Thermally-driven heat pumps (TDHPs) are an emerging class of efficient heating equipment, capable of reducing the fuel consumption of a building served by furnaces or boilers by 35%-55% with seasonal heating efficiencies of >140% AFUE, depending on the application and region. These TDHPs, like other heat pumps, are commonly air-source heat pumps and reduce energy consumption by upgrading ambient heat via a refrigeration cycle. Advantageous for colder climates, these TDHPs employ internal heat recovery such that the reduction in efficiency and output capacity with lower outdoor temperatures is milder compared to other heat pump technologies, with TDHPs retaining >85% of their nominal output capacity from 47F (8.3C) to 0F (-17.8C) outdoors. Additionally, TDHPs frequently put this heat recovery to use during coil defrost events, minimizing the impact on indoor thermal comfort.

For multifamily buildings in cold climates, commonly served by a central boiler plant and standalone water heating equipment, TDHPs drive energy efficiency and emission reductions via a "hybrid" heating system approaches wherein the TDHP is installed to operate in conjunction with boilers and/or water heating equipment to jointly serve thermal loads. A "hybrid" approach assures that the TDHP efficiently meets most of the thermal loads most of the time, but utilizes efficient conventional heating equipment (e.g. boilers) to assure capacity to meet peak demands. In this paper, the authors describe different approaches to hybrid heating with TDHPs in multifamily buildings, while outlining findings from a technology demonstration, pairing an air-to-water TDHP with hot water boilers to serve the space and water heating loads of an apartment building in the Chicago, IL metropolitan area. Also, the authors examine different approaches to system controls, balancing component performance with overall system cost and emissions reductions.