When an effective dual check valve, or a check valve on a water meter is installed, you no longer have an "open" system. The backflow preventer becomes a very effective non-return barrier, and the system is now a closed system just as a hot water space heating system is "closed". Expanded water can not find its way back to the city main. So in periods of water heater recovery when there is no hot water draw, expanded water volume creates a tremendous pressure increase, until the heater's safety relief valve pops and hot water spills on the basement floor. This thermal expansion will occur on the domestic hot water system in every home equipped with a dual check or water meter check valve. Thermal expansion can be a source of serious liability for the water authority or supplier.