I have a boiler with a single zone feeding cast iron radiators. Their is a large conventional expansion tank in the ceiling (I figure 25 gallons or so). To drain it I off the valve to the tank and let it drain. It takes about an hour to get all the water out with the vacuum lock but I am positive that it is empty because I can lift if up and feel it. As soon as I turn the valve back on and run the boiler a few hours she is full again. Am I using the wrong procedure to drain the tank?
Yes I know I can replace it with a diaphram tank. I'd rather get this one working properly.