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MVHR and Approved Document B

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I'm doing a self-install of an MVHR (got a design done) and have suddenly got worried about Approved Document B of the Building Regs (2019 incorporating 2020 and 2022 amendments). 


Does anyone have any experience of this for a standard dwelling? 


Building regs are a challenging read... My interpretation is that there is no requirement for fire dampers or fire hoods on the vents, the dwelling in question has no floor above 4.5m and no protected stairwell is necessary.  The only bit that concerns me is the general classification of lining D-s3/C-s3 and d2.  I am using Zehnder plenums which have a fire protection of class E.


I can't find that anyone has had an issue but things do tighten up.  Anyone know the situation?




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The Building Regs certainly are a challenging read and are only getting worse to understand!


Requirement B2 says:


B2. (1) To inhibit the spread of fire within the building,
the internal linings shall—
(a) adequately resist the spread of flame over
their surfaces; and
(b) have, if ignited, either a rate of heat release
or a rate of fire growth, which is reasonable in
the circumstances.
(2) In this paragraph “internal linings” means the
materials or products used in lining any partition,
wall, ceiling or other internal structure.


That's my emphasis, I can't see a plenum being used to line a partition etc so they are not "linings" and thus not covered by B2.


If your ductwork is not penetrating a "fire separating element" of which in a simple two storey dwelling is likely only to be a floor over a garage (assuming first floor bedrooms have escape windows) and the party wall (if not detached) you don't need to close off that ductwork at the penetration.

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Thanks kandgmitchell,

That is a very reassuring point about the word "internal linings"... that slipped past me.  I doubt anyone could afford to line a ceiling in Zehnder plenums.

I haven't seen it mentioned that anyone does it for MVHR but then someone selling ceiling speakers was adamant they need fire hoods and it got me thinking...

And yes no penetration of any of the listed areas.

I'll post again in the future if building control are unhappy!

Thanks again,


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Posted (edited)

Ceiling speakers need fire hoods for two reasons. One is to help the sound quality because they are open backed (generally some are enclosed) The alternative way to fit them is to put them inside a box thus enclosing them. The install guides normally always say they don’t work properly if they aren’t enclosed in some way. 

The other reason is indeed for fire safety which is less of an issue if they are in a bungalow, single storey extension, beneath a loft or in a garage. If they are being installed professionally they will insist on using them regardless. 

They probably also dampen the sound travelling to the floor above but I doubt by much. 

Edited by Kelvin
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