Struggling with ducting routing and 75mm vs 90mm?

[SuperPav]

So time has come to start the ducts for MVHR and it's giving me a big headache as the runs need to cross over some beams and through some stone walls.

 

Related to this - how much of a difference does 90mm vs 75mm make? I know 90mm semi-rigid is preferred - longest runs I've got are around 15m, one of which is to an en-suite, but using 75mm would be easier (but not if I have to double up lots of them).

 

Also, I can't find much of an answer to this:

- Walk-in wardrobe - supply? extract? Or neither

- Hall (double height space with stairs so all the warm air in the house will collect at the top of this stairwell) - supply, extract or neither?

- Utility room off kitchen, as walkthrough to garage, *without* washing machine/dryer - supply, extract, or neither? Kitchen will have 2 extracts in it.

 

Thanks

Pav

[markc]

Ducting is all about cross sectional area so a reduction in diameter from 90 to 75 doesn’t seem much, the area is reduced by almost 1/3rd requiring a much faster flow to get the same volume of air .. more speed requires more power and increases noise

[Nickfromwales]

1 hour ago, SuperPav said:

So time has come to start the ducts for MVHR and it's giving me a big headache as the runs need to cross over some beams and through some stone walls.

 

Related to this - how much of a difference does 90mm vs 75mm make? I know 90mm semi-rigid is preferred - longest runs I've got are around 15m, one of which is to an en-suite, but using 75mm would be easier (but not if I have to double up lots of them).

 

Also, I can't find much of an answer to this:

- Walk-in wardrobe - supply? extract? Or neither

- Hall (double height space with stairs so all the warm air in the house will collect at the top of this stairwell) - supply, extract or neither?

- Utility room off kitchen, as walkthrough to garage, *without* washing machine/dryer - supply, extract, or neither? Kitchen will have 2 extracts in it.

 

Thanks

Pav

W-I-W defo an extract, if sizeable. If a big wardrobe, prob nothing required / necessary tbh.

Who's design are you working to? BCO will demand one in the utility regardless.

Hall? I did the same in a project in Oxford were we installed an AHU after the MVHR, and introduced fresh ( cooled / tempered air in at the foot of the stairs and extracted stale air at the head of the vaulted landing seating area ). MVHR on it's own will do little to nothing there, unless boost rates are used in the peak of the summer months but will still only scratch the surface of moving stale warm air.

 

 

[Petrochemicals]

At the airflow rates I use 75 is fine double it up if you need to or put 2 vents in. I I have just done mine, I thought about 90, for one reason it seemed cheaper, but when you actually look the 90 stuff is less common and would have probably turned out more expensive to source everything. I think 75 is winning, so it is far more common and that must be because it is adequate, plus the fact it is easier to run,  bends can be easier, narrow spaces under the bath etc. 

 

To be honest I do not like the rigid or semi rigid ducting, it seems to convey the noise very easily as an eco chamber, even around bends, I would prefer it to be done in closed cell foam.