Laboratory and Field Evaluation of a Gas Heat Pump-Driven Residential Combination Space and Water Heating System

Combination space and water heating systems have historically offered end users and installation contractors numerous benefits over conventional, standalone equipment - typically a gas furnace or electric heat pump paired with a standard gas or electric storage water heater. Combination systems, typically driven by a gas-fired potable (tankless water heater) or non-potable (boiler) water heating system, offer benefits including reduced equipment costs with one "thermal engine", reduced installation costs through requiring only one vent/gas line/condensate drain, and when deployed effectively, they can yield consistent high operating efficiency and reduced cost. Concerning high operating efficiency, these "combi" systems often cost-effectively improve the operating efficiency of domestic hot water (DHW) production to a condensing efficiency (>90%), where the economics of a standalone DHW system at a similar efficiency level are often difficult. For the majority of U.S. homes that are heated by natural gas, particularly in colder climates of the Pacific Northwest, Midwest and Northeast, the authors have demonstrated a gas heat pump-driven residential combi system, through laboratory and field testing, a step up in combi system efficiency. The gas heat pump (GHP) component is based on a low-cost single-effect absorption cycle and, through prior testing, has demonstrated an operating efficiency of a projected AFUE of 140% for Climate Region IV as defined by ANSI Z21.10.4. The GHP has a nominal output of 80 kBtu/hr at 47 degrees F and is capable of 4:1 modulation, to load follow when necessary. Developed to provide space and water heating to a residence, integrating with a forced-air heating distribution via a hydronic air coil and heating an indirect storage tank for DHW, the GHP-driven combi system is shown to meet a home's space and water heating loads simultaneously, through simulated- use testing in a laboratory and through a field demonstration over 12 months at a residence in Tennessee. The authors outline the performance of the GHP-driven combi system as a function of loading, operating conditions (e.g. ambient temperatures), and system control strategies. Additionally, the impact of system design considerations for component sizing and control, such as the indirect storage tank size, and system performance during defrost events are explored.