CH-15-C044 — The Effect of Piping Heat Losses on the Efficiency of Solar Thermal Water Heating in a Net Zero Energy Home

This study evaluates the thermal performance of a dual-tank domestic hot water (DHW) system designed for a net-zero home at the National Institute of Standards and Technology, Gaithersburg Maryland campus. Hot water demand for a virtual family of four in this facility is met with an active, indirect solar thermal water heater and a heat pump water heater that serves as the auxiliary heating method. Testing under a representative water-use schedule between February 2014 and June 2014 has shown that heat losses in interconnective plumbing between the two hot water storage tanks can significantly diminish solar fractions at times when solar thermal water heater output is high. For June, the monthly solar fraction of the DHW system was 86%, while a 95% solar fraction would have been achieved without heat losses. This highlights one known drawback of dual-tank hot water storage systems, where constant heating of interconnecting plumbing results in energy loss to the ambient environment and should be considered part of the total hot water storage volume.