operating the modes separately. Defrost testing showed faster completion times and lower peak power than typical single-speed heat pumps, improving compatibility with existing electrical infrastructure.
Path forward
WCEC’s next steps include evaluating next-generation MFHP models featuring variable-speed compressors and improved heat exchangers for enhanced efficiency and load flexibility. The team will collaborate with San Diego Gas and Electric (SDG&E), the California Technical Forum (CalTF), and software developers to support measure development, ensure accurate performance modeling, and promote MFHP adoption in efficiency programs and building codes.
RESOURCES
PUBLICATIONS
- Residential Multi-Function Heat Pump Laboratory Testing (2024 | Final Report)
- Load Flexibility of a Residential Multi-Function Heat Pump Using Dynamic Pricing (2024 | Technical Report)
This work was supported by CalNEXT (Contract for Services No. 9211).
Villara Corporation provided equipment for testing and technical support.
