Here typical connection to horizontal and vertical hot water unit heater including hot water supply , hot water unit , unit heater , boiler will have a pipe attached to the water heater. This pipe circulates hot water from your boiler to a heat exchange within the water heater. This exchange then heats the water in the tank and gives you anywhere from 30-100 gallons of water to use when you need it. From the Manufacturer. Eastman's Steel-Flex 3/4" FIP x 3/4" FIP x 24" Water Heater Connector is the standard in water heater supply lines. for more details of Typical connection to horizontal and vertical hot water unit heater download this file.