American Solar, Inc. installed an air to water heat pump to deliver hot water to the domestic water loop in the Freedom Barracks building at Fort Meade, MD. The heat pump received a thermal assist from warm air drawn from the attic to boost heat pump performance and deliver heated water. The heat pump also simultaneously delivered cooler drier air that could reduce the air conditioning load when heating water. The objective of this project was to demonstrate and validate heat pump performance with and without attic heat recovery and to compare performance to the existing heating and air conditioning systems.

Air to water heat pumps have been introduced for residential and commercial use in the last several years. These systems use air as the thermal resource to heat water to temperatures as high as 160 degrees Fahrenheit (160F). The warmer the air entering the heat pump, the more productive and efficient it becomes at heating water with a minimal increase in electricity use. When simultaneous delivery of cool exhaust air with hot water is accomplished, with no added electricity use, compared to just water heating, the performance of the system can produce up to 5+ times the heating and cooling energy of the electricity consumed.

Both the recovery system for solar heat from the attic and the heat pump performed as expected, delivering a combination of higher heating performance than the heat pump alone would deliver. The additional energy benefit of cool dry air from the heat pump exhaust stream added to the overall efficiency of the system. At peak performance, the systems delivered over seven times as much heating and cooling energy as the electrical energy required to run the system. The economics of the system is dependent on: the existing heating and electric energy costs, the installation cost, and the local climate, which drive the energy use of the particular installation. Where the existing facility is heating with high cost heating energy sources and inefficient equipment, the heat pump can reduce heating costs by as much as 85%.

The best opportunities to deploy thermally assisted heat pumps are those with simple configurations of equipment. Systems with minimal ductwork and piping, providing local hot water heating and simultaneous cooling, minimize installation costs and maximize economics. One example would be in a commercial kitchen environment with high hot water load and a need for air conditioning. Heating, or preheating, the coldest incoming water also improves economics compared to using the heat pump to heat high temperature hot water to very high delivery temperatures.