Denning HPWHThe Daily Journal of Commerce just reported that the Denning, a 46 unit apartment building, is fully leased. The project was designed to achieve a very high level of energy and resource efficiency for a relatively modest budget. The building includes an innovative design for the central hot water system. The equipment is installed in a rooftop direct-gain solar space with South-facing glazing and skylights. Heat pump water heaters in the sunspace harvest the solar energy and use it to preheat the domestic hot water. Final heat demand for the water is provided by high efficiency gas water heaters. The system is tuned to use the most efficient system at all times. When the mechanical room is warm the heat pumps provide the primary heat for the hot water. If the mechanical room drops below 40F then the heat pumps are no longer efficient and the high efficiency gas system takes over.