The addition of a boiler in your water system may make a difference, so hopefully HJ who is highly knowledgeable in boilers will respond to this, too. In a basic water system you need an expansion tank if the system is closed. This means there is a check valve located in the cold water supply. This can be a backflow preventer or a PRV. In a closed system, the exansion tank is placed in the cold water supply line between the device that checks the backflow and the intake to the hot water tank.