Recharging Diaphragm Expansion Tank

After changing a leaky pressure relief valve on my furnace, I noticed that the diaphragm expantion tank in my hot water heating system was filled with water.
I shut off the supply and opened the pressure relief valve. I then pushed on the pin in the bicycle-tire valve on the expansion tank and let all the water drain out of it through the boiler and out the expansion tank.
So far so good.
I am not sure if the diaphragm failed or I need to recharge the expansion tank with air.
Can anyone tell me the procedure for recharging it?
Bill
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Bill Smith wrote:

That last sentence is a little confusing. It reads like you let the water drain out of that little "tire valve", but I'm assuming that's not what you really meant to say...

You probably should have pushed in on the pin of that "tire valve" BEFORE you shut things off and drained the tank. If water came out of it you'd have known the diaphragm was "holy".
But, you can try pump air in through that valve now with a bike pump, checking the pressure with a tire gage as you go. If you can't build up any pressure at all, then the diaphragm is shot. If you can build up say 20 psi or so and it "holds" for an hour, then the diaphragm's ok.
Assuming it IS ok, just pump more air in until you reach a pressure about equal to the pressure you normally see on your heating system's gage and then refill the heating system.
That's the way I do it.
HTH,
Jeff
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My name is Jeff Wisnia and I approved this message....

(W1BSV + Brass Rat '57 EE)
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Bill Smith posted for all of us....

I believe (am no expert) that a leaky PRV is a symptom of a bad diaphragm in your expansion tank. So you you spent time & $$$ and my have not fixed the problem.
Other replies are good.
--
Tekkie

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Thanx for the replies.
When I try to pump it up, I head gurgling sounds when I shut off the air pump. So I suspect you are all right that the diaphragm is shot so I'll replace the tank.
I'm pretty sure the PRV was shot anyhow. It was not just discharging under pressure, it was dripping quite a bit too.
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There should be no water in the lower (air) chamber. It's shot. And since it's no longer absorbing the thermal expansion, that's why your relief valve tripped. Buy a new tank (mine was $40 at Menards a few years ago). It came pre-pressurized to 40 psi. You may have to replace the relief valve as well -- in my limited experience, an old valve does not reseal well after tripping.
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