High pressure problem; old expansion tank.

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Old 02-20-20, 11:54 AM
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High pressure problem; old expansion tank.

20 year old gas boiler but 60 year old plumbing with old style expansion tank. Until recently, system ran consistently around 20 psi, as long as rads were regularly bled and the expansion tank was emptied. Then the pressure gradually increased to above the safely valve 30psi limit. At the same time, there was little or no water in the expansion tank to drain. I donít think the tank is blocked as I can allow water in thru the shutoff valve and it flows out of the tank. Any suggestions? And thanks for reading my post.
 
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Old 02-20-20, 12:34 PM
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Dave,
With a conventional steel like yours the heated expanded boiler water must have an open path to the tank for pressure purposes which I'm sure you are aware of. There are only a couple of things that will make the boiler pressure rise and that is if the heated water cannot get to the tank possibly by a bad isolation valve between the tank and the boiler or if the valve is closed. The valve of course must be left open. When you drain the tank and you have the valve between the tank and the boiler shut off isolating the tank from the system fresh water should not be feeding into the boiler as you drain the tank. If that happens you have a problem with the isolation valve.

The other reason for high pressure is if the tank is full leaving no room for expansion. You said the tank has little or no water which sounds like maybe something happened to the valve before the tank not allowing water to enter and increasing boiler pressure. One other thing that could cause high pressure is a faulty feed valve leaking by but you check that just by shutting off the manual cold water valve going to the boiler.

When you tap the tank does it sound empty or are you assuming it is because all of a sudden it stops draining and you might be pulling a vacuum. You must let air into the tank to break the vacuum so the water will drain.

If pics are possible it would hel to see what you have.

Hope this helps a little.
 
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Old 02-20-20, 02:20 PM
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Thanks for the input

Thanks Spott; very useful suggestions. I checked the tank isolator valve and the tank drain valve; both appear to function ok. I cannot tell if the tank is full by tapping on it; I might shut down the system, remove the drain valve and poke around.

I suspect that the feed valve is faulty; if I release the pressure with the relief valve, then the feed allows the pressure to go up to 35psi. Itís rated at 15-25. The tank is too far downstream to absorb the sudden pressure increase maybe.

I didnít understand your comment on the possible vacuum. Previously the tank drain has allowed air to replace the draining water.

Appreciate any more comments you might have. Dave
 
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Old 02-20-20, 05:26 PM
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The only way to be sure the tank is properly drained & air charged is to drain the tank COMPLETELY. This may mean forcing air into the tank once you think it' empty.
 
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Old 02-20-20, 05:29 PM
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Ok. Thanks Grady. Iíll remove the valve.
 
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Old 02-21-20, 06:13 AM
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if I release the pressure with the relief valve, then the feed allows the pressure to go up to 35psi.
^^^^^ This is your problem, the feed should stop at around 15 - 20, try adjusting the feed valve or just replace it.

 
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Old 02-21-20, 07:15 AM
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No need to remove the valve.
Close the valve between the boiler & tank.
Open the tank drain valve.
Once you think the tank is empty, force air into the tank. If you hear bubbles, the tank is not empty.
Repeat until tank is empty.
Cljose drain valve & open valve between the tank & boiler.
 
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Old 02-21-20, 11:01 AM
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Iíll replace the feed valve first; see if that also restores the normal flow of water into the expansion tank. Thanks for the comments. Iíll keep you posted.
 
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Old 02-21-20, 11:03 AM
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Just to clarify the vacuum statement. If you put a straw in water and put your finger on the top and remove the straw from the glass, the water in the straw will not drain out as long as your finger is covering the top of the straw. This is vacuum. Once you remove your finger the straw empties out because you released the vacuum by letting air in. It's the same thing with your tank. It must have air to release the vacuum. They make tank valves that allow this to happen but everyone doesn't install them.

If you are draining your tank and it starts to gurgle and come out in small spirts then you must break that vacuum by getting air into the tank.

As far as your high pressure goes, is the relief valve going off at that pressure. If not you may just have a bad gauge. The relief valve should be letting go at 30 psi. If not it could be frozen and should be replaced.
 
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Old 02-22-20, 03:54 AM
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The pressure relief valve is opening, but at maybe slightly higher than the rated 30psi. Iíll have that checked as well on Monday when Iím scheduled to have the feed water regulator replaced. Iím pretty sure the tank is empty, as I get water flow from it if I briefly crack open the expansion tank drain valve when the system is running. Thanks again for your inputs.
 
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