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 in some scenarios, room and home comfort may be sufficiently sustained year-round even in the case of a partial-coverage system where the heat pumps either a) lack direct distribution into certain rooms or b) are sized to only provide between 70-90 percent of the full 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. This study would attempt to account for the common market error of Manual J load calculations overstating the actual load of the home.  Using a combination of energy modeling and field testing, this project will identify specific conditions where this is true, quantify the comfort impacts of various partial-coverage scenarios, and quantify first-cost and operating cost savings of partial-coverage applications that still fulfill sufficient year-round heating and cooling. The project will also measure operating costs of partial-coverage heat pumps to assess if they operate more efficiently due to less short cycling and more operating hours within the heat pump’s modulating zone. Heat pumps sized for 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. We are expecting to see benefits and lower annual energy usage as a result of lowering the minimum capacity and maximizing the time spent in the heat pumps modulating zone. Specific applications include the following: 1. Indirect Coverage: Base case is an ASHP system providing direct airflow with heads or registers in every room. Test case is individual rooms with no ducted registers, or ductless heads, but with certain thermal-transfer pathways to the rest of the home such as door undercuts; transfer grills; transfer fans; low amounts of external wall/ceiling/floor exposure relative to internal partitions; and highly insulated external wall/ceiling/floors 2. Partial Load: Base case is 100% of the design load. Test case is ASHP systems whose heating or cooling capacity at the home’s design temperature are only 70-90 percent of the full design load but for which the short duration of insufficient capacity combined with solar gains and internal gains (for heating) or fans (for cooling) may not lead to substantial comfort impacts. The study will use energy modeling to predict the temperature drift in rooms or homes with Indirect coverage and/or partial load under the most extreme outdoor weather conditions in various climate zones. The project will use field studies to verify thermal comfort impacts with indirect coverage and partial load ASHP systems in real-world scenarios.