Thermal expansion tank maintenance

It’s important to understand that all expansion tanks will fail over time. Usually you can expect an expansion tank to last for at least 6 years as long as it was installed correctly and the PRV is also working. When an expansion tank starts to fail it is only a matter of time before it ruptures and causes a leak or flood.

The thermal expansion tank is filled with compressed air and has a bladder or tube on the inside that separates the air and water.

Thermal expansion tank failure

Inspecting a thermal expansion tank

The easiest way to check a thermal expansion tank is to simply tap on the end of the tank to get a feeling for the weight or density. An expansion tank that is functioning properly will be 95%-100% full of air. A failed expansion tank or under inflated expansion tank will be filled with water.

The proper way to inspect a thermal expansion tank is to turn off the water to the entire house and open a faucet to relieve the water pressure. Using a tire pressure gauge (must go beyond 75 psi) you can unscrew the bottom cap of the expansion tank to reveal a valve stem. If the tank does not have any air pressure whatsoever, the expansion tank has failed and needs to be replaced. If you do have pressure inside the expansion tank you then need to check the water pressure. If the expansion tank does not have similar pressure to the water pressure then you will need to add air to the expansion tank. This should be done with the water off and the water pressure relieved.

high water pressure from thermal expansion

Checking the water pressure

A water pressure gauge (pictured above) can be purchased for about $15 and connects just like a garden hose. This is an absolute must for anybody that has a thermal expansion tank. You want to inflate your expansion tank to within 7 psi of your water pressure. If your water pressure is 75 psi, your expansion tank can be set anywhere from 68-75 psi. 

When checking your water pressure you will first need to let the water run for about 10 seconds, this will drain off any excess pressure from thermal expansion. You can check the pressure from the bottom of the tank or even the outside of your home at an exterior hose spigot. You may also want to check your water pressure again 2-3 minutes later. On occasion, a PRV will initially reduce the water pressure and then gradually releases excessive pressure.

You may also be able to check your water pressure before your PRV. This will help you determine the importance of having a working thermal expansion tank. Generally any incoming pressure over 100 psi it is recommended to have a thermal expansion tank even if it’s not required by your city code.