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Re: Normal Boiler Pressure Range

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Posted by Ziad on March 17, 2008 at 21:07:49:

In Reply to: Re: Normal Boiler Pressure Range posted by Steve R. on March 16, 2008 at 16:22:14:

: 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.

That's a good one! I had not thought of the effect of the drop in pressure on the suction side of the pump. Unfortunately for me, the problem happens even with the supply line shut. I had shut it for the past few days to be sure there were no leaks.

: 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.

Thanks for the detailed explanation, now I understand the reasoning behind your boiler's connections.

: 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.

I got lazy and went with the quick sizing charts. Unfortunately I do not know how much water my system holds. Maybe I should bite the bullet and figure it out.

: Hope this all helps.
Many thanks again, you've been most helpful.


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