> What qualifications do you need?

For designing it, to be able to read a plan, work a spreadsheet and have a bit of common sense with regards to wind speeds in the ducts. If you have a cluster of rooms that need higher flow rates, positioning the manifold of a semirigid system near those rooms means the duct lengths act to balance the system ; shorter ducts lead to higher flow rates by simple friction

For installing it, chiefly the ability to read the installation guide that will accompany the system (mine came with a 6 page A4 stapled sheet with tips like "make sure you fit a rubber seal between the pipe and connector - amusingly something the first builder did not do, because he didn't read the sheet) and have some common sense about building, tool use, minimising bends and uses of flexible foil sections

For commissioning it, the ability to hold a device shaped like a megaphone over the vent and read a number off the back, turn a screw and have a bit of patience going round and round all the vents (adjusting one affects the others so several passes are required), plus a working understanding of percentages (for any given fan speed you may not hit the flows in the spec but as long as the proportionate percentages are equivalent you can then adjust the fan speed to reach the targets. But then you'll be running the system at half spec anyway to avoid over ventilating... my BI just wanted to see a sheet of measured flows vs soec'd flows and even then he didn't care too much about them matching because on a conversion project they don't have to. On a renovation I'd be surprised if you even have a building inspector for this aspect let alone him taking an interest in it