ET23SWE0050 - Comfort Impacts of Partial Coverage ASHPs
A limiting factor to rapid adoption of air-source heat pumps (ASHPs) for residential retrofit applications is the first costs associated with installing a heat pump system that provides the home’s full heating and cooling loads with direct distribution into all conditioned rooms of the home. This is true with both ducted and ductless technologies and impacts low-to-moderate income customers disproportionally. Anecdotal evidence (from previous utility programs) suggests that systems are often oversized to 140% of peak design load.
Homeowners who installed heat pump systems to meet 70-90% of the heating loads engaged through TRC’s various heat pump program implementation efforts, have indicated that they do not use their backup/supplemental heating systems and that their homes maintained personal comfort ranges even with outdoor air temperatures below design conditions.
Using market assessment and thermal comfort modeling this project will quantify the comfort impacts of various part-load scenarios, and quantify first-cost and operating cost savings of part-load scenarios in CA.
Heat pumps intended to satisfy the full design load are sized to provide heating and cooling at the 1% and 99% temperatures seen annually. However, smaller capacity heat pumps may operate in their efficient modulating zone for more time annually. The net result is more annual hours of efficient operation and less annual hours of inefficient low-load cycling.
Air source heat pumps (ASHP) are essential for meeting California’s climate goals, but high initial costs limit adoption in existing homes. This study evaluates two alternative ASHP configurations—suitable capacity and indirect coverage—to determine their potential for reducing upfront costs while maintaining occupant thermal comfort.
Suitable capacity ASHP systems are designed to meet 70-90% of a home’s heating or cooling design load, enabling smaller equipment and lower first costs. Indirect coverage ASHP systems heat and cool homes without directly distributing conditioned air to every interior space, reducing installation costs. Ducted indirect coverage systems eliminate the need for vents and ductwork in every space, while ductless systems avoid installing indoor units in all conditioned spaces. Both configurations were compared to baseline systems, which meet 100% of heating or cooling loads and distribute conditioned air directly to all spaces.
The project team developed prototypes for a single-family home and a multifamily unit, and used Manual J and Manual S calculations determined design loads and heat pump sizes. Energy simulations predicted space temperatures and humidities, which were used to model occupant comfort.
Results showed that both suitable capacity and indirect coverage ASHP systems will reduce first costs compared to a baseline ASHP system. The suitable capacity system is estimated to save less first costs than the indirect coverage system, while still maintaining occupant comfort. The indirect coverage ASHP system is estimated to reduce first costs more than the suitable capacity system, but occupant comfort will need to be maintained with clothing adjustments or a transfer fan.