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A better ducting option for Passive House

The Passive House concept is rapidly growing in popularity, and naturally this has resulted in an increase in an output of high-quality materials and components to meet the Passive House standard.

Innovations in mechanical ventilation with heat recovery (MVHR) have played a pivotal role in achieving Passive House certification and continue to be at the forefront of the Passive House movement.

The installation of branch steel ducting in Passive House design has long been held up as the most reliable method for MVHR. But while this tradition has many advantages, it does not mean it is necessarily the most beneficial for Passive House. Having worked closely on delivering new concepts in ducting technology over the past 14 years, I have witnessed first-hand the benefits of radial semi-rigid ducting – which I believe far outweigh their steel counterparts – and feel that key specifiers, architects and consultants should now be looking to this as a linchpin for their future designs for Passive House.

Why is this? Well, if you take the example of a two-bedroom residential property, the installation of a radial semi-rigid ducting system averages at around a maximum of 20 pieces of component parts using a 10 port distribution manifold.

Compare this with branch steel ducting, which averages at over 100 pieces for the same property, and it is easy to see that instantly labour intensity and therefore expenditure are dramatically reduced.

While all these savings are of great benefit, the semi-rigid ducting will also deliver almost no possibility of air leakage – absolutely fundamental to Passive House design and passing the certification.

Looking at the design credentials for a traditional branch steel ducting system versus the radial semi-rigid ducting, there are many reasons to opt for the latter wherever possible.

The dedicated duct run to each room delivered by a radial ducting system allows individual rooms to be commissioned to extract or supply air at exactly the correct rate without having a secondary effect on other rooms – a function not possible with a traditional ducting layout.

And while installing steel ducting requires numerous labour-intensive cuts, cross-talk attenuators and solid formed bends, semi-rigid ducting – currently supplied in 20m or 50m lengths – can be cut to length with a craft knife or small hand saw and negates the need for clumsily navigating ceiling joists allowing for lazy bends.

This means less resistance to air movement, a finding that has been proven by independent tests carried out by the BRE.

This ultimately results in a reduction in the motor speed, meaning less noise, more energy efficiency and lower running costs – all essential for achieving the Passive House standard. The size of rigid ducting relative to the velocity limitation of 3m/s for main runs also means it is unlikely to fit into larger properties, which is where semi-rigid ducting comes into its own. I still believe rigid ducting has a place, but only in much smaller plots where small ducting can be accommodated.

Although the style of ducting system installed in new properties is often down to preference or cost considerations, the benefits of radial semi-rigid ducting are especially pertinent for EnerPHit retrofit projects.

It offers the flexibility to solve even the trickiest of installation issues, and along with its robust qualities and simple maintenance – you can even clean it – I believe semi-rigid ducting is an innovation set to expand far beyond the future of Passive House.

Rupert Kazlauciunas is a senior technical adviser at Zehnder Comfosystems

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