Loop system presure loss

Ted_6

My boiler runs at about 20lbs. of presure.When it is shut off when not needed hte presure drops to 0lbs.The auto fill will bring it back up to 20lbs. but when the system is turned on the presure will increase and the relief valve will blow.If the system is shut down with the auto fill off the presure will decrease but come back up to normal when turned on.The expansion tank was replaced and there appears to be no leaks. Can anyone help?

Guy_5

Pressure Loss

I would begin by looking at the expansion tank. If it is an older compression tank it probably needs to be drained. The newer type diaphram tanks can have bladder failure and react just like a full compression tank. That being the case, I would just replace the tank.
Beware that full bladder tanks are always heavier than you expect when you remove them. And wetter.

Radiant Wizard

Can You

draw a piping diagram of the boiler along with all check valve, circulator and feed valve placements?

Paul Mitchell

gauge

sounds like maybe the gauge is bad.

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HP80

Has anyone checked for air in the system?