[rohrmann]

That VALVE opens up the line to the suction side of the water pump so when winterizing, and you have already bypassed the water heater tank, a hose into a bottle of antifreeze and the water pump turned on, will put antifreeze into your plumbing. You of course have to open individual faucets to allow the antifreeze to travel to the various faucets. The picture below is a plumbing diagram I drew up some years ago of the plumbing looking at the rear of the convenience center. The blue lines in the pictures of the valves indicates normal water flow, so on the Winterize valve, when turned to Winterize, you can follow the blue water line directly to the suction side of the water pump.