The decarbonization of heating hot water (HHW) systems is an important and necessary step in the building industry’s effort to combat climate change. Existing heating hot water systems can be decarbonized by replacing existing fossil fuel-based heating systems with high-efficiency electric heat-pump-based heating systems in parallel with national and regional efforts to improve the source efficiency of the electric grid as a whole.
Replacing fossil fuel boilers with electric heat pumps is a significant technical challenge, particularly in cold climates. Perhaps the most critical consideration when designing a heat pump system is the lift the heat pump will be subject to. Heat pump lift is defined as the temperature difference between the heat source and the water temperature being produced. Minimizing lift to the greatest extent possible allows for significantly higher heat pump coefficients of performance (COPs), as shown in Figure 1, and in cold climates, increases the number of manufacturers and products available to meet the performance specification. The best option for minimizing lift in cold climates is to use a relatively high-temperature heat source, such as a ground heat exchange loop or wastewater source. In applications where outside air is the only available heat source, the only variable left in the designer’s control is the heating hot water supply temperature (HHWST) the heat pump is producing, which leads us to the focus of this article.