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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    • CommentAuthorluz13827
    • CommentTimeOct 28th 2021 edited
     
    We are looking at installing either continuous dMEV or a humidity-sensor extractor fan in the bathroom, and possibility also the kitchen. We also will have a kitchen extractor fan to use when cooking. This will be combined with manual monitoring of humidity and temperature/opening windows everyday, etc.

    My question is... with a lot of air potentially being extracted continuously, and intermittently (with the kitchen hood), how do we ensure that there is enough incoming air to balance out the air pressure... (but the rest of the windows are closed)? I know trickle vents aren't ideal, but would we need to add one or two, to allow for incoming air to be sucked in when the fans are running? Or a wall vent?

    Just to add that we are improving insulation and materials throughout, etc. We aren't purposely trying to be 100% airtight, but I would hope that we do a good enough job that there aren't gaps/cracks. So a lot of natural air infiltration isn't likely.


    (I know MVHR is the gold standard, but we don't have the space for MVHR in this project as it's a small conversion flat. The decentralised MVHRs all rely on Wifi, which we do not want. I am mostly worried about extracting humidity - less worried about the incoming air quality - we don't live on a polluted street and we use air purifiers/monitor air quality)
    • CommentAuthortony
    • CommentTimeOct 28th 2021
     
    Your problem will be solved automatically, as internal air pressure is reduced so extract rate declines 🙂. