Builder only supported on "top" and "bottom"
MVHR unit installed in loft

humblebee hive – retrofit #7

Seal tight, Ventilate right

It’s the retrofit mantra. And we did notice how quickly it got stuffy once the windows were in and the EWI was all but sealed. We were still getting chilly inside, in part because in order to have fresh air we had to have the windows open, while the rest of the work was being carried out.

Because we were meant to start in November but got delayed, I thought I could maybe bring forward the MVHR (Mechanical Ventilation with Heat Recovery) installation and have it done before the spring. But I struggled to find installers and it turned out that the person I had in mind, Erol (whom I’d met at a Carbon Coop matchmaking workshop before lockdown), had joined forces with Richard as a member of his Retrofit Hub, so really it made sense to do it as part of this current phase.

At least I had some time to fit in the calculations, size the ducts and valves – circular adjustable grilles that either push air into a room or suck it out – work out a sensible route with very little floor to ceiling height and research possible high-efficiency units (it had to fit through the loft hatch). Needless to say we ummed and aahed about most things.

Fast forward and today we part-commissioned, and I have to say I was pretty pleased: overall flowrates and pressures were fine (extract fan speed had to be adjusted up, but apparently that’s not uncommon as they’re not always set the same by default), and with all the grilles open we were getting roughly the right proportions in or out of each room. Patrick (from Heat, Space and Light) was happy with the installation which is, of course, very neat (team Windows had turned into team MVHR, performing another form of joinery).

He’d not come across the Adroit DV96 before, a Passivhaus-certified British-made unit from Airflow Developments, which we paid a small fortune for (VAT really does add a massive chunk, doesn’t it? That was a bit of a shock). But P liked the robust looking filter section and the LCD controller with a tidy little graphic showing outside and supply air temperatures. It also has built-in CO2 and RH sensors, and 4 modes: “away” (low), “at home” (medium), “boost” (high), and you can set a “open fire” mode if you need to (supplies more air than it extracts). An automatic 100% bypass is also a key feature.

There was a kind of anti-climax – not to mention heart-stop – when P switched on the unit and nothing happened. Daz the electrician had come round to connect everything very efficiently the week before, a day that I was out, so I hadn’t dared try to switch anything on (this included the ethernet connection – and so I guess the hub is staying in our bedroom for the foreseeable then… :-D). Therefore I didn’t even know if any of it was working.

But in fact, the LCD controller* – located in the landing cupboard – had been activated, lit up and was ready to be set up.

Brief interlude: the roof

Had woken up before dawn that day to the sound of dripping: delays to the roof works (a number of factors) meant we’d been left with just the felt for a good few weeks, but a heavy downpour (and lapped up sections of felt) meant that water had found its way down some areas of the cavity wall through a couple of plywood boxes. Even though forecast was dry – local farmers had obviously been praying too effectively – we should have known: it was a Bank Holiday in the Northwest of England after all…

I had also been chasing cheeky jackdaws off the roof all weekend as they had pecked merrily at the felt, making visible holes and tears. They had the gall to keep coming back, and even perch on scaffold bars while the workers were up there repairing the damage! As Chris said, they have the advantage of being able to fly up there and fly off at will.

Lindab galv steel gutter set off by cedar fascia
Temporary roof cover

So water had also trickled down the rafters and soaked the woodfibre layer, which then came through the wafer-thin plasterboard above the window in our bedroom (signs there of historic damage, so there had probably been a leak and problem lintel in that location before).

And to top it, Andy had just started a new roofing job that day so wasn’t going to be available for another week at least (can I cry at this point?). Time to extend the scaffold again, and delay the solar PV company once more – after all the “we need you on site next week” talk… (and so much for “and while we’re up there”).

Another night of rain, and although the cedar fascias and lovely Lindab galv guttering were up, it obviously wasn’t enough because we were still getting drips in our bedroom, and the team went and got some Visqueen sheets to protect the roof further. The work continues…

But back to the MVHR…

Chris connects the MVHR ductwork (while Erol finishes off our loft insulation, the tricky bits)… strangely mesmerizing.

