The Negative Impact Of Cold Water Bypass On Solar Domestic Hot Water Systems

A report prepared by Bright Power, Inc. for NYSERDA and the New York City Economic Development Council. 

August 2011

Supported under NYSERDA Agreement No. 19712 and NYCEDC Agreement 4262000.

 Download the report (2MB)

A solar domestic hot water (SDHW) system is designed to preheat the domestic hot water for a building. It saves fossil fuels or electricity by reducing the runtime required of the conventional domestic hot water (DHW) appliance, because the conventional appliance is not required to supply as much heat. The proper functioning of the SDHW system is predicated on the fundamental assumption that the vast majority of cold mains water to be heated for DHW will flow through the SDHW system, and will not enter the conventional DHW appliance some other way. 

Post-installation monitoring of a 24 collector SDHW system recently installed in the Bronx revealed that the overall performance of the system is far lower than expected. The initial hypothesis for the cause of this reduced performance posited that less water is being drawn through the preheat tanks than designed for, thereby impeding the distribution of the thermal energy collected and reducing the efficiency of the solar thermal system. To investigate this theory, a study was commissioned to analyze the internal dynamics of the system through a joint effort between Bright Power, the New York State Energy Research and Development Authority (NYSERDA), the New York City Economic Development Corporation (NYCEDC).

Using a 10-point temperature and 2-point flow sensor setup for six months, together with hourly energy simulations, Bright Power analyzed the thermal and fluid dynamics of the domestic hot water (DHW) system. We found that the primary cause of this problem is cold water bypass, whereby cold water makeup from the mains plumbing line is circumventing the solar preheat tanks.