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In this picture you have a Weil McLain gas boiler, showing is the filtrol expansion and fill system with the water line connected to it to the left of the red handle valve is a check valve, a check valve prevents water from going the wrong way in a pipe, the person who installed this must have thought it was ok to use this flow check valve in place of a back flow preventer that is required for connecting a water line to a boiler, but a flow check valve is not the same as a back flow preventer, and does not have the required air gap. water could back up into the drinking water with this type of setup. A real plumbing and heating contractor would not have made this mistake.