Replacement Flapper Gasket (Air/Fuel) for Vitodens 100-W WB1A (24/30) Series Residential Wall-Mounted Gas Condensing Boilers. Any adjustments, installations, service or maintenance must be completed by a licensed professional heating contractor who is qualified to work with hot water boilers. When ordering replacement parts, check with the manufacturer and provide the 16-digit boiler serial number or the 12-digit ASME/NB serial number to ensure a proper fit before purchasing.