Beware of hidden disclaimers

Very few MVHR suppliers offer proper ‘installable’ designs because very few install their own systems. Most transfer accountability to subcontract installers, who then inspect and commission the system, effectively marking their own homework. Not many designers can guarantee their systems will perform as designed after installation, or that the process of installation will comply with all associated regulations.

Compliance doesn’t just Part-F (ventilation), if you need to drill holes you it’s Part-A (structure), if you have more than two floors it’s Part-B (fire safety), if you’re close to a neighbour it’s Part-E (acoustics), if your pipes exit the building near a flue or chimney it’s Part-J (combustion appliances), if it’s a heavy MVHR on a suspended floor or is hung below a ceiling it’s Part-K (protection from falling), and if it uses electricity it’s Part-L1 (energy conservation). Regulation 7 applies to Workmanship and requires you to evidence the competence of those who work for you, and since 2022 there’s a new Part-O (overheating) regulation that may impact the ventilation.

There’s also a new Building Safety Act that says if any future occupant suffers a physical or financial loss as a result of something that doesn’t comply, the principal designer, builder, or more likely the home owner is liable for damages. Since 2022 there’s a lot more at stake, so maintaining design accountability is particularly important. The last thing you need is a hidden disclaimer that says “These drawings are for indicative purposes”, “Placement of [the materials] is for guidance only”, “company [name] disclaim any liability for work carried out”, or the old favourite “The design may need to be altered due to site conditions”, followed by “alterations are the responsibility of the installer”. These are all get-out-of-jail cards for designers more interested in the initial sale than the final outcome.

After you’ve checked their reviews, just ask; can you guarantee the installed system will consume less than 1.5 Watts per litre per second? Without the perfect design and then the perfect install, this mandatory requirement is the first thing to go, not that anyone will be checking.

2D vs 3D designs

Ask any experienced installer why they don’t like indicative designs, and the usual answer is ‘they just don’t fit’. Indicative designs are two-dimensional. They give you a plan view only and don’t account for varied floor levels or intersections, window and ceiling datums, often the direction and type of joists, steels, and other impenetrable structural elements. They’re just lines on a flat drawing to help someone work out what materials to sell you.

Even if it looks 3D when you get it, if it was designed in AutoCAD or any other 2D drawing package, it’s only a ‘3D render’ of a two-dimensional design. It’s not the same. For a system to fit exactly as intended, and therefore work as intended too, it has to be designed in three-dimensions from the outset. That means you need to start with a 3D model of the building in a design package like Revit. It adds a bit of cost upfront, but saves a lot of cost later when things start to unravel on site.

A 3D model means the designer can see all potential obstacles from all angles, effectively they can install the system virtually before pallets start landing on your driveway. Without that ability there’s an element of guesswork, and that means nasty surprises when you’re on site working to a tight schedule with one trade following another. As soon as the installer is forced to deviate from the design, they effectively become the designer and you lose accountability.

It’s impossible to guarantee the outcome unless you already know it’s going to fit perfectly.

Avoid cheap designs

Lets assume you have two options, a £300 design fee or a £3,000 design fee, which one saves you money? You might think the first option saves £2700, but when the installer discovers it won’t fit, it needs to go back to the designer, architect, structural engineer, or building control, holding up the next trade and upsetting the build plan, the £2700 you thought you’d saved will soon evaporate.

What happens if a neighbour complains about the noise from the intake or exhaust terminals because they’re too close to their property, all because the cheap designer didn’t study the site plan? Dealing with noise without compromising performance is expensive and moving the terminals is next to impossible. You can’t just ‘turn it down’ to make it quiet because you’ll be under-ventilated.

What happens if the cheap plan shows ducting running through an un-insulated void, across stairwells or other structurally challenging areas, or through fresh air where you planned a vaulted ceiling, all because the designer didn’t study the sectional drawings, or overlay the joist/steel plan on to the 3D model? Insulating duct work is expensive and not particularly effective. Moving it is difficult and normally compromises performance. Drilling through joists or burning through steel is normally impossible, and lowering ceilings to accommodate pipes can spoil the aesthetics and cause knock-on effects with things like the window datum.

What happens when the individual responsible for signing off Part-B (fire safety) asks to see fire collars that should have been installed but weren’t? If you’re lucky you may only need to knock holes in your finished ceiling to show they were installed. If you’re not the ceiling may need to come down.

What happens if Building Control ask to see details of your mechanical purge system because you have habitable rooms with non-opening windows? The guy selling the MVHR didn’t mention this and you thought the standard MVHR would tick all boxes. Anybody who is willing to sign off Part-F in the knowledge that ALL requirements have not been met should check the new penalties under the Building Safety Act.

When you discover why the design process was so cheap, the normal solution is tell the installer to ‘make it fit’ – see image. The science goes out the window, along with any design accountability. All those extra bends then double the system pressure (air resistance), which doubles the running cost (forever) and knocks it over the mandatory 1.5W/l/s. This strains the motors, shortens their life, and increases noise.

The problems above are not hypothetical, they’re based on experience. Not only are they costly to attempt to fix, they’re painful if you have to pay someone undo and redo a job, like the architect or trades on site. They’ll potentially spoil the end result without saving any money.

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If you prefer to design problems out rather than fixing them on site, work on the basis that every pound invested up-front saves two pounds worth of headaches later
Eliot Warrington
MD and Founder
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