Enclosing a twin wall factory metal chimney

Brian Bailey, senior product evaluation officer Answers Common Questions on Enclosures for Twin Wall System Chimneys

Twin wall metal chimney systems enclosures: fire-stopping & the use of ventilation openings in the enclosure wall

There are instances where installers want to know if it’s ok to have ventilation openings in the walls of the enclosure as these are sometimes seen as providing a heat source in an upstairs room.

For several years now HETAS Technical Helpline position is not to install an air vent in an enclosure if a ventilated fire stop is required by the flue manufacturers as part of their system design. The issue for us is that ventilated fire stops comply with Approved Document B; preventing fire transmitting from one room to another in the event of a fire, but they do not prevent smoke passing through them. ADB prior to the 2019 amendments smoke was never a consideration, but it was for us.

Approved Document B Volume 1 2019 edition page 49 states: –

B3 (4) The building shall be designed and constructed so that the unseen spread of fire and smoke within concealed spaces in its structure and fabric is inhibited.

Intention (e) States: –

Inhibition of the unseen spread of fire and smoke in cavities, in order to reduce the risk of structural failure and spread of fire and smoke, where they pose a threat to the safety of people in and around the building.

Enclosures serve two main purposes (1) as a barrier to protect occupants from touching the hot surface of the chimney and (2) to provide an aesthetic surround to hide the unpleasing appearance of a bare metal chimney surface. Their secondary function may be to maintain fire-protection between room compartments within a dwelling, as described overleaf.

When installing an internal system chimney in a storied dwelling it is always necessary to make holes in floors and ceilings to pass the chimney up through the floor(s) to exit through the attic and roof.

The act of making a hole in the property like this requires the fire integrity of the original structure to be maintained, i.e. it should not be compromised. The installer can make the penetration holes fire-safe by way of fire-stopping as supplied by the chimney manufacturer.

Enclosures can be made from combustible materials such as plywood or plaster board with timber struts and/or batons. As explained previously, their purpose is to protect the occupants from touching the surface of the hot chimney and for cosmetic purposes.

As long as designated clearance distances to combustible materials of the chimney system are maintained between the materials of the enclosure and the outer surface of the chimney system then this is an acceptable construction. Care should be taken observing Gxx distances, as these may differ for the system in ‘free air’ and within an enclosure.

Where the enclosure is made like this it will be necessary to fire-stop the penetration of the floors and ceilings at each end of the enclosure to maintain fire safety compartment integrity. This must be done using the tested firestop supplied by the system chimney manufacturer. In such cases we advocate the use of an access panels/ hatches to inspect any concealed joints within the enclosure. One access panel/hatch should be sufficient in a run of 2.4m – floor to ceiling dependant on the type of inspection equipment being used, but more may be required.

The HETAS training manual and Approved Document J reference means of access:

Any means of access should not impair any fire, thermal or acoustic requirements of the Building Regulations. Where necessary, inspection panels or hatches should be fitted with resilient seals and provide a similar standard of thermal and acoustic isolation to the surrounding structure.

Under no circumstances can the inclusion of ventilation in the enclosure walls be used to reduce the established clearance distance to combustible materials as declared by the manufacturer in their Declaration of Performance in the designation of the chimney.

Use of manufacturers solid fire stop:

Where chimney manufacturers supply a solid fire stop for their system, it may be possible to either have a boxing with air vents installed in the box or alternatively have the twin wall flue shielded by a perforated sheet metal guard. This is the only method that will allow heat from the flue system to enter the room (normally a bedroom) and at the same time protect the occupants or any combustible material placed by it from accidental contact with a hot surface. Ref BS15287.

On a final point when in doubt, registrants should contact HETAS Technical helpline or consult manufacturers for further guidance.

Leave a Reply

Your email address will not be published. Required fields are marked *