There are several manufacturers of tankless hot water heaters, among them Bosch, Rheem, Rinnai, Noritz and Takagi. Tankless water heaters are rated by the number of gallons per minute they can heat. A heater that can effectively warm more water per minute is more suitable for a larger home or a home with more inhabitants. Tankless water heaters use either a natural gas burner or an electric heating element to warm the water; those using natural gas typically have higher heating capacities. Some tankless heaters carry the Energy Star certification.