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.