Why is it in the (cold) loft? It’s probably one of the first questions anyone asks me: “couldn’t we put the unit in the loft?”. In typical doctor’s “do as I say not as I do” fashion, I normally advise against it. This is because the loss of heat exchange efficiency due to the supply and extract runs being in a cold space is significant: the ducts lose heat to their surroundings (a drop of nearly 10 points in our case), however much you insulate (2-3% uplift with each 25mm layer of insulation, with diminishing returns). And when you’ve invested so much in the efficiency of the unit, it’s a shame to be losing so much. But space-wise, we really had no other choice.

Radial or branched?

P would normally design radial systems and you can read more about why here. In an “octopus” configuration, main ducts to / from the unit connect to plenum boxes and then individual semi-rigid plastic ducts to each room, all small diameter. It has the advantage of easy installation – only a stanley knife is needed, connections are push-fit, and you can avoid crosstalk attenuators.

At first I’d envisaged running in ceiling bulkheads and my choice was one, originally, of materials – both type and quantity. I wasn’t comfortable with that amount of plastic or limited longevity.

The decision to run a fair amount of ductwork in the attic space was a rather late one though. Had that been my starting point, the idea we could have mainly distributed within the loft insulation layers and possibly had better locations for the bedroom supply grilles, not to mention ease of installation may have just swayed me towards a radial solution.

I do like the galvanised steel: it certainly feels like the sturdier choice, but it’s good to keep an open mind and evaluate each project on its own merits, including materials + installation costs (I’d got an exceptional deal on materials, but labour-time cost was inching towards the upper end).

How I imagined connections off the unit in my head (but in double line / How it actually looks – definitely wouldn’t fit in the landing cupboard!

Is it worth it?

It’s easy to be put off by the cost, but this is a value choice. As Jan Rosenow tweeted the other day (in relation to heat pump costs, and I’ve expressed similar sentiment in the past), no one ever asks what the return on investment is on a new car (it depreciates) or a marble kitchen worktop (none, though it will last longer than MDF and veneer). They just say “oooh, how lovely!”. Nor do they ask why you spent so much on those curtains / that piece of furniture / <insert luxury item>. As soon as it’s to do with energy, or health & comfort, “what’s the payback on that, then?”.

So the cost proved significant, despite my efforts at being lean, and I must confess, the process of actually procuring a MVHR system is far from straightforward.

Our industry is severely fragmented, and it’s the same in commercial projects: engineers do the calculations and design the system, installers are brought on board by a main contractor to finish the design, order and fit, a different team does all the fire stopping, another the commissioning, another the boxing in, making good and decorating.

It seems the same is true in domestic applications: rare indeed is the one-stop shop. I only know of Solarcrest in Macclesfield who do the complete service – and my thanks to Eliot for his input and for sourcing the unit. Green Building Store and HSL will design, supply and commission, and suggest installers, although Patrick can do a “guided install” if you fancy another bit of DIY, or you’ve found a joiner/plumber/handy person who’s willing to roll up their sleeves. All-in, it could be considered expensive, and/or they supply only a preferred (limited) choice of MVHR units.

Having persuaded my in-laws to get this in their own home, we are now grappling with the reality – months after I sketch-designed it for them – that they will probably not have it in this side of summer, and that it will cost nearly double what we’d envisaged. And we’ve already missed the start of the pollen season (my mother in law has such severe hayfever she pretty much shuts herself indoors all summer so the filtration would have been early relief for her).

I’m convinced… how do I get this done?

There will be a few routes you could take, which we’ve touched on, but here was the overall process in our case:

  • I did my own calculations, and outline designed the system: determined the volume flowrates to meet both Passivhaus and Part F of the building regs, sized the ductwork, determined the locations of grilles, worked out rough routes for ducts.
  • Researched and ordered the unit; forgot about VAT which clouded my final choice – as much as it’s a great unit, the Adroit is definitely one of the more expensive options, similar to the Paul (classic top-of-the-range Passivhaus).
  • Worked out a parts list to order direct from Lindab, to-and-fro’d with the cash account rep at Trafford because a) I can’t count b) he can’t count c) some items are in stock, some are not, so d) can you do this instead? and e) did I mention I can’t count?
  • (Russel had warned me what a drudge this part was, as he did it for his own Passivhaus in the Lake District**.)
  • Ordered separate kitchen extract grille with removable filters (+ spares) and a combined inlet/exhaust grille. Could have paid several hundred for the latter!
  • Team MVHR (Chris and Erol, of Team Windows fame) measured out the route and we agreed final positions together, so I sketched the plans for them. This was possible because they were on site anyway.
  • Worked out pressure drops and updated the input data in the PHPP model.
  • Returned some attenuators and swapped them out for smaller ones (because a 600 mm drum silencer is in fact 665 mm long plus couplings etc… look at the bloomin’ manual!). I was charged a stock return fee – in future I would just hang onto these and use them on the next job / flog them on Gumtree.
  • Rectangular attenuators would have fitted better in some spaces but were specials and had longer lead-in times. (By the time we got the installation done, that might have been OK, so perhaps worth finding out.)
  • Ordered more parts which were originally missing (did I mention I can’t count?) and because despite my staring at the diagram for weeks on end, I still got the handing of the unit wrong and we ended up needing more duct lengths…
  • (At this point I think it’s appropriate to mention I am peri-menopausal.)
  • Team MVHR installed – this was a bit stop-start because of other work going on.
  • Patrick from HSL got the system running: set fan speeds for the different modes, checked pressures, measured fully open valve flowrates (using what’s known as a “balometer” – a large 600 x 600 square hood with an array of pitot tubes – which needs enough backing to create a seal behind). He has to come back to fine tune, probably after our “new-to-us” (reclaimed) kitchen is in.
  • Team MVHR are boxing in and lagging the ductwork in the loft.
Adroit LCD controller “grinning smiley”


Adroit DV96 £2,297, LCD controller £211, Boost switch £20, Spare filters £48.72, add VAT, Total £3,092.06 incl. VAT.

Lindab Ductwork, valves and components £1,319.35 incl. VAT.

Kitchen extract grille (with replacement filter) £97.80, Intake/Exhaust combined grille £149.70 incl. VAT

Installation, £4,500 (includes builderswork holes, excludes making good / boxin in).

Commissioning, £475 +£125 (return visit) + VAT, £720 incl. VAT.

Sub-total MVHR, £9,880 incl. VAT.

Total Retrofit to date, £63,021 (£573/m2 GIFA).

Note a Zehnder CA200 can be purchased for £1,484 + VAT, but expect to pay approx. £500 (ex VAT) for design.

My advice? We do this for our offices and commercial spaces (provide good indoor air quality) – why wouldn’t we do this in our homes. And with the pandemic and more working from home, those are even more reasons to invest in these systems. So I’d say just do it, because you’re worth it! (and please don’t sue, big corporates whose tag lines I’ve just plagiarised).

A rather long post – I was going to touch on airtightness but it merits a whole article (spoiler alert: we had a royal visit – the guru of air testing himself – and by the way, you don’t need an airtight building to install an MVHR system, despite what some might say).

*To be honest, I’m not sure why the controller is sold as an extra but this did make commissioning straightforward as I couldn’t see any “on board” controls that would have allowed setting the fan speeds and modes. The alternative, according to the manual is to connect to the Airflow Cloud with a computer and use the ethernet/WiFi connection. In any case, I definitely recommend ordering it, along with a boost switch (or two) for wet rooms. We’ve placed just the one between kitchen and WC as a bit of saving.

**I heard this week that Tethera has been formally certified! Our first Passivhaus, whoop whoop! As an aside, we’d spec’d the Systemair unit for R’s job, because of dimension constraints, but you need to unscrew the panel to change the filters, and commissioning wasn’t straightforward, so I’ve steered clear since.

#MVHR, #retrofit, HealthyBuildings

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.

(c) 2019 Humbeblee