Rheem RTG-64XLN Outdoor Natural Gas Tankless Water Heater Residential Tankless Water Heaters - Rheem tankless gas water heaters offer you the perfect solution to heat your water at home. Imagine never running out of hot water again. Rheem provides to you innovative, new tankless products that provide a continuous supply of hot water from energy efficient and space saving design units. Specifications - 6.8 gal./min. at 35A-Degree F rise max. - 5.3 gal./min. at 45A-Degree F rise : - Outdoor Installation Only. - New. Next Generation Burner Technology - Industry Best. .26 GPM Minimum Flow Rate.