I have a Kerr wood fired boiler in combination with an Old Wel Mclain oil boiler(coal fired at one time ). I'm using the existing 12X60 steel expansion tank. Ive always drained a few gallons from the expansion tank every year. In the past air would enter the tank through the drain valve after the water stopped draining. Now I continue to get some flow which seems to be coming from the system since it does increase some when I open the fast fill. This started when I first noticed a couple drops coming out of my relief valve, although the pressure never exceeds 18 psi with the system on and water temp at 140. The relief valve was set at the factory at 30 psi and was recently installed with a new regulator which is keeping the system at 12-15 psi with the system off. The instructions that came with the relief valve state that if the valve leaks some it could be caused by a waterlogged tank. Can the old type steel expansion tanks be serviced or perhaps the expansion tanks drain valve isn't working. When I tap on the tanks, it sounds empty. Any suggestions might be helpful, Thanks

 

There should be a valve to isolate the tank from the system. If there is, close it, then drain the tank completely. Do not be fooled into thinking the tank is empty just because water stops coming out. If there is no other way to get air into the tank, here is a way to insure the tank gets emptied: Use a piece of hose only as long as you need to get into the bucket you are using to catch the water. When the water stops, take a deep breath & blow into the hose, forcing air into the tank. Quickly kink the hose & put it back into the bucket. Repeat until all you get out of the hose is air. Be aware that you, at some point are likely to get wet or a mouthful of foul tasting water. Been there, done that.

 

hello, i also have an expansion tank directly above my furnace, i believe its about a 30 galloon tank, how much water/air should be in this tank? would blowing air into the tank using an air compressor help?
thank you

 

In theory, the tank should be 1/2 - 2/3 full of water. Using a compressor will insure you have drained the tank completely. Two cautions: (1) Do not exceed 25-30# of pressure (I wouldn't even go that high. 5# is ample).
(2) Do not try to "pre-pressurize" the tank. If you do, you can force air into the system when you open the shut off valve between the tank & boiler.
Footnote: If you have automatic air vents, close the taps tightly. Auto vents & conventional expansion tanks are a bad mix.