Water Hammer coming through auto feed valve?

[MarkB]

Hi cattledog
The hot water tank loop and the heating tank loop do tie in together at the return. I don't see a check valve anywhere in the hot water tank loop. There is a circ pump but with no check valve installed.
With the ball valve fo the auto feed closed the only place domestic water use can influence the system is through 1-the hot water outlet on the tank and 2 the cold water inlet on the tank. The pressure spike must be transfering from the inner tank to the outer one and into the heating system. Maybe the pressure is pulling the swing disk in the return check valve. The banging is followed by an echo...BANG,Bang, bang. which is why I thought it may be that check valve.

[anna_student]

A silly question. I just got installed a brand new hot water heating system in my house. I used your picture to ask the question. Could you tell me what is the device that I circled in green. I know the other valves, etc, but forgot to ask my plumber what is this thing doing. Greatly appreciated. Thanks.

[Furd]

Your "thing circled in green" is a pressure reducing valve that reduces the pressure of the domestic water to about 12 psi to fill the heating system.

[anna_student]

Where in this picture is the auto fill valve that MarkB mentions in one of his posts? What I am trying to work out is whether I have an auto fill valve in my system. In my old steam system it was obvious what part was responsible for automatically adding water to the system. I am not familiar with the hot water heating systems.

What would happen if the pressure in my system dropped. Would the system automatically draw water from the domestic water circuit via pressure reducing valve. My ball valve to the pressure reducing valve is open just like in the MarkB's picture. Thank you.

[Furd]

The pressure reducing valve is often called the auto fill valve.

[anna_student]

Thank you so much for clarifying it.

[Furd]

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.