We self-installed ours (I did most of it myself) and found it fairly easy. But we were a new build, with metal web joists etc. Installing the unit itself is very easy; just a couple of screws for the bracket and some wiring, ducts and a drain. I plugged mine into the ring main although I think most people have a fused switched outlet. Installing the duct work was reasonably easy with the semi-rigid stuff, just dragging it through the joists. The terminal vents are screwed up where the plan says (it is important to have a proper plan with drawing and schedule) and the ducts just push on.

External ducts were built in as we built the house. Ceiling plaster was put in after the internal ducts and noise insulation. IIRC I put the ducts in before we put the resilient bars up, which simplified dragging the ducts through.

Commissioning is the slightly tricky bit. I did that with my architect, who was interested. Mostly it's just checking flowrates with a calibrated meter and filling in the forms, but there was the odd adjustment we had to make.

Thanks all. I'm an engineer so I'm sure I can more than manage it but I just to factor in time needed so I don't slow the project down. I should have my final build plans today so I'll send them out to suppliers when I receive the plans.

We bought ours from the Green Building Store (who offer a very good design service) but they were too far away for it to be economical for them to install. So they recommended a more local installer (Earthwise in Bristol) who did a great job with our fairly complex arrangement.

Notably though the installer couldn't offer the commissioning at each terminal to ensure that the system was balanced etc. So we ended up with a third party to undertake this work and provide the evidence for our passivhaus certification.

As others have said you can do parts of this yourself, but I'd recommend getting the final commissioning bit done by a professional so you can be sure it is all working as designed.

One final point is timing - we got our ductwork put in quite early as we had other things to work around it, but the general recommendation is to only actually start the MVHR working once all the dusty jobs on site have been done.

Inevitably some things end up happening later (with plasterboard dust and sawdust being particular culprits), so make sure you have a good way of masking over terminals to hand. The design of our ceiling outlets meant that those disposable plastic overshoes worked well as a temporary cover!

They may well lend you a manometer too

I bought from BPC, ok but not impressive, got the impression everything they did was at breakneck speed, they clearly didn’t read emails to them and i think i had 3 different contacts over the time between initial enquiry and order. Never did get calculated flow rates that were promised, when you get their list of components , go over it carefully and price stuff on line, some items were marked up considerably , also measure out the length of ducting they suggest you need , i have over 100m left over and i was pretty generous with allowances at each end especially the manifold end. All that said, as a supply only service , everything fitted and works.

I bought a Titon unit they gave me a rough sketch of layout and position of outlets. I used Domus rectangular rigid they do a range of sizes in rectangular round and adapters. There is a special adhesive for them as well. I bought them from BES. I found Lindab outlet metal valves best some plastic ones I originally bought were rubbish. When fitting the valve try and get a solid termination at the ceiling so you can fit the valve into it. As I was doing the plaster boarding I was able to get a precise hole in the right position for the end of the duct. Installers tend to use a flexible pipe with worm clips which to my mind is a poor job. You can balance the system yourself if you get a kit like this.
https://www.test-meter.co.uk/extech-an300-anemometer

Thank you all. Some great advice and tips included. Thank you again.

BPC also for me, we have a Vent-Axia Sentinel.
I found self-installing easy but then I consider extreme DIY to be one of my hobbies, plus I'm too tight to pay anyone to do anything that I think I can do as well or better - unless it needs certifying for safety (plumbing, electrics)
This was in a stone barn conversion, so not straighforward.
The hardest part for me was making a hole for 4 x 75mm pipes through a 450mm thick stone wall (to avoid having 4 lots of 90 degree bends on each pipe, which was my initial plan - to use an existing opening)
Also chiselling the sides off the floor joists to accommodate the manifolds in the f/f floor & g/f ceiling void. This was my choice to do this and to put them there, I got the exact measuremtns before I purchased them.

Luckily I had 2 existing holes through the wall for the supply and extract ducts (insulated) they just needed widening a tad to take a round pipe.

I ran all the pipes parallell to the f/floor joists above the utility & hall, then along the walls inside the depth of the interior wall insulation to each room. It does mean that there are places where you can't screw into the walls!

For commissioning I hired an anemometer. There is some good info around the internet or I can send you the info I collected. YouTube was a godsend for how to access the control settings on the Vent-Axia, I don't recall seeing a user guide with it.
Once you have it all set up and all the info to provide to Building Regs, you will probably want to turn down the 'Normal' setting but leave 'Boost' as it is.

Not much to add but
- BPC (who did me a design). Did get a design from a second company for comparison/pricing.
- self installed with semi rigid
- hired a meter to set up.
- Vent Axia unit

Even the semi rigid pipe takes up a lot of space especially as it congregates around the unit. Ours was well planned into the design but still had to make some on site alterations for it to fit.

What I'd change if doing again?
- Vent Axia unit is very plastic-y. But it's 8 years old and still working fine. Uncle's Genvex is like a Mercedes compared to a Dacia. But they're a multiple of the price. So not sure....

- my connections to outdoors are much longer than optimal but was unavoidable. I used 150mm Domus plastic pipe with their preformed polystyrene insulation around it. I think there are much better options now - and metal actually looks like it could be cheaper. Then can use various forms of flexible wrap insulation.