Water Heaters - Thermal Expansion

If a check valve is present on the inlet water line, it will create a “closed system.” 

Heating water in a closed system creates an increase in pressure within the water system because the pressure is not able to dissipate in the main supply line. Referred to as “thermal expansion”, the rapid pressure increase can cause the relief valve to operate (releasing water) during each heating cycle, potentially causing premature failure to the valve or even the water heater. 

The suggested method of controlling thermal expansion is to install an expansion tank in the cold water line between the water heater and the check valve. Contact your installing contractor, water supplier, or plumbing inspector for additional information.