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I have a closed loop 6 zone heating system in 2 storey home. Circulating pump is on inlet side and air scoop and expansion tank are on supply side. Boiler reads 22psi. There is a pressure gauge at inlet of circ pump that reads between 12-14 psi depending on how many zones open(sucking). But when system shuts down this gauge slowly drops to zero, why? There are no visible signs of leaks anywhere. Any help would be appreciated.