gas fired water boiler expansion tank filling question

[MasterPlumbersKeymaster]

I have a gas fired water boiler with an expansion tank. The expansion tank is connected to the furnace by a “T” off the water supply line to the furnace. There is a valve (#1) after the “T” that goes to the expansion tank – to shut off water to the furnace. There is also a valve (#2) on the line to the expansion tank.

I recently replaced the plumbing in the basement, including the supply to the furnace prior to the “T” that goes to the expansion tank. I had the valve (#1) on the water supply line closed, as well as the main supply to the house closed, while I was doing the work.
When I was done I turned the main supply to the house back on. Next I opened the valve (#1) to the boiler/furnace. When I opened valve (#1) the expansion tank began to rapidly fill with water. I closed the valve (#1) and the expansion tank stopped filling. Prior to my beginning the plumbing work the expansion tank was empty (it is summer and we haven’t used the heat for months). I opened valve (#1) again and the expansion tank began to again fill rapidly. Just to see what would happen I then shut off the valve (#2) (the line to the expansion tank) and the overflow/pressure release from the boiler started to dump water so, I opend the valve (#2) and closed valve (#1).

Is it normal for the expansion tank to fill up like that? I thought it was just to allow for the expansion of the heated water while the furnace was in operation.

Thank you anyone who can help. [email protected]

[DUNBARParticipant]

Tank will fill with water until it satisfies the matching pressure of the pre-charged bladder pressure, which should be the same as the normal working pressure of this system.

Only time an expansion tank is defective,,is when water comes out of sifter valve where air is applied to set bladder pressure.

[Author]

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