Ventilation Ducting by Polypipe Ventilation
By Jon Hill, Technical Manager, Polypipe Ventilation
Polypipe Ventilation manufactures the Domus range of high quality ventilation solutions for domestic and light commercial applications.
The Domus range has been established for 25 years and during this time we have been providing expert ventilation advice to designers and contractors who come to us for technical help. As a result, we often get asked the same questions on a regular basis. We have answered below some of the most frequently asked questions, some as a result of new developments in the market and some of a more traditional nature. We are sure you will find them all beneficial.
Q: The NHBC have recommended I fit a condensation trap to my ducting; how does it work and where does it go?
A: When extracting warm moist air, for example from a shower room, through a cold space such as a loft, condensation will form on the inside of the duct, run back down and may damage the electrics of a fan or cause damage to decorated surfaces. The trap has a gutter inside that collects the moisture and an overflow port then allows condensate build up to drain to the outside. The traps are installed in the loft space as low as possible on vertical ducts but high enough to allow for a slight fall in the overflow pipe. To avoid condensation build up in horizontal ducts, insulation with an equivalent or better thermal conductivity value of 0.04 W/mK should be applied to the ducting.
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A: It’s possible that you have fitted the wrong size duct. If the duct is too small for the volume of air that is being moved, intrusive noise will be generated. CIBSE has determined that to avoid this occurring, air in domestic ventilation systems should move at maximum velocities of 3m/s for bedrooms and 8m/s for kitchens. By using a simple calculation of maximum fan extract volume divided by maximum acceptable velocity, the cross sectional area of the duct is easily determined. It is a simple step to work out the diameter of a duct from its area; always remember to use common units of measurement and to round up to standard diameters or their rectangular equivalents.
Q: How are plastic ducting joints sealed?
A: If the duct doesn’t need to be dismantled then the joints should be sealed using an appropriate acrylic or silicone mastic; this gives a superb seal and stops any condensation leakage. If the duct has to be dismantled at regular intervals, for example cleaning in a commercial environment, then the duct should be sealed with duct tape. On no account should any sort of mechanical fixing be used as this could corrode and cause leakage around the screw thread. Also, due to the variety of plastics used in domestic ducting it is inadvisable to use plumber’s solvent weld. For standard installations the push fit connection does not need to be sealed providing high quality duct is used, manufactured to exacting tolerances ensuring a tight and secure fit between joints. High quality duct will help prevent moisture and air leakage. The fabric of the building will be preserved and the full efficiency and effectiveness of the extraction appliance is achieved.
Q: I am installing an exhaust air heat pump and need to have provision for replacement air into the property. How is this achieved?
A: Building Regulations consider these to be System 3. They state that provision for make up air should be made in every habitable room other than wet rooms at a minimum of 2500 square millimetres of equivalent area per room inlet, ideally mounted at over 1.7m to avoid draughts and discomfort; generally it should be left permanently open. This can be achieved using traditional window mounted trickle vents, ensuring they are not blocked by curtains or blinds or by purpose designed wall mounted, draught free, air supply kits designed to match exactly the air usage requirements of the heat pump.
Q: Can I connect two individual fans into one duct run?
A: This is permissible under current Building Regulations; however care must be taken to avoid exhausting through the other fan instead of to outside. This can be achieved by installing fans that have backdraught dampers built in, such as domestic centrifugal surface mounted fans – ideal for this type of application. Alternatively, duct system connectors with built in gravity non-return flaps can be installed after each fan. It should be noted that the single duct should have a geometrical free area of the sum of the two feeder ducts to provide sufficient flow if both fans are running simultaneously.
Q: I have to pass a duct through a fire rated wall. What fire-stopping methods are available to me?
A: Current Building Regulations state that when a duct or any other service passes through a fire rated wall, or to use the correct terminology, a fire-separating element, it should be adequately protected by fire-stopping so that the fire resistance of the element is not impaired. Furthermore, only fire-stopping systems which have been shown by test to maintain the fire resistance of the element may be used. In a masonry wall, intumescent wraps may be used as the masonry will contain the expansion of the intumescent. Alternatively, steel intumescent-filled collars can be fitted around the duct on both sides of the wall. For stud walls the only suitable solution is to employ steel sheathed intumescent duct connectors; the steel sheath contains the expansion of the intumescent within the stud wall cavity. This type of product is the most suitable for all applications as it forms an integral part of the duct system as well as providing fire-stopping. It is vital in this safety critical field of application that the fire-stopping products used have been tested by a reputable test establishment such as BRE on both the size and type of duct being used and in the type of fire wall being constructed. Make sure you ask for those supporting test reports and check carefully if the test data matches your intended installation; you will be surprised how often they don’t, especially with wraps and collars. Contractors can be held liable for installing a product that is not fit for purpose.
Q: Why does fitting a tight bend adversely affect the performance of a fan?
A: Like a car on a road, for optimum extract performance a duct run should be as straight and as short as possible; however, we realise that this is rarely the case. When using bends to avoid an obstacle, consider using multiple gentle 45 degree bends rather than sharp 90 degree bends as these have approximately a quarter of the resistance to the passage of air as a 90 degree bend and will improve your fan’s overall efficiency.
Q: How can I prevent cold draughts entering a room through the extract vent?
A: There are a number of ways of preventing this. The first is by fitting a simple gravity flap outlet in place of an open fixed louvered grille on the exterior of the building. However, if the outlet is mounted on a very exposed wall, a more robust single gravity flap outlet with protective cowl should be installed. If access to the outside of the building is difficult, a gravity flap built in to a duct connector should be employed in the duct system. These products are often used as non-return valves in larger duct systems and are readily available.
Q: The extract fan is not ducted to the outside and is venting directly into the loft space. Is this OK?
A: The simple answer is no. All extract ventilation must exhaust to the outside, as clearly stated in Building Regulations. The exhausted warm moisture laden air will condense forming water droplets in the colder environment causing mould to form in the long term and other associated moisture damage to both the roof structure and the insulation.
Polypipe Ventilation, manufacturer of the market leading Domus range of domestic and light commercial ventilation products, has been at the forefront of the ventilation ducting industry for almost 25 years and is well known for its quality and innovation.
For more information, please go to: www.polypipe.com/ventilation or contact Polypipe Ventilation on 01799 540 602.
