Schiedel B Vent flue Vent Increaser - 100mm to 125mm Part of the twin wall gas venting system for residential and small commercial gas appliances with input up to 60 kW. Suitable for internal and external use, either as a complete system or in combination with flue blocks or brick chimneys. Must not be used for oil-fired or solid fuel systems. Note that for external runs exceeding 3 metres, BS 5440-1:2000 requires the use of a stainless steel insulated flue. The ICID Flue range meets this requirement.