Hayward H400FDN Universal H-Series 400,000 BTU Pool and Spa Heater, Natural Gas, Low Nox Standard Cupro nickel heat exchanger for superior value and reliability Patented polymer header and bypass valve design for the ultimate in hydraulic efficiency and reliability Hayward Universal design for easy and versatile professional installation Environmentally responsible low NOx emissions to meet or exceed air quality emission standards in all Low NOx regions