I have a zoned in floor hot water heating system that is hammering. The system hammers in the boiler when I have only one zone on. When more than one zone calls for heat, then there is no hammering.
* I have installed spiro vents to remove all of the air.
* I have checked the pump, and it is fine.
* If I open a bypass valve to increase the circulation through the boiler, the hammering goes away.

I am thinking that I am not getting enough circulation through the pipes in the floor. Water is turning to steam in the boiler and hammering.

If the bypass works set it and forget it. Has this done this since new? Where is the professional company that installed it for you? You might call them and ask the same question.