Very few MVHR suppliers offer proper ‘installable’ designs because very few install their own systems, most delegate responsibility when you’ve paid for their materials. Even fewer guarantee their systems will perform as designed after installation, and comply with all associated regulations. That’s not just Part-F (ventilation), if you need to drill holes it’s Part-A (structure), if you’ve more than two floors it’s Part-B (fire safety), if you’re close to a neighbour it’s Part-E (acoustics), if you’re near a boiler or flue it’s Part-J (combustion appliances), if it’s a heavy machine on a suspended floor or something need hanging from the ceiling it’s Part-K (protection from falling), and if it uses electricity it’s Part-L1 (energy conservation), or L2 if commercial.
Indicative designs are easy to spot because there’s normally a disclaimer hidden in the small print or the corner of the drawings. “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. But if you’re unsure, just ask them – can you guarantee the installed system will run below 30 decibels and consume less than 1.5 Watts per litre of air movement per second? i.e. Part-F and Part-L. No harm reminding the designer it’s now also their duty to identify overheating risks under Part-O.
NB, It’s not the ventilation designers responsibility to identify fire risks, only to mitigate risks identified by someone else with the necessary expertise, qualifications, and insurance. Someone is responsible for Part-B compliance on every job. If it’s your job and you don’t know who that person is, take a guess..
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.
Even if it looks 3D when you get it, if it was designed in AutoCAD or any other 2D drawing package, it’s 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 something like Revit, and producing that model normally costs more than most quick indicative designs.
A 3D model means the designer can see all potential obstacles from all angles, because without that you’re relying on guesswork. Guesswork means nasty surprises when you’re on site working to a tight schedule, surprises force the installer to deviate from the design, effectively the pipe fitter becomes the designer, and that means you lose accountability. 3D designs make it easier for the installer to see every component from every angle, so providing the design is correct there should be no reason to deviate from it.
Lets assume you still think a £200 design will save money compared to a £2,000 design, the person producing the cheap design will cover enough bases to get the job done, and you’re happy to take the responsibility off the designer. Consider what happens when things start to unravel on site.
The MVHR design shows a straight pipe running from A to B. But it can’t go from A to B because the designer missed a steel, a chimney breast, a gas flue, underfloor heating manifold, electrical consumer unit, or some other impenetrable object. The installer is forced to add four more bends to circumnavigate the issue. Each bend adds 15 pascals air resistance, so you just added 60 pascals to a system that should ideally have less than 150 pascals in total. More pascals means more energy and more noise (forever), and could take a year off the life of your motors.
While the installer is waiting for the supplier to send more materials, they’re wasting their time and normally someone else’s. Your time, the site managers time, the electrician, plumber, floor screed, etc. If that deviation from the design affects other fixtures and fittings, ceiling heights, or window datums, now you’re architect may need to get involved. All of a sudden that cheap design doesn’t seem so cheap any more.
It’s always easier to plasma-cut holes through steels, or pillar-drill holes through a SIPS frame, before they’re delivered to site. But you need to know exactly where to cut the holes, and that means you need an accurate design before you order anything structural.