Hayward Universal H-Series H400FDNASME ASME Natural Gas 400K BTU. Hayward Commercial Pool's Universal H-Series ASME gas heaters are designed and certified for commercial use. The flagship of our H-Series line encompasses high-end features and construction materials as standard. They combine industry-leading hydraulics, exceptional performance, energy efficiency and reliability, with planet-friendly low-NOx emissions. Flexible connection options make them ideal for new or existing applications.