Comfort Impacts of Partial Coverage ASHPs

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.