Thermal Expansion Tanks

Water heater thermal expansion tanks are safety devices that reduce the stress on your home’s piping system. Because water expands significantly as it heats up, it creates pressure on the water heater tank as well as all connected piping. That “extra” water has nowhere to go. Sometimes it leaks out of faucets or causes toilets to run, sometimes it shortens the life of plumbing. It’s still the same amount of water, of course, but it takes up more space. Similar to how water expands when it is frozen. A thermal expansion tank is a tank that initially holds no water, only air. When water in the water heater expands, it can compress the air in the thermal expansion tank and occupy some of that space. A thermal expansion tank simply provides an overflow for the expanded water to temporarily live in, until it cools down and returns to the pipes or tank. From there, it will heat up again and restart the process.

Depending on how cold the water was when it filled the tank, it is estimated that a 50 gallon tank of cold water heated to 120 degrees Fahrenheit would take up about 52 gallons of space. Of course, if the water is warmer to begin with, it is already expanded – so the amount can vary. This is why two gallon expansion tanks are common for 50 gallon water heaters. Whether that number is sufficient for your home is something to discuss with your plumber.

Expansion tanks are especially important if you have pressure regulating valves, check valves, backflow preventers, or other components that prevent water from moving back through pipes and distributing the pressure elsewhere. Without the ability to expand back into pipes, your water heater itself can become dangerously pressurized. Of course, this is when a TPR valve would protect you – but then you have scalding water to contend with. TPR valve opening is better than an exploding water heater, but not as good as preventing those problems with an expansion tank.

Too Long, Didn't Read Version: Water expansion tanks can reduce stress on your water heater and water pipes due to the expansion of water as it heats up and creates pressure inside the closed system.

-Marshall Tramp, CPI, RMP


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Marshall Tramp, CPI, RMP
Author: Marshall Tramp, CPI, RMP
Top Notch Inspection Services llc
Serving Eastern Washington & Northern Idaho