Heat Recovery UK's top ten MVHR installation tips

Recently I saw an article on LinkedIn called something like “Top 12 mistakes when installing MVHR”. Actually the list was mostly mistakes in design – not in installation. So given that Heat Recovery UK specialises in MVHR installation, I decided to compile my own list of tips, which you could also call errors when not done properly. Note that this post is mostly about rigid steel systems. I will follow up with some tips for installing semi-rigid systems soon.

1. De-burr.

   I am putting this first because I see it so often, and it is a real problem! When cutting steel duct with an angle grinder, the cut edge will be sharp and jagged. If you do not make this edge smooth before you push the duct into another steel part, there is a high risk of tearing the rubber gaskets which are there to ensure airtightness. This is compounded when you have to pull the parts apart again – which you will often have to do to make adjustments.
   

   

   Use a de-burring tool, a metal file, or the angle grinder to remove at least the inside jagged edge.
   

2. Tek screw.

   Although Lindab safe duct duct is fairly secure when pushed together, some of the joins will try to pull apart as you build the system, which you will notice after doing tip 10. Therefore you should use 2-3 tek (self-tapping) screws on every join to mechanically secure all the ducts together. The entire duct network now becomes one homogenous, secure part instead of many insecure parts.
   

   

   Make sure the screw is not more than 30mm from the end of the outer part, otherwise you risk only screwing through one duct instead of two.
   

3. Fix duct solid. I see too many systems installed with loose ducting. Not only does this look rushed and is just bad practise, the pressurised duct may move or vibrate and cause a noise. The last thing you want when the house has been decorated.
   

   

   All ducting should be immovable by hand. Simple strap banding does the job nicely.
   

4. Level and vertical.

   It could just be my OCD but I like ducts to be at right angles in all planes. If a duct runs in the joists and is not level / parallel to the floor, then the air valve on the end of it may sit on the ceiling at a jaunty angle, which can be difficult to rectify at second fix.
   

   

   Use bits of timber to pack around the duct to make it level and secure. Note, the duct joins do allow a bit of flex, Lindab say up to 3 degrees per join. You can use this to your advantage in some situations.
   

5. Sufficient clearance for air valves.

   A ‘100mm’ air valve will typically be around 140mm in diameter on the room side. Therefore any duct in the wall or ceiling should be at least 50mm off an adjacent wall or ceiling; this would leave ~30mm space from edge of air valve to wall/ceiling, which will allow the anemometer hood to fit over the valve at commissioning.
   

   

   Also, an air valve tight up against a wall or ceiling just looks like someone didn’t know what they were doing. The above photo is about the closest you should go.
   

6. Design and installation drawings. A good MVHR design company will provide installation drawings which will show each part in sequence. It is quite clear that many installers hardly look at the drawings. Unless you need to make slight changes, you should be referring to these throughout your install and following to the letter. (pic to follow) The result will be fewer or no parts wrongly installed, which is quite a common issue.
   

7. Fill internal holes.

   Holes made in internal walls should be filled with an appropriate filler to reduce sound transfer between rooms / floors.
   

   

   Mineral wool, expanding foam, cement etc will do. No newspaper!
   

8. External penetrations. Quite often I can feel cold air at the junction between intake / exhaust and wall or roof penetration. Patchy expanding foam doesn’t cut the mustard. The best install is one which does not affect any of the hard work done on insulation and airtightness.
   

   

   PHS expanding foam, strips of airtight tape, and an airtightness grommet is the best approach here.
   

9. Wrap your intake and exhaust.

   The total thickness of your intake and exhaust ducts should be around 40mm, to stop any heat loss near the ducts which will be carrying cold air for most of the year. If you are using Zehnder Comfopipe Plus, this has a thickness of 43mm - so you are fine. If you have Comfopipe Compact, this is 15mm thick so you must wrap with 25mm of (ideally) closed cell insulation like Armaflex. (pic to follow) Often this won’t be done because the installers didn’t follow tip 6, and / or were lazy!
   

10. Check, check and check again. It is easy to get disoriented when installing complicated duct runs. A short lapse in concentration can see you installing the wrong duct to the wrong place – all installers have done it, and the result can be quite disastrous behind nicely skimmed and painted walls. That is why you should constantly check the ducts are running where they should do. Better to keep checking than to realise at the end you have messed it right up.
    

    

    As I install, I write “E” and “S” (for extract and supply) on the ducts with a sharpie.

And that concludes my top ten MVHR installation tips. Did I miss anything? Do you agree or disagree? Let me know. Cheers!