Feasibility of a booster for DHW circulation in apartment buildings

With increasing focus on the performance of district heating systems, a concept is developed to obtain low district heating return temperatures from domestic hot water systems with a high share of circulation loss. For these systems, it is challenging to realize a low district heating return temperature by direct heat exchange only, due to the high flow of circulation return water at 50 °C. The concept is termed Circulation Booster. The purpose of the Circulation Booster is to boost the domestic hot water circulation temperature and at the same time secure a low district heating return temperature from this part of the service. The domestic hot water circulation temperature is heated in two steps: direct heat exchange and a heat pump. The heat source for the Circulation Booster is district heating, and the heat pump itself is driven by electricity. The paper includes the field experiences from a 1-year test period, concluding that the concept is operating as intended. Further, the performance results regarding electric consumption and district heating return temperatures and an economic feasibility study are presented. The current tariff structure in Denmark related to the district heating return temperature and electric costs gives a feasible economic case for the Circulation Booster concept with a direct payback time of 5,1 years. An increasingly progressive tariff scheme for low district heating return temperature or lower electric costs could further improve the economic feasibility of the Circulation Booster concept.