As policymakers, builders, utilities, and end users seek to reduce the greenhouse gas (GHG) impact of our homes, it is critical to address the low installed efficiency of equipment serving what is often the most significant load, space heating and domestic hot water (DHW). This is apparent for cold climate regions with greater than 5,000 heating degree days per year, such as in Chicago where residential buildings are responsible for 28% of the city’s GHG emissions, the largest of any single category. However, this is also true in California with a mild climate but a disproportionately large fraction of gas-fired heating equipment, where 40% of GHG emissions from all CA homes are from gas-fired heating and DHW equipment. Thus, in order for individual homeowners and governments to meet GHG emission reduction targets, it is essential to address the inefficiencies of these equipment cost-effectively. As described in a previous paper, the authors outlined an effort to develop and demonstrate a residential combined space and water heating system ("combi" system) driven by an efficient gas-fired absorption heat pump (GAHP), with the goal of reducing a home’s energy and emissions impact by 45% or greater (depending on existing equipment). At the core of this combi system is a low-cost GAHP, using a direct-fired single-effect absorption heat pump, using the ammonia-water working pair. Prior laboratory testing of this component show high-performance, with an operating efficiency (AFUE, projected) of 140% at 47°F (8.3°C) and with a nominal output of 80 kBtu/hr (23.5 kW), capable of 4:1 modulation for load following. In prior laboratory testing and a single-site field trial, this performance was verified, including sustained operation at ambient temperatures below 20°F (-6.7°C). In this paper, the authors outline additional findings and lessons learned from these preliminary field trials of the GAHP combi system, at sites in Tennessee and Wisconsin. Specific attention is paid to the controls of this system, including balancing DHW priority with thermal comfort, maximizing GAHP runtime for operational efficiency. The authors outline the opportunity for further improvements to system design and controls, to maximize emissions reductions while maintaining thermal comfort, and extrapolate these findings with general guidance on system sizing across multiple climate regions with building energy simulation.