A. O. Smith claims its new water heater offers the advantages of on-demand and traditional heating appliances without the disadvantages. The Next Hybrid gas water heater is powered by a tankless engine that supposedly can deliver 189 gallons of hot water in the first hour. A 25-gallon buffer tank ensures a constant flow rate during periods of peak demand. Though smaller than a conventional 50-gallon model, the Next Hybrid delivers twice as much hot water, the maker says. It sells for $1,800. A. O. Smith Water Products, 800/527-1953, hotwater.com