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 mean Part-F (ventilation), if you need to drill holes you need to comply with 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 you need to install a heavy MVHR on a suspended floor or it’s hung below a ceiling it’s Part-K (protection from falling), and if it uses electricity it’s Part-L1 (energy conservation). Regulation 7 also applies to the 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.
The new Building Safety Act states that if any future occupant suffers a physical or financial loss because of something non-compliant, the principal designer, the principal contractor, or more likely the home owner is liable for damages for a decade or more. Design and build accountability is therefore more important than ever. The last thing you need are disclaimers hidden in the small print; “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 get-out-of-jail cards are to protect the salesman not the customer.
Always check their reviews and ask can you guarantee the installed system will consume less than 1.5 Watts per litre per second? in line with regulations. Because if they can’t guarantee it then who can? This and the acoustic performance is how you recognise quality.
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 as a native 3D BIM/Revit design. 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 can only be done if you start with a 3D structural model of the building. It adds a bit of cost initially, but it’ll save a lot of cost later if 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 you can’t avoid an element of guesswork, and that normally means nasty surprises when you least want them, on site working to a tight build schedule with one trade following another. As soon as the installer is forced to deviate from the design to ‘make it fit’, the installer becomes your designer and you lose accountability.
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, because time-limited designer didn’t study the site plan? Dealing with noise without compromising performance is expensive, and moving the exterior terminals is next to impossible. You can’t just ‘turn it down’ to make it quiet because then you’ll be under-ventilated.
What happens if the cheap plan shows ducting running through an uninsulated void, across stairwells or other structurally challenging areas, or through fresh air where you planned a vaulted ceiling, because the designer didn’t study the sectional drawings, or overlay the joist/steel plan on to a 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 often impossible, and lowering ceilings to accommodate pipes can spoil the aesthetics and effect other 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 they were missed the ceiling may need to come down.
How about 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 ventilation 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’. When that happens the science goes out the window along with any accountability. All those extra bends needed can soon double the system pressure (air resistance), which then doubles the running cost and knocks it over the mandatory 1.5W/l/s. This also puts unnecessary load on the motors and adds noise, meaning the system that should have lasted ten years now lasts five.
These problems are not hypothetical, they’re difficult to fix, they’re painful if you need to pay someone to redo a job, and they can spoil the end result without saving any money.
