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Water Heaters 101 > Water Pressure Issues

What you'll find on this page: Water pressure that is too high is a hidden killer of water heaters and piping that most people never think of. Here is how to test for it and what to do if you find it's a problem.

Water heaters are manufactured to withstand pressure up to 150 pounds per square inch (psi), but anything above 80 psi is very bad for them and plumbing in general.

We've seen situations where water heaters failed before their warranty was up and piping developed leaks because of pressure spikes. Too-high pressure can also damage powered anodes.

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Water pressure gauge set on a water heater drain valve

Temperature/pressure relief valves don't open below 150 psi. Thus, people may have pressure problems without knowing it.

A secondary issue is thermal expansion. This occurs when a water heater comes on to maintain temperature although no hot water is being used.

If the system is closed, perhaps as a result of a check valve, a pressure reducer or a backflow preventer, then as the water heats and expands, it has nowhere to go and that results in pressure spikes.

Testing is simple. You can buy a water pressure gauge at most hardware stores for about $10. Some have a feature where the needle will hold at the highest reading.

Screw the gauge onto the water heater drain valve or any hose bibb, such as above right, then open the valve. That will give you static line pressure, which should be below 80 psi.

Next, making certain nobody is about to use hot water, make the water heater fire up by raising the thermostat. Watch for pressure spikes.

If everything stays below 80 psi, you don't have a problem.

But if you do, the solution is to buy an expansion tank, such as the one below, which is installed between a boiler and its storage tank. The device is a steel tank with a rubber diaphragm inside. It is charged with air, and when thermal expansion occurs, the water flows into the tank, compressing the air. When pressure returns to normal, the air pushes the water back into the water lines.

Expansion tank mounted between a boiler and storage tank

Because of federal regulations, expansion tanks come precharged only to 40 psi. You must use a bicycle pump or something like it to pump the tank up to match line pressure. Otherwise, it won't work properly.

The tank can be mounted anywhere in the piping, although they are often found next to the water heater.

If hot water often flows upward into the tank by convection, it will shorten the life of the diaphragm, so it's best not to mount it upside down right above the tank, as we've seen done a few times.