Water Hammer coming through auto feed valve?

Thank you so much for clarifying it.

Please understand that while it might be called an auto fill valve there are situations where it will NOT keep water in the system.

The most obvious one is in case of a "runaway" burner that does not shutdown at the appropriate temperature. As the temperature climbs so will the pressure until it lifts the safety valve and releases some water causing the pressure to drop. This will continue as long as the burner continues to operate and eventually the water will be changing to steam and the boiler will go into a low-water condition. During all of this the pressure WILL remain high enough that the "auto fill" valve will NOT open.