Question about Amtrol ST-5-C THERM-X-TROL ASME Expansion Tank, 2 Gallon

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I have an ST-5 Therm-X-Trol when I tap on it it appears to be completely filled with water. Is this normal or is there something that should be done with the air pressure?

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  • emcnama01 Nov 08, 2010

    Thanks for pointing me in the right direction



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You are correct... the tank should not be full... usually the problem is that the bladder inside breaks and then the air escapes....or more accurately is disolved into the water.

The ST tanks are usually used for expansion on domestic water systems to make up for hot water heater expansion. If there is a check valve at the entry point then the tank is needed to allow for water expansion upon heating in the hot water heater. Therefore the bladder pressure is usually about 100 psi +or- depending on the setup. the main thing is that the pressure is lower then the relief valve stting usualy about 125 psi.

The tank can be bought online or at many big box stores and it is easy to replace.
Simply shut off the wwater...isolate as much as possible and then relieve the pressure at the closest point to the tank...
Then just unscrew the tank... dope or tape with thread sealant, the threads of the new one and screw in back into the fitting...

There you are back in business again...


Posted on Nov 08, 2010


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Once these start to leak, usually the best solution is to replace the tank. It is corroding and will only corrode more with time. You could try epoxy for a short term fix, something like liquid metal.

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My A.O. Smith 8 year old water heater drips water out of the expansion valve but only when it is heating water. The builder of my home mounted the Amtrol THERM-X-TROL ST-5 in the horizontal position and I...

It seems that you have this figured. In their literature all units are shown vertical, either inlet up or inlet down. Also, it appears that if the unit is full of water, (water logged) pressure will build up in the system. Temporarily, you could shut off water to the water heater, open some drains and use a pump to put some air back into the tank. This would at least get air into the upper half of the tank for a while until you can get set to make a repair.

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I am loosing water pressure in my shower so I checked pressure at my ST-12 Therm-X-Trol tank hoping it would be 40 but water comes out of the test point and if you tap the tank it feels like it is full of...

you are correct....the rubber bladder inside the tank has leaked by and filled the expansion tank. it will need to be replaced and you also may find boiler or water heater relief valves may blow off until its replaced

Oct 21, 2010 | Amtrol ST-5-C THERM-X-TROL ASME Expansion...

1 Answer

I have a Therm x Trol on my water system. It has a small leak in the top. I'm on public water in a subdivision. Do I really need this device? Can I remove it with out harming my home?

depends on why it was installed in the first place...

most of the time these are installed to reduce thermal expansion... "hot water into the cold pipes"

I don't know your system, but I would just replace it. removing it could potentially bring back, or cause other problems we are not aware of, the reason it was actually installed.

Aug 29, 2010 | Amtrol ST-5-C THERM-X-TROL ASME Expansion...

1 Answer

My st-5 seems to be just filled with water, my hot water relief value discharges now. I went check if there was any air pressure and rusty water came out.

The diaphram in the tank has ruptured. This is not repairable.
You will need to replace the tank.
To purchase one find your nearest plumbing/HVAC supply and they will have one or can get one for you.

I hope that this will help you to solve your problem!

Thanks for using Fixya!!


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Short buzzing noise every few seconds from hotwater tank

it maybe that there is a small leak at the connection.try to remove the tank and put teflon tape on the connection.this may fix the problem.if this does not fix the problem,it may be possable that the amtrol has a pin hole in it.this may hve to be replaced.good luck with your project

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1 Answer

Therm-x-trol st-12 leaking

Never seen one leak other from the seam or the top or bottom.

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