Thermal expansion is a relatively new problem in household plumbing systems. It is the result of installing water meters with check valves or dual check valves mandated by local and national plumbing codes. Although the purpose of the check valves is to isolate the municipal water supply from potentially contaminated household water, the resulting thermal expansion creates a separate, often under-appreciated problem.

In a typical domestic residence the largest use of potable water in the home is usually for hot water for bathing, washing, dishwashing, etc. A gas or electric water heater heats water in the domestic hot water system. The water heater uses a continuous supply of fresh water. Hot water is used at a rate dictated by the demand at the water fixtures. As hot water leaves the system through the faucets, showerheads, dishwasher, etc., fresh, cold water from the city main is supplied to the heater where it is heated for continuing use. On average, a water heater in a home will heat water two or three times a day.