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Two weeks ago, I had to replace the expansion tank on my hot water heater due to a small pinhole on the top. Ever since then, it seems like there is increased water pressure when I flush either of the two toilets in the house. In the master, you can hear like a banging in the wall as you flush. In the other bathroom it sounds more loud with an abrupt stop once the tank is filled. I have not noticed significant water pressure or pulsating water from any of the faucets. This was not issue before the expansion tank was replaced.

Is this normal? Does this usually take care of itself? Is there something I can do to reduce the pressure or does this mean there is something wrong with the expansion tank?


The pressure on the tank came precharged at 40psi. It was not adjusted at the time of the install to match the incoming water pressure of the house which measured 60-65psi. Due to this, I pressume that is why I may be having the issues. Will this be resolved if the expansion tank pressure is increased? Is it an easy fix to just add the air, or is it more complicated than that?

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The banging in the pipes may be water hammer.

You can test your water pressure with a simple gauge. Some are sold with a garden hose fitting for easy connection to an outdoor hose bibb or to a clothes washer supply valve. Consider choosing a gauge that has two needles: one needle shows the pressure in real time while the other needle shows the maximum pressure. That second needle tells the story when there's a water hammer or thermal expansion problem. In the USA residential water pressure is often regulated to no more than 60-70 psi. Many water pressure regulators are adjustable by turning a screw on the regulator.

Although expansion tanks exist primarily to absorb thermal expansion from a tank-style water heater, they can also help mitigate water hammer. Did you confirm the air pressure in the expansion tank was correct at the time of installation? It may have been shipped with some pressure in it, but the air pressure should be adjusted to match the setting of your water pressure regulator. If the air pressure isn't correct the expansion tank can't perform its job as well.

Also, the air can leak over time, so the tank pressure should be checked occasionally.

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