• In this article we are looking at the correct installation of extract ducts. These can be either rectangular or round. We will also be looking at the correct installation of flexi-ducts where they are used as part of an extract system.

    These articles reflect defects which are currently and commonly picked up by FCM’s Clerks of Works during their site inspections.

    Our Clerks of Works are your "eyes and ears on site", helping to get it right first time.


    Correct installation of extract ducts


    Manufacturers installation instructions are very often overlooked or ignored which leads to poor system performance and later defects.


    Rectangular duct

    Unsupported duct without vapour sealed joints.


    Manufacturers and NHBC state that rectangular duct must be fixed at 750mm centres on runs and a maximum of 300mm from any bend.

    Also all joints must be vapour sealed. This is usually seen as silver tape.
    Sub-contractors will often claim that they are glueing the joints but this in not satisfactory unless the system is designed for glueing. There are systems with air tight metal joints but these are less commonly found due to cost.


    Correctly sealed duct

    Vapour sealed joints.


    Fixings should not crush or distort the duct which will cause air-flow leakage and there should be no fixings into the duct which would cause the same or condensation leaks onto ceilings below.

    Ductwork  should  not be in direct contact with other surfaces, such  as plasterboard ceilings,  that may transfer noise  to the dwelling.


    Damaged duct

    Screws fixed into duct


    The main photographs and the one above show incorrectly used fixing straps (cut short) which have been fixed into the duct (causing condensation leaks) and inadequate fixings to support the duct.

    The only fixings into the duct should be those supplied to join sections together.

    Extract ducts should be insulated to avoid internal condensation if the ducting is running through an unheated void. Above MF ceilings are usually acceptable as heated voids.

    Round ducts

    Round duct with too many bends and a condensation sump


    Ducting (round and rectangular) should be installed with a slight tilt towards to outlet to avoid the risk of condensation. Consequently there should be no low points like in the above photograph.

    Bends and corners must be kept to the minimum. Excessive bends will severly reduce the ettectiveness of the system.

    The above could have been installed with far fewer bends with a bit of thought.



    Friction is the enemy of air flow


    Manufacturers fixing instructions state:

    • Use the minimum length of flexible duct to make connections and fully extend the duct.    30% compression of the duct can increase resistance by 400%.
    • Avoid bending ducts across sharp corners or incidental contact with metal fixtures, pipes or conduits.  Radius at center line shall not be less than one duct diameter.
    • Sag between support joints will have serious effects on system performance due to the increased resistance each introduces.
    • Flexi ducts must be fixed to the soffit in the same way as rectangular duct unless they are fully supported from below.


    Below shows a flexi-duct connection onto a rectangular section.

    Flexi-ducts with bends


    If flexi ducts must be used to form a bend, the length must be kept to the minimum and be as as fully extended as possible.

    The internal diameter of the duct must remain constant.



    Flexi ducts must not pass through walls


    They must be terminated and connected to a proprietry fitting or re-connected to a length of duct.


    Below is a short video on flexi-duct and it's drawbacks.



    Due to the number of systems in use on sites it it most important to check the particular manufacturers installation instructions for your system.

    It has been difficult to obtain up to date data from manufacturers recently. The above is based upon information from Manrose, Nuaire, NHBC and internet rersearch.
    If you disagree with anything in the article please let me know and I will update as necesary with new information recieved.



    Steve Farrell
    T: 07714234380
    E: stevefarrell@fcmltd.co.uk
    L: linkedin.com/in/stevefarrellfcm


    Fox Curtis Murray are a building consultancy specialising in providing Clerks of Works services and Quality Control inspections.

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