The Elementary School Building Committee (ESBC) will not be making the decision between a ground versus air sourced heat pump system for the new elementary school until later this summer. The committee needs to secure the Amherst Board of Health’s permission to drill test wells and the drilling companies that the designers are comfortable with are scheduling out to the end of this summer. In addition, the designer (DiNisco) indicated that Eversource is about to announce a new “generous” incentive program (“GSHP Heat Pump Adder”) for geothermal systems.
The Net Zero Subcommittee was joined by several members of the DiNisco Design team and their engineering consultants from Thornton Tomasetti at their July 6 meeting to compare and contrast ground and air source heat pumps (GSHP and ASHP). Presentation materials are available here.
Both systems are all electric but include a generator on-site for back-up. The ground source system’s higher upfront costs are offset by both lower operating, maintenance, and photovoltaic costs, as well as anticipated incentives available for initial installation. The air source system would require a dedicated preheating unit for domestic hot water, specialty maintenance for the refrigerant system, and would use ducted fan coil units in classrooms rather than the chilled beam units planned for the ground source system. The ground source system is noted to be extremely quiet, although the air sourced system is quieter than the traditional units.
The engineers described the energy modeling they will be using, which accounts for different room types and thermal zones and follows ASHRAE 209-2018 guidelines. The current thermal envelope assumptions (window to wall ratio of 23%, R-40 for the roof, and R-25 for the walls) may be modified in ground versus air sourced modeling to compensate for the lower Energy Use Intensity expected (i.e. more energy efficiency) with the ground source system. Any adjustments here would add to upfront costs to the air source model.
DiNisco team members pointed out that the Life Cycle Cost Analysis (LCCA) done on both systems represent the capital costs only, not maintenance costs. According to Designer Donna DiNisco, the anticipated “GS Heat Pump Adder” incentive, not yet included in the cost analysis, could cover nearly the entire difference in capital costs between the two systems. DiNisco said that they would post information on this program as soon as it becomes available; inquiries by the Indy at time of the writing of this article revealed no updates as yet.
There are other differences between the two systems. The GSHP is expected to achieve the ESBC’s goal of an EUI of 25, which is then eligible for additional utility incentives. Refrigerants, of interest for environmental impacts because of the potential leakage of greenhouse gasses, are only used by the GSHP system in the mechanical room, as opposed to throughout the building in the ASHP system.
A vote to recommend GSHP versus ASHP is on the agenda for