Hayward HAYH400FDN Universal H-series Low NOX 400k BTU Pool and Spa Heater. The Universal H-Series is an energy efficient, high performance, installation adaptable commitment to the pool service professional. Standard cupro nickel heat exchanger. Excellent salt water and chemical corrosion resistance. Exclusive “Totally Managed” cupro nickel heat exchanger water flow. Same water velocity through all tubes for faster heating. Patented header by-pass design. Up to 18 percent savings by reducing pump run-time. 2in x 2 1/2in CPVC union plumbing connections. Common union for installation or service, adapts to larger plumbing recommended for greater efficiencies. Low NOx emissions – meets clean air quality standards.