What is water heater thermal expansion and do I need to worry about it?

Thermal expansion refers to the expansion in volume that happens when water is heated. What this means for your water heater is that every time a tank full of water is heated the water may require more space than is available in the tank. The extra water volume creates excess pressure in the plumbing system. This extra pressure can cause damage to the water heater, fixtures, and water piping over time.

So what happens if the pressure is not relieved? . . . Hot water tanks contain a pressure relief valve to protect them from rupturing under such a scenario. The valve opens when the tank pressure reaches the valve set point. After enough water has passed through the valve the pressure in the tank drops and the valve closes again. Sometimes in older hot water tanks the valves might be plugged with scale or corrosion and not function properly. For this reason, a thermal expansion tank is recommended.

A water heater expansion tank is designed to help prevent fluctuations in water pressure. The water heater expansion tank serves as an overflow receptacle by absorbing excess water volume that the water heater creates when heating water. . It is important to note that even with a properly selected and installed thermal expansion tank the plumbing pressure will still increase, but not by enough to cause a failure in the plumbing system.
Expansion tanks are normally installed above the water heater on the cold side before the water comes into the water heater. The extra hot water from the thermal expansion can be stored in the expansion tank and when hot water is demanded it is put back into the water heater. Having an expansion tank helps prevent dripping faucet and running toilets by keeping the extra built up pressure from reaching the fixtures and prematurely damaging them.
Water heater expansion tank prices start at around $40 and the tank is easy to install with a normal 3/4” threaded connection. See illustration below.  Contact your local licensed plumber for more information.



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