A closed water-supply system (with a backflow preventer or check valve at the meter) has a water heater but no expansion control. As the water heats and expands, what device is required to prevent pressure buildup?

Explanation

A closed system cannot push expanded hot water back into the main, so a thermal expansion tank (or similar expansion control) is required to absorb the volume increase and prevent the T and P valve from weeping. Water expands roughly 2 to 3 percent between cold and hot, which spikes pressure in a sealed system. The expansion tank is charged to system static pressure.