Hydronic Heat Boiler noise

[Warren Hilderbrand]

All the 4 heating zones (2 bsbd up and 2 radiant down) work fine. In fact the system seems to run ok except for one thing. Every time the boiler fires to heat the water a sound like a tea kettle starting to steam happens followed by a few loud bangs inside the boiler. The psi gauge jumps down and back with every bang. I cannot detect any air in the system. (all bleeders just producing water.Any ideas as to some common causes of this? I understand that por circulation is a common cause but this shouldnt be unless there is a resriction inside the boiler itself. What would cause that?Any help would really be appreciated.John

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