Water, when heated, expands. The expansion increases its volume. For example, water heated from 90ºF to 140ºF in a 40 gallon water heater will expand by almost one-half gallon. Because water is not compressible, in closed systems, the increased water volume must go somewhere. This thermal expansion increases water pressure. Since pressure seeks the path of least resistance, annoying and potentially dangerous problems can occur. In an “open system” or a system that does not contain the aforementioned valves, the increased volume is able to find its way back into the municipal supply.
Thermal expansion can cause pressure surges, chronic or continuous dripping of temperature and pressure valves (T & P Valve), faucets and/or leaking fill valves in toilets.
More serious problems can also occur due to thermal expansion. When pressures within a water heater surpass the intended operating range, internal parts may fail such as internal flues, fittings or water connections. If a flue way collapses it can lead to the potential release of toxic gases, such as carbon monoxide into living spaces. Thermal expansion can also cause the water heater’s tank to rupture or explode. Minimally, this may require replacing the heater. However, there are many documented incidents that have caused major property damage and fatality.
Plumbing codes require that expansion tanks be used whenever a potable water system contains a Pressure Reducing Valve (PRV), a Backflow Preventer or a double check valve. When water flow is static or not flowing through the system, the aforementioned valves are closed which prevents water from reversing its intended course. This is referred to as a “closed system”.