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Posted by Steve R. on March 16, 2008 at 16:22:14:
In Reply to: Re: Normal Boiler Pressure Range posted by Ziad on March 16, 2008 at 09:44:47:
Well, Flexcon is correct in stating that the expansion tank should connect to the supply "hot", but they don't show how the fill system is to be connected in relation to the expansion tank.
So, here's the problem as far as I can tell:
When the pump that is on the boiler return (just downstream from the fill line (1/2") runs, it creates a drop in pressure where the fill line attaches. If the PRV (Pressure Regulating Valve) is set to, say, 15 psi and that pump, when it starts up, drops the pressure on its suction side to 12 psi, or anything less than 15 for that matter, the PRV (or we call it a fill valve on boiler systems) allows water in until it sees 15 psi at the point it ties into the boiler system. When the pump stops, your static pressure has gone up 3 psi (in this instance). Your static pressure is now 18 psi instead of 15.
So, in looking at your picture of the (old expansion tank), shut of the ball valve just above the green tank and remove the tank. Place a tee fitting there (below the ball valve). Pipe the fill line into the branch (side) of the tee and put the new grey expansion tank on the bottom of the tee. Make sure to double check the tank pressure before you install it again. Then cap off the 1/2" line that used to feed the boiler where it ties in (just before that pump I talked about). This should fix your problem.
As far as the size of the expansion tank, I think the one you have will work. If it doesn't, it will over pressurize the system again. Then you will need to size it up. BTW, expansion tanks are sized based on system size not BTU of the boiler.
Hope this all helps.