NVS Limited

Twin Wall Stove Pipe, Gas B-Vent, Installation of B-Vent


Mandatory requirements

Connection to an appliance which is not connected to the fuel supply may be carried out by a competent person. However, connection to an appliance that is connected to the fuel supply must be carried out by a Gas Safe Register installer. The flue system must be installed to comply with Building Regulations Document J (in England, Wales and Northern Ireland) for gas appliances having a flue gas temperature of 250°C max, and the Building Regulations for Scotland. The installation must also comply with BS5440 pt 1 : 2000 for gas flues up to 60 kW in the UK and IS813 Domestic Fuel Installations in Ireland.


All pipe lengths and flue gas carrying components are joined together by a twist lock, bayonet system. The system should be installed with the visible male collar pointing upwards, this is reaffirmed by the directional arrow pointing upwards, indicating the directional flow of flue gases. Taping of joints is unnecessary. B Vent is for atmospheric appliances which have negative pressure flues meaning that when the system is primed and running at normal operating temperatures, air is drawn into the flue via the joints, assisting safe evacuation of flue gases.

Adjustable Length

Within the range is an adjustable length which is used to telescope over standard pipe lengths to provide the exact flue lengths required. It should not be used directly after a bend since there is insufficient overlap to insure a sound joint. A wall band must be used above an adjustable length as this component is not loadbearing.

Connection to Appliance/Flue Block

Always use an appliance connector, sealed using fibre rope and fire cement or high temperature sealant. The liner should not project below the appliance outlet spigot and can be cut to length if required.

Appliance Removal

Use of a pipe and an adjustable length immediately above the appliance enables removal of the appliance later without dismantling the full system.

Painting of B Vent

If required to be painted, simply clean the surface with a solvent cleaner (White Spirit), apply a coat of primer and a top coat of high temperature paint e.g. enamel.

Recommended distances to combustibles

In accordance with the Building Regulations, a minimum of 50mm distance to combustibles must be maintained. B Vent support components provide a 50mm clearance. At the maximum flue gas temperature of 260°C, the outer case will be in the region of 75-85°C.

Support components

Internal systems should be supported by using a support assembly fixed on top of the first floor/ceiling joist. A Firestop plate is also required fixed to the ceiling below. The clamp plate and firestop have tags fixed to ensure 50mm distance to combustibles. In a normal house, when passing through the second floor the only requirement is two firestop plates because the system is adequately supported at first floor level.

Wallbands are not load bearing and give lateral support only. Wallbands should be fitted every 3m to ensure the system is rigidly held. The system should be braced with a wallband immediately below passing through the roof line to ensure the flashing does not suffer lateral pressures.

Ensure that no joint occurs within the floor space. A roof plate (2 piece) should be used on the underside of roof trusses where the system is terminated via a flashing.

The maximum height unsupported above the roof line is 1.5m. Where a joint is above the roofline it should be determined that in extreme wind conditions this joint would not be over exerted. If there is any doubt then a guy wire should be used. Beyond this guy wires should be installed every metre.

Loft and Ridge Connection

Use a flue block adaptor to start from the flue block. The connection to the ridge terminal should be sealed using a gasket or high temperature sealant to provide a gas and condensate tight joint.

Connection of B Vent to an existing chimney

B Vent is not for use as a chimney liner, however, it can be used to connect to and from a Wonderflex/Triplelock flexible flue liner which may be lining an existing chimney, when used on appliances described in previous section.

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