I have been servicing a water source heat pump system for over four years now and have not had the issue that I am having today. To give you some background information this system uses a three way valve, a plate exchanger, 2 wells (one for water in and the other for water out), expansion tank, well pump and controls. This is used for heat rejection from the main circulating loop. The heat addition is done by an on site boiler. My question is when the BMS calls the heat rejection to begin operating there is a loud water hammer sounds that occur. I am thinking that it could be the pump is being activated before the bale is fully open causing the hammer. New pressure tank was installed for years ago. Time delay on pump controls? Any thoughts would be great...