Plumber Installed Thermal Expansion Tank Without Adjusting Pre-charge - Plumbing - DIY Home Improvement | DIYChatroom

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Old 01-18-2010, 01:29 PM   #1
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Plumber installed thermal Expansion tank without adjusting pre-charge


I had a plumber come out to install a thermal expansion tank near my hot water heater as my internal water pressure was reaching 160psi (which is my municipal water supply pressure) when the hot water heater fired.

The tank is a Watts PLT-5, which comes set from the factory with the bladder at 20psi.

The plumber did not adjust this charge and seemed surprised when I asked about it. I got the impression that he did not adjust any expansion tanks when they are installed.

My understanding was that these tanks need to be set to an internal pressure near that of your pressure reducing valve.

My pressure reducing valve is set to 60psi. I called the plumber's office and the tech called me back and said he can stop by to "try to pump it up," but I think I'd rather finish the job myself at this point..

So, how can I do this correctly? Do I need to remove the tank from the plumbing, or is shutting off the water and draining the lines sufficient? What should I pump it up to?

Am I in the wrong or right for thinking that what the plumber did is incorrect?

Is the tank possibly now damaged?

Thanks for any and all responses!


Last edited by markwo; 01-18-2010 at 01:33 PM.
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Old 01-18-2010, 04:51 PM   #2
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The tank will not be damaged by being underpressurized.

I am just guessing that the tank should be prepressurized to between 1/2 and 2/3 the desired water system pressure which I assume is 60 psi in your case. I believe that most homes have the regulator set to between 30 and 40 psi.

To prepressurize the tank without dismounting it, turn off the main water supply, open a faucet or two, wait for the water to stop flowing, and then slowly pump air in or release air.

What you need to do also is monitor the pressure as measured at a faucet. Should you still get excessive pressure as the water heater runs, try pre-pressurizing the tank again, this time to a higher starting pressure. But be careful, some models of tanks have a maximum permitted prepressurizing.

When you turn the water on, water will enter the tank and compress the bladder until the bladder pressure rises to match the water system pressure. With prepressurizing at 1/2 to 2/3 the desired system pressure, the amount of water that enters the tank will be between 1/2 to 1/3 the tank capacity. The more water that enters the tank to reach normal pressure, the less room there is in the tank for expansion as the water heater kicks on.


The good conscientious technician or serviceperson will carry extra oils and lubricants in case the new pump did not come with oil or the oil was accidentally spilled, so the service call can be completed without an extra visit.

Last edited by AllanJ; 01-21-2010 at 04:33 PM.
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Old 01-21-2010, 12:41 PM   #3
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With no water pressure present to the tank,the air pressure should be the same as the PRV.
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