Hayward UHXNEGVT13501 Negative Pressure H350fd Indoor Vent Adapter Vertical Replacement Kit for Hayward Universal H-series Low Nox Gas Heaters 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% Savings by Reducing Pump Run-Time 2inx 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