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

Compliance doesn’t just mean Part-F (ventilation), if you need to drill holes through structural elements you need to comply with Part-A. 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 hang it below a ceiling it’s Part-K, protection from falling. And every system must comply with Part-L1, energy conservation.

Regulation 7 applies to Workmanship and requires you to evidence the competence of those who work for you, meaning the MVHR installer should be qualified by a Competent Person Scheme, not just the commissioner. And then there’s Part-O (overheating) to navigate for new builds approved since 2022. It may not be possible to rely on opening windows for thermal purge at night because of external noise, meaning you may need a lot more mechanical ventilation than you think. Part-O can significantly impact your ventilation spec.

The new Building Safety Act states that if any future occupant suffers a physical or financial loss because something is non-compliant, the ‘Duty Holder’ is liable for damages for a decade or more. And if it’s your property then you’re the duty holder until someone else is nominated.  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”
“The design may need to be altered due to site conditions”, or
“Alterations are the responsibility of the installer”

These get-out-of-jail cards protect the material supplier, not the customer.

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.

Some two-dimensional design tools will produce a 3D render, but that is not the same as a native 3D BIM/Revit design. It’s just eye-candy for sales purposes. For a system to fit exactly as designed and therefore work as intended, it must be designed in three-dimensions from the outset. But that’s only possible if you start with a 3D structural model of the building. Something that allows the designer to see potential obstacles from all angles, so changes can be made to the building design before things are actually built. Installing a system virtually is how you avoid guesswork.

Working in 3D may add cost initially, but if it avoids delays and problems on site the overall project will be less expensive not more expensive.

Let’s say you have two options, a £300 design fee or a £3,000 design fee, which one saves you money? You might think the first saves £2700, but when your installer discovers it won’t fit and needs to go back to the designer, architect, structural engineer, or building control, therefore holding up the following trades and upsetting the build plan, £2700 will seem like small change.

What happens after the build if a neighbour complains about the noise from your intake or exhaust terminals because they’re too close to their property. The £300 designer didn’t study the site plan or consider road noise, local air pollution, or the proximity of other buildings, chimneys or flues. Dealing with any of these problems after-the-fact can be very expensive. Moving exterior terminals is often impossible, and you can’t just turn the system down a bit to make it quieter because then you’ll be under-ventilated.

What happens if £300 design shows ducting running through an uninsulated loft or eaves 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 a 3D model? Insulating duct work running through a cold space is expensive and not very 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.

What happens if Building Control ask for details of your mechanical purge system because the windows in your habitable rooms don’t open sufficiently? The £300 designer didn’t mention that MVHR is only part of the ventilation strategy, and for full ADF(1) compliance your window apertures will need measuring and certifying too. Retrofitting mechanical purge through MVHR is practically impossible. You may need a completely separate system.

Sadly these are not hypothetical problems, they’re common problems that occur in the interests of saving money. But when you consider the expertise and experience it takes to avoid them, and the amount of time £300 actually pays for, it shouldn’t really come as a surprise.