Hello! I was wondering if anyone was able to share photos if you have built a frame/insulation shed/igloo for your MVHR unit, especially if the unit is suspended from the frame?

We are retrofit installing in a cold loft bungalow (not ideal but no other options sadly). It’s a Zehnder ComfoAir 180 Luxe with preheater and enthalpy exchanger.

The plan is to build a frame (still working on the design - any suggestions would be amazing especially for an alternative to the counterbalance beams) from CLS to suspend the unit and both manifolds (no gable walls to hang the unit from). Then wrap OSB then PIR board, not sure yet whether it needs to be airtight or not?

The ducting will then travel down inside the ‘igloo’, and exit below loft boarding/within the loft insulation layer across joists (each duct will also be individually insulated from the ‘igloo’ exit point too).

I’ve seen this recommended a few times but I cannot find any guides/visual examples, and the exact details are throwing me a little bit!

- Even though it’s an insulated box, will it still be cold inside or will the heat from the mvhr unit be enough to keep it comfortable (ours has a preheater if that makes any difference?)

- Not sure how to PIR insulate underneath without impacting the frame, it cannot be a continuous layer as the frame is going to have to be secured to the joists and counterbalanced so it doesn’t fall with the weight of the unit. I believe the PIR has to be on the outside rather than the inside of the box, though it would be much easier to get a continuous fit.

- I’ve read mixed things on airtighting. We have put an airtight membrane down on the loft floor, up and over the joists but below the insulation level. Not sure if we need to include the ‘igloo’ in this airtight layer (remove membrane where the ‘igloo’ sits, then up and over), or if that would just compromise the layer? Or do we airtight the ‘igloo’ as it’s own separate entity? I’ve also read that the unit needs airflow and ventilation, not sure if that’s needed from outside the ‘igloo’ or if it’s more about making sure the unit has breathing room and the frame isn’t flush against it?

- For the door of the ‘igloo’, again how best to make sure we can access the unit without compromising the insulating properties?

If anyone has done anything similar, advice would be much appreciated! Thank you!

Just build from the joists, make a box with access door and floor across joists (you need to maintain the MVHR). Do not insulate the floor to house section and house heat will keep the space in the box warmer. Insulate box and door. Do MVHR avm at the mount of the MVHR or mount on 2x layers of plywood (2x 18mm) this will dampen the noise.

Your current design looks very poor access for maintenance. Not a fan of MVHR in a loft, to far out the way and will never get maintained, plus you need full walkways and lay down area for tools etc otherwise no one will maintain it.

Is there really no way to fit it somewhere in the main bungalow? There are several ducting options for the ComfoAir 180. My bungalow is quite small, and I was able to fit mine into the space previously occupied by the hot water cylinder in the airing cupboard. If condensate drainage is a problem you can fit a pump, or go with the enthalpy core (edit: just re-read and see you've gone with that anyway). Also don't get caught out if you're reconfiguring a standard CA180LUXE to ERV, there are two parts to the condensate drain cap you need - there is the cap itself (part 400300130), but you also need part 400300078 to replace the standard drain assembly.

I'm not sure if vibration dampers are needed with the ComfoAir 180. I too was concerned about it, but in reality the motors are very smooth and balanced. It might get worse as the unit ages but it's fine initially.

If you go with the box-in-attic option, you might find it easier to move the manifolds outside of the box. That way, you only need to seal the 4no large ducts to/from the MVHR unit itself, rather than sealing around all the individual radial tubes. Just locate it flat on top of the ceiling joists and cover in insulation. Be aware that around 50mm vertical clearance is needed below the manifold face (at least for the DN90 faceplate with two rows of ducts) in order to slide the lower duct retaining clips in (assuming you're going with the Zehnder plates with the plastic red clips rather than the metal U-clamps). This might determine where the manifold needs to sit relative to joists.

Edited 3 hours ago3 hr by hauntedicecreamvan

There's a huge difference between - 'As Planned' MVHR installations and 'As Built' installations.

Our installer took one look at that As Planned drawings and grinned.

"They dunno wot they're talkin' about do they..... "

12 hours ago, JohnMo said:

Your current design looks very poor access for maintenance.

The main cost is not the box or even the installation: its the cost of ownership. Or to put it another way, cost of maintenance. Yet another - as John says above - the cost of not maintaining it.

16 minutes ago, hauntedicecreamvan said:

Is there really no way to fit it somewhere in the main bungalow?....

It would help us help you more if you could give us the measurements of the the box and the manifolds . They're easily accessible from the manufacturers manual / working drawings.

Our installer looked at what I thought was insufficient space to fit our bits and bobs and remarked 'Easy'. Because they do it all the time.

I'd keep the box outside your airtight layer rather than trying to bring it inside the envelope. Leave the membrane on the loft floor unbroken and only seal where the ducts pass through it, with proper grommets or tape collars. Cutting it away to wrap it over the box usually ends up with more leaks, not fewer.

On temperature, the box will still get cold on a frosty night if the floor under it is insulated, as the unit gives off very little heat. The preheater stops the exchanger icing up but it won't warm the box. That's why JohnMo's point is a good one: leave the ceiling under the box uninsulated and the house heat keeps it well above freezing. You then only insulate the walls, lid and door, which also gets round your problem of the frame fixing through the PIR.

For the walls and lid, PIR on the outside of the OSB with foil taped joints works fine. The door can be OSB with PIR bonded on, compression seal round the frame and a couple of toggle catches so it pulls tight.

The unit only needs clearance for filters, servicing and the condensate route, not fresh air, so a sealed box is fine. Insulate every duct, and use a vapour tight wrap on the two outside air ducts, as they run cold and will sweat inside a warmer box. Board a proper walkway and working area in front of it too.

2 hours ago, JpManning said:

I'd keep the box outside your airtight layer rather than trying to bring it inside the envelope.

Very poor advice, sorry!

100% of me says you should make the effort of creating an insulated ‘hat’ and the MVHR lives in that; made large enough for all of the ducts to be also sharing the same heated and airtight(?) envelope.

@oliviaaa999 , if you’re not making the house airtight I would not bother with MVHR, the natural rates of infiltration will delete any possible heat recovery gains, so effectively you’ll be installing a system that pumps cold damp air into all of your habitable spaces all winter.

Airtightness comes before insulation, eg fabric heat loss is massively trumped by ventilation heat loss; you can have 1000mm of EWI and IWI, but be freezing cold if the house is draughty.