Part O and MVHR

[Little_Miss_Tidy]

We're having our building regs drawings done at the moment.  I'm somewhat annoyed that the architect didn't think about this at the planning stage, but that's a moot point...

 

The architect is now saying that due to Parts O, B and F, the upstairs (bedroom) windows have to be exactly 1050mm tall (to meet the various criteria for fire escapes and fall prevention scattered among those documents).  However, the windows are shown in the planning drawings at 1350mm tall, and we like them like that.

 

The problem seems to have arisen because Part O says they have to be left wide open all night.  But we're having MVHR.  The architect is saying MVHR doesn't matter, it doesn't count for Part O ventilation.  Is he right??

[Mr Punter]

MVHR will make no difference to overheating.  The windows open give purge ventilation.  MVHR does not.  In bedrooms and living rooms it will normally just feed air in, not extract it.

[Little_Miss_Tidy]

OK, thanks @Mr Punter.  Nice and clear.

[Dunc]

Are you certain they window size is limited to 1050mm (you used the word "tall")?

Sounds rather close to the requirement that the bottom of an escape window should be no more than 1100mm above the floor. Nothing you can do about this requirement other than provide access to a protected stair instead.

 

I'm not aware that the window size itself is limited to prevent falls, only that it either can't open more than 100mm, or a barrier is provided.

[Nickfromwales]

11 hours ago, Mr Punter said:

MVHR will make no difference to overheating.  The windows open give purge ventilation.  MVHR does not.

 

11 hours ago, Little_Miss_Tidy said:

they have to be left wide open all night.

If the occupants can tolerate the MVHR being boosted from the late afternoon and into the very latest part of the evening, then a slightly oversized MVHR system will suffice here, 100%.

 

@Mr Punter is correct in that it will not be able to combat solar gain whilst the sun is shining, the contributor to the problem, but MVHR very much will give you a resolution that will allow you to not fry like an egg when it's bedtime. It is simply down to compromise, education, understanding, and effort.

 

For my current client (a certified Passiv Haus) I proposed an opposing solution, for the dreaded Part O demon, to the PH certified architect which dissolved this 'window & doors open' purge ventilation nonsense; I say nonsense because the actuality of being perpetually self-employed as a human room(s) thermostat is completely rubbish for a regular way of life IMHO, so is not an option at all in my M&E proposals. Houses (our homes) are to be lived in for the benefit, not detriment, to living eg they should not be made with inherent flaws that impinge upon us getting on with day-to-day business.... aka "life". Don't get me started on the opposite issue of being too cold in winter and having to put heaters in rooms and monitor that also.....ffs.

 

Your architect fails to impress me, sorry.

 

[torre]

Clarify with your architect whether the sizes are also to meet percentage limits of glazing using simplified model, or whether it's only for guarding.

 

If it's guarding when open then there are compliant guard bars that can be fitted in the reveal, or consider switching to dynamic modelling which may give you an alternative without some windows having to be fully open at night.

 

MVHR doesn't help when using the simplified method for Part O but it will if you use dynamic modelling. Summer bypass mode is bringing cooler outside air in.

 

(We squeaked through on simplified but were getting quotes heading up to around 1k for dynamic modelling on a small build)

[Nickfromwales]

I was going to add, the windows can have bats fitted for sign off and then just remove them, if for safety etc. 

 

If there’s an ASHP being installed then get one that does cooling also, add that into the mix, and then the issue gets massively reduced (bringing this back to reality). 

[Little_Miss_Tidy]

22 hours ago, Dunc said:

Are you certain they window size is limited to 1050mm (you used the word "tall")?

Sounds rather close to the requirement that the bottom of an escape window should be no more than 1100mm above the floor. Nothing you can do about this requirement other than provide access to a protected stair instead.

 

I'm not aware that the window size itself is limited to prevent falls, only that it either can't open more than 100mm, or a barrier is provided.

 

Sorry, I could have been clearer.  Yes, the architect was talking about the size of the window - by raising the sill sufficiently to prevent falls, the overall height of the window is then limited by the ceiling (and it is the shrinkage of the window itself, i.e. daylight entering the room, rather than the sill height per se that we're unhappy about) - but it all boils down to the same thing.

[Little_Miss_Tidy]

Thank you for your posts, @Nickfromwales and @torre.  We're just on the verge of going down the dynamic modelling route because we fail the simplified method in other ways - so I'm glad to hear that the MVHR could have a positive impact here.

[Alan Ambrose]

>>> the windows can have bats fitted

 

Suggest much easier without bats, newts, otters, voles, crayfish, owls, badgers etc.

[Redbeard]

15 minutes ago, Alan Ambrose said:

Suggest much easier without bats, newts, otters, voles, crayfish, owls, badgers etc.

 

Not just me, then! On a related note while cavity walls could, I suppose, have bats, it worries me when architects specify them...

[Gordo]

For guarding as long as the opening is above 800mm fromFFL you could have a fixed pane below. To make it non climbable you need 600mm to sill or 600mm from sill to openings 

[kandgmitchell]

Part O changes the traditional guarding rules though. If you need to keep a window open wider than 100mm in order to avoid overheating, then the cill height (to avoid falls) becomes 1100mm if there is no other guarding to the opening.

[Gordo]

14 hours ago, kandgmitchell said:

Part O changes the traditional guarding rules though. If you need to keep a window open wider than 100mm in order to avoid overheating, then the cill height (to avoid falls) becomes 1100mm if there is no other guarding to the opening.

Nonsense. The minimum guarding height at windows is 800mm in a dwelling (except for loft conversion skylights) 

 

 

edit

scrub that. I wasn’t aware of part O in English regulations for overheating. Seem like tall openings are an issue for you guys without shading. Got to a have to look into that some time

Edited Friday at 01:24 by Gordo