Lessons from an MVHR ventilation installation

Whole house balanced ventilation is relatively new in the UK and many of the problems reported by users relate back to poor installation and commissioning.

To help me design better systems and to find out first hand about reality on site I decided to dust off my tool bag and got the opportunity to install this system at Cocreate’s Whipley Lodge Passivhaus project. I’ve put down some of my experiences to help me remember, and to help other designers and anyone considering installing.

Design lessons

  • Make sure the rigid connection between the unit and the external wall allow for installation. At least one bend is required here. Very short ducts have to be installed before the unit and this needs to be clear in design information.
  • Preference for fan units that have internal frost protection, the external box and associated cabling seemed an add on which wasn’t well thought through.
  • Make sure the control panel for the system is on the drawings and there’s a spec for the cable.

Installation lessons

  • Insulate the exhaust and inlet ducts through the wall with a short length of insulation at first fix. Carry out all sealing and air-tightness measures back to the external wall at this time.
  • Provide very robust temporary sealing to the duct ends post first fix.

System overview

I used a design from Green Building Store who also supplied the kit; a Paul Novus 300 heat recovery ventilation unit, and Lindab Safe spiral wound duct system. Green Building Store were excellent, provided lots of drawing revisions as we tweaked the design, and someone useful was always available on the phone.

The duct system is a commercial system with low pressure drop fittings, and is of a very high quality. All the ducts are completely rigid and have to be carefully integrated with the building structure during design. We allowed for distribution between the open webs of the floor joists at ground floor, above a suspended ceiling at first floor level, and left a dedicated space in the floor plan as a riser to transfer between floors.

First fix

The duct system is fairly time consuming to install, but comparable to a plumbing installation. Getting the distribution in took me three long days for this four bedroom house.

I didn’t have too many issues with getting the ducts in place, however even though the design left lots of space there still seemed to be tight areas where the timber structure had to be modified. The framers had put extra timber in quite a few places and metal gang nail plates encroached on the open web. Upstairs the suspended ceiling also had to be modified in some places but was generally easier.

Cutting the ducts

Its awkward and the worst bit, ‘measure twice, cut once’ was repeated a few times. One of the bricklayers on site worked for a commercial duct installer and he swore by a hacksaw, but I found more mechanical means faster.

We had a large stone cutting saw which I used as it was quick and cut perpendicular, but I tried other methods for interest.

  • Grinding with a very thin cutting disk was quick and safe but had to be done outside, is classed as hot works on a bigger site, and left a burr to file off.
  • A jigsaw with a fine metal blade worked well but was very noisy, less accurate and still left a burr.
  • The ideal tool is a ‘nibbler’ but these seem unreasonably expensive, you can get hand ones for panel beating which I’d like to try but I’m not sure they would get over the spiral ridge in the duct.
  • Tin-snips or side cutters only work for trimming small sections off as the waste side can’t peel away so gets in the way.

All these methods required accurate marking out around the duct to make sure it was square and there was substantial time saved by setting up a good jig, I believe there are commercial ones available that you can mount the cutting tool on.

Keeping it clean

The ducts were all stored in the house from day one which is a prime example of how not to do it. Despite dust sheets they were quickly covered in cellulose insulation, cement, and saw dust inside and out. I swept out each duct before it was installed and then kept all the working ends sealed, a box of latex gloves was really good for quickly doing this.

Dirt in the ducts disrupts the air and can also catch other particulates once the system is running and means it gets prematurely dirty and noisier. This sort of linear duct design would be tricky to clean in the future.

Hanging ducts

As the ducts are all hidden in the ceiling void and were mostly supported by the floor structure we decided the threaded bar commercial duct mounts were unnecessary. The duct manufacturer and our timber framers recommended steel strip as a fixing but I found this awkward and very time consuming. Instead I used very large cable ties screwed in place which worked well.

With this system there is very little adjustment in the distance from the duct to the room terminal, coupled with the depth of the flanges on the joists this made it difficult to get them low enough to sit flush with the plasterboard. I found some places where I had to use another bend to bring the duct parallel with the joist so I could reduce the height.

On all the extract ducts I gave a slight fall back to the terminal to help any condensate run back into the room, although unless its really steamy this is unlikely in a Passivhaus.


Each joint is fixed with stubby self-tapping screws that need pre-drilling. Despite the clear instructions not to use self-drilling screws I can see the huge temptation to swap to these on site, it would save a lot of time. This simple and seemingly innocent change can have a big effect on the noise in the system as the drill ends protrude into the duct making the air flow turbulent, it would be almost undetectable by inspection, the system in general would get blaimed for the extra noise when operational, and I imagine this happens quite often.

The manufacturer recommends assembling sections of duct on the floor and then lifting into place, but this was only possible in a few places as the ducts had to be threaded into the ceiling structure.

Plant room

I temporarily mounted the fan unit on the wall to line up the ducts, carefully sealing all the inlets with polythene to again avoid dust. A template would have been better here. I found the design – in an effort to keep the external ducts as short as possible – left no room for adjusting the position of the unit in relation to the ducts going to outside. This meant the holes had to be very accurately positioned through the wall without the unit in place. Not ideal and some careful measurement was necessary.

The ducts to outside need to be sealed to the airtight barrier and insulated with a moisture impervious insulation. The sequencing of this is really important, as I learnt to my cost. Although I did the airtight taping at first fix I realised too late that it is also best to install a short length of the insulation and seal this back to the air tight barrier as well. Then when the plasterboard goes in it is cut around the insulation and not around the duct. At second fix I had to cut away the plaster board and squeeze the insulation against the air barrier which was difficult and messy. Contractors not used to insulating ducts would certainly need this sequence explained in the design.

Lessons from second fix

Checking the ducts

Returning to a completely boarded out house two weeks later, and with only the terminal stubs now showing, I couldn’t help a bit of doubt creep in. Did I miss a joint? Are the extracts extracts and the supplies supplies? I wasn’t able to pressure test the ducts before they were covered but it would be good to do this, I imagine it happens in commercial installations but I’m not sure how. A miniature blower door setup would be one way, perhaps even a vacuum cleaner as the fan, maybe a future Arduino project for someone.

The plaster boarders had devastated the temporary sealing of the duct ends, some were completely missing as presumably the tape had got in the way. Despite this there seemed to be very little dust in the duct work I could reach or photograph.

Installing the Paul Novus fan unit

Hanging the unit was very easy, the rest a bit awkward. It was made worse by the very short duct runs especially to the exhaust side external, but also to the internal connections which were one piece vertical silencers. I’d fixed the duct ends coming through the ceiling and really needed some sort of sleeve joint to give me room to get the duct in. A telescopic duct, similar to used on wood burner flues, would be a good addition here, I don’t know if they are available. This will also be an issue if the ducts need to be removed for maintenance.

The external frost protector wasn’t great and doesn’t have any points for mounting supports, it relies on the ductwork which could be a bit vulnerable on a different installation. On this one it meant that the frost protection unit isn’t installed level and looks messy. It also means there is an assortment of flex cable running from the unit, to a control box, to the frost protector, to a temperature sensor, and to the frost protection heater. I found this virtually impossible to get neat, although most of it is hidden behind the ducts. Luckily we have a plant room, but in a domestic setting I think this sort of thing should be in the box.

The controller wasn’t marked on the drawings the electrician used, and it is an extra requirement in most domestic settings meaning he didn’t notice and it got left out. In the end this didn’t matter too much as we chose to put it in the plant room anyway, but is worth checking on projects with a larger proportion of ‘sub-contractor design’.