DIY MVHR retrofit

I want to install a ceiling mounted MVHR system with spiral metal branch ducting. I have a 3 bed, 2 storey semi including a loft room. I’m currently fitting IWI before the airtight membrane.

I’m awaiting a quote from Green Building Store but I suspect it will be more than I can afford considering they insist on commissioning it themselves and I live 4.5hrs drive away.

I have the layout pretty much decided but I struggling to find out what type of vents, silencers/attenuators and cross-talk silencers required.

Has anyone successfully fitted a DIY system? Or do you have any drawings I can look at please?


I fitted an MVHR in 2007 which meets your description. A couple of years ago I replaced the unit and modified the ductwork in places with plastic foam preformed duct. This improves airflow, being smooth, and reduces airflow noise.

I have no cross talk attenuators fitted but do have supply and extract attenuators. When I fitted the system there were only two people in the house and we were never bothered by noise from other rooms. Now there are 5 of us and we still aren’t bothered by sounds coming through the system. This may be thanks to ear phones instead mega speakers in the children’s rooms!

My Zehnder ComfoAir Q350 is in the roof and when I am standing next to it I cannot hear it operating (a wonderful change from our original Villavent 400R). The Q350 has 160mm ducting and the old system had 150mm. The new ducting is 160mm with step down to 150mm. For me, that is no problem as the airflow requirement for our house is not high enough to demand the full 160mm. There are further step downs to 120mm where only a single vent is being supplied/extracted. There are rising/falling ducts concealed in fitted cupboards on the first floor and insulated with glass wool, mainly to stop sounds being transferred into the ducts.

I bought my replacement unit from GBS but had it commissioned by a relatively local company. I asked GBS to do the airflow calculations for me, which they did for a fee. It turns out the commissioning company would have done that if required.

If the above looks interesting I can root out more details.

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Patrick Chester at Heat Space and light does a very useful blog on DIY and is local to Manchester if you want a local commissioning. DIY Archives - Heat, Space and Light Ltd. I was recommended him by Flo Collier who is part of the contactor team for People Powered Retrofit.



Thanks @Tim_Gilbert, that’s very helpful. How long and wide are your attenuators? Are you able to share your airflow calculations/document please?

Am I right in saying your system has a 2 main ‘trunks’ of 150mm with ‘branches’ of 120mm going from the trunks to the vents?

I think the connections for a ceiling mounted unit are 120mm maximum.

There is one trunk rather than two. I would prefer one for upstairs and another for downstairs. The downstairs one could then have an added heater at some point, leaving the bedrooms cooler. The trick is to minimise the total duct length and thus air drag and heat loss. It is particularly important to minimise the exhaust and supply to/from outside as heat loss to them is lost to the house. Those two also ought to be well insulated to reduce heat loss and reduce condensation risk.

I have taken a photo of my unit with the attenuators in place to give an idea of size. Both are for the 160mm duct, as I didn’t have any before replacing the unit. The longer one is for supply and the shorter (largely hidden behind the long one) for extract.

I will route out the airflow table for you. There are different ways to calculate the figures and I asked for the Passivhaus (= EnerPHit) method in the hope of eventually getting there.

You will need a system capable of delivering the combined boost amounts plus some. I say “plus some” if you will be getting it professionally commissioned as the fan speeds can then be reduced, making the system quieter. This isn’t a user task.

I attach the airflow chart as a photo:

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I just found this article with advice on MVHR. It is hardly local but that doesn’t negate the sound advice.

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Tim Have a look at the various blogs on website. It has a lot of good info it you want to DIY a MVHR. They are based in Chester and I’ve been using them to design and supply my new system as the existing one was under powered for our house. Patrick, whose business this is, has been really helpful producing a detailed specification and design and answering any installation questions I have.

That is an interesting site. Too late for me (by about 17 years). But could be useful for @Ben_Partridge, the OP for this thread.

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