Field monitoring and data analysis were conducted at six (6) residential sites to evaluate a hybrid natural gas-solar hot water system that provided domestic hot water and space heating. This particular system includes a storage water tank, a gas-fired condensing tankless water heater, and an evacuated tube solar collector. Installed according to manufacturer and contractor recommendations, the test protocol allowed the system to operate as required by the site to meet the hot water and space heating demand profile of the residence. The Data Acquisition System (DAS) enabled remote monitoring of the system performance and hot water demand including water flows and temperatures. Data was uploaded weekly, and analyzed on a monthly basis. Parallel to the field monitoring, a similar system was installed and monitored in a laboratory. This test plan used water heating demand profiles developed by the Solar Ration & Certification Corporation (SRCC) under actual seasonal conditions to calculate the system’s fuel and gas only efficiencies. This approach provided real-life operational loads on the system, and allowed not only performance analysis, but also insight as to how the system could be adjusted for improved performance in meeting the end user domestic water/space heating needs. Under the current system configuration, the storage tank temperature is maintained by the tankless water heater. Both field and laboratory data suggest that the systems, as configured and operated, are not performing to optimum efficiency. Additional laboratory testing suggest a reconfiguration of the equipment with modification to the system operation - by dedicating the storage tank to solar preheat and moving the tankless heater downstream - will result in improved overall system efficiencies. This paper will discuss the performance of the systems in the field, the laboratory validation of the current field configurations, the results from the laboratory assessment of a reconfigured system, and finally the potential impact a reconfiguration could have in the field.