Despite their benefits, tankless water heaters have been a tough sell in retrofits due to high installation costs, according to Noritz, a hot water heater manufacturer. The NRC661, part of Noritz’s Eco-Tough series of gas-fired residential water heaters, delivers 6.6 gpm using a 1/2”-diameter gas line up to 120’ in length. Conventional tankless heaters typically need gas lines with a minimum diameter of 3/4”, which existing homes may not have readily available, according to Noritz.