How to Install a Thermal Expansion Tank

Step by step guide on how to install a thermal expansion tank in the home water heating system. How it works and why to install one.

What is an Expansion Tank and Why to Install one

Thermal expansion tank is the device used in potable water and hydronic space heating.


The main purpose is to accept the increase in the volume of the heated water, reduce the pressure and protect the system from failure. The device contains the air membrane, usually made of rubber, which is pressurized with air on one side while on the other is water from the home plumbing. The air bladder can expand or contract making the room for water expansion from the water heater. 

The device is used in closed loop systems where the flow preventers and check valves are installed, and since the expanded water doesn't have anywhere to go, the expansion tank is there to take this extra water. The expansion tank should be sized properly and the installed capacity depends on the size of water heater.

If the the expansion tank is not installed, one of the most common problems you can have is the dripping from the temperature and pressure relief valve or TPR valve.

Much bigger problem, due to the extremely high water pressure is bulging water heater, tank rupture, dislocated air vent or gas and water fittings, leaking and other.

How to Install a Thermal Expansion Tank

  • Check the water pressure in your home. It should be close to 50 PSi. If it is bigger than 80 PSI install the pressure reducing valve.

  • Check the pressure of air inside the thermal expansion tank. The pressure should be equal to the maximum water pressure in your home. Use the hand pump to increase the air pressure if needed.

  • It is recommended to install the thermal expansion tank on the cold water line, horizontally and close to the water heater.

  • Use the T-fitting with the thread, teflon tape and pipe wrench to tighten the connections.

  • Once installed and to remove the trapped air just open the hot water tap.

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