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Ventilation requirements for pipework in ducts or voids

Do you know how to calculate the amount of ventilation required to prevent a build-up of gas in a floor void or duct? Gas Safe Register technical support officer Katrina Martin sets out the requirements.

This article was originally published in Registered Gas Engineer’s December 2021 issue.

We have all come across gas pipework that runs in floor voids, ducting and similar circumstances. In many of these situations, ventilation is required to prevent a build-up of gas within the void, as this can become dangerous.

Gas pipes in ducts and voids need ventilation to ensure that any minor leakage of gas – which may be within permissible limits and therefore not raise a concern during a conventional gas tightness test – can be cleared to prevent a build-up reaching dangerous levels. BS 6891 advises that the ventilation provided in a duct or void is intended to clear minor leakages and it is NOT practical to provide ventilation that would clear a gas escape caused by a major failure of the pipe. The level of ventilation is normally dictated by the cross-sectional area of the duct, as shown below.

Fire safety
Any ventilation provision must be considered carefully to ensure that it does not conflict with any fire safety measures and other building standards in place for that building, such as noise or air transfer. This means that any ventilation provided for the duct or void should not communicate with any protected area. You should contact the building’s designer or relevant local authority Building Control to confirm whether a specific space or area is classified as being fire-protected.

Protected area
A protected area is defined in Annex D of BS 6891 as an area that has been specifically designed so that the occupants of the building – for example, a block of flats – can escape without being affected by smoke or fire.
Gas pipework installed or passing through protected areas must be one of the following:

  • Steel pipework with screwed joints
  • Steel pipework with welded joints
  • Continuous length of copper with no joints
  • Continuous length of pliable corrugated steel manufactured to withstand fire test A of BS EN 1775:2007 Annex A.

Gas pipework installed in a protected area within a duct that contains an inspection hatch or opening that communicates with that protected area must be at least the same fire rating as the area in which it is fitted. Pipework installed in a protected area shall be ventilated to outside air at both high and low level. The ventilation requirements depend on the size and cross-sectional area of the duct, and therefore Table 6 of BS 6891 should be consulted.

Note: Natural ventilation shall be used in this instance, not mechanical ventilation.

Pipework is not considered to be contained within a protected area if it is separated from the protected area by a fire-resistant duct. The fire resistant properties of that construction must be as recommended by the relevant guidance document. Fire stopping must also be taken into consideration when pipework is installed in a protected structure. If pipework is running in floors or walls, you must ensure that openings are kept as small and as few as practical so as not to impair fire resistance.

Let’s examine how to measure and calculate this:
To calculate the cross-sectional area, multiply the height (A) by the width (B). The length of the duct is not relevant when calculating the cross-sectional area.

So, for instance, if the height of the duct is 50mm (0.05m), and the width is 800mm (0.8m), to calculate the correct cross-sectional area, you multiply 0.05 by 0.8, to give you the answer of 0.04m2.

By referencing the table, you can see that the ventilation requirements for this sized duct is the cross-sectional area, or 0.04m2.

So, the calculation is: 0.05m (A) x 0.8m (B) = 0.04m2.

For pipework exceeding 35mm in commercial-type premises, the ventilation requirements for larger size ducts can be found in IGEM/UP/2 Edition 3, Table 16.


The standards and technical guidance that apply to pipework in ducts or voids

  • BS 6891: Specification for the installation and maintenance of low-pressure gas installation pipework of up to 3mm (R1¼) on premises
  • IGEM/UP/7: Gas installations in timber framed and light steel framed buildings and, where necessary, manufacturer’s instructions, as appropriate.
  • Guidance for gas in flats and other multi-occupancy dwelling buildings can be found in IGEM/G/5 Edition 2 – Gas in multi-occupancy buildings. For installation in commercial and industrial premises, use IGEM/UP/2 Edition 3.
  • Technical Bulletin 113 gives guidance on research into ventilation in modern floor voids.

Work must also comply with the relevant geographical Building Regulations.
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