Hi Peeps.

In short:

I need ducting that is 40mm. What are my options?

Long:

Our bungalow was built in 3 stages over about 80 years. The front part has pitched roof. The rear part has flat roofs, 2 of them at different heights.

The back part has all the wet rooms of kitchen, bathroom, and shower room etc.

Right, where the roofs meet there is a gap of about 45mm maximum. And due to the way it was all put together I can't realistically just cut pieces out of the roof or walls or do anything to it at all.

So to get the stale hair out of the wet rooms I will need to go through that 45mm gap to the mvhr unit that has to go in the front part of the house. Or not at all.

Do I have any options?

Obviously I could put in 3 decentralised single room types, but I really do not want to do that. All the insulating and air tightness work we've done (underfloor, walls, lofts) were based on getting an mvhr!!

Cheers

You can get flat rectangular ducting. If there is only room for round I don't think it will work.

3 hours ago, Mr Punter said:

flat rectangular ducting.

The gaps are about 350 to 400mm wide by 45mm.

I'll have a look for those flat ducts. Thanks.

Some random ideas:

1. Use 2 separate smaller MHVRs (one for each roof area)

2. Don't try and go through the roof, go through a wall (needs boxing in of some sort, but maybe at the back of cabinetry)

3. Build your own manifolds either side of the tight area. Connect with multiple pvc waste pipes. Need to be careful to make sure enough area as will be more air resistance with multiple smaller pipes (Different situation but others on here have built similar sorts of boxes for other purposes - have a search)

Based on your comments I guess you know this but just in case: Remember that if the pipes are transiting a non-insulated area you need extra room to insulate them or you will have problems. Same goes for air tightness.

3 hours ago, Mr Punter said:

You can get flat rectangular ducting. If there is only room for round I don't think it will work.

I used 25x225 in an MBC TF job in Gravenhill. Worked a treat.

They were 35mm service voids, and all commissioned and ran perfectly (and silently). 👌😎🫡

2 hours ago, -rick- said:

Some random ideas:

Nick from Wales as given me a solution. If that doesn't work then I think I'll have to go with your idea of making my own insulated adapter.

Thanks.

1 hour ago, Nickfromwales said:

I used 25x225 in an MBC TF job in Gravenhill. Worked a treat.

They were 35mm service voids, and all commissioned and ran perfectand silently). 👌😎🫡

Thanks, Nick. The adaptors and ducting that you used in the pictures are exactly what I need. Thank you so much!!!

On 12/11/2026 at 22:33, Dazz71 said:

Thanks, Nick. The adaptors and ducting that you used in the pictures are exactly what I need.

The 225 x 25 stuff was all Screwfix, cheap as chips too. The adaptors that take Ubbink 92mm semi-rigid ducting (the green stuff) are not compatible................unless you're able to "adapt and overcome". With some kit on site I set about working out a solution to round pegs and square holes and came up with a hybrid solution; the green Ubbink couplers and beds went into my large chopsaw and got reduced down the the female receptacle part which you can see in the pics (basically the fittings are chopped in half with the middle discarded) and they got bonded in and a ribbon of airtight tape wrapped around for good measure.

Seems some of the products have gone / been superseded, but starting here should help:

https://www.i-sells.co.uk/product/manrose-round-to-rectangular-adaptor-100mm-to-225-x-25mm/