In larger hot water systems, when the circulation pump suddenly stops, the water's inertia force can cause a sharp increase in water pressure in the return pipe before the pump, generating a strong water hammer wave. This may lead to valves or the pump being cracked and damaged, and it could also be quickly transmitted to users through the pipes, causing radiators to burst. A way to prevent water hammer is to connect a bypass pipe between the circulation pump's discharge pipe and the return pipe, with a check valve installed on the bypass pipe. During normal operation, the discharge pressure of the circulation pump is higher than the return pressure, so the check valve remains closed. When there is a sudden power outage and the pump stops, the pressure in the discharge pipe drops while the pressure in the return pipe rises. The circulating water then pushes open the check valve on the bypass pipe, thereby reducing the force of the water hammer. At the same time, the circulating water flows into the boiler through the bypass pipe, which not only reduces the pressure in the return pipe but also prevents the boiler water from vaporizing.
How does the Hermann wall-hung boiler prevent water hammer in hot water boilers?
Source: http://blog.tianya.cn/blogger/blog_main.asp?BlogID=3778681