Energy & Cost Impacts: Heat Pump Water Heaters

Thermal Storage and Tank Sizing

Most commercial and multifamily HPWH installations require a properly sized hot water storage tank. The tank provides thermal storage that allows the heat pump to run steadily and efficiently while still meeting short-term peaks in demand. A sufficient first-hour rating is critical to ensure reliable hot water availability during high-use periods. Tank sizing has a direct effect on overall performance, efficiency, and the system’s ability to support load shifting or demand response.

Too small a tank leads to frequent cycling and reduced efficiency, while an oversized tank adds cost and can increase standby losses. Insulation quality and standby heat loss should also be considered when selecting a tank. Because sizing depends on occupancy, draw profiles, and the characteristics of the selected HPWH units, calculations should be performed by a trained engineer with expertise in plumbing and hydronic system design where possible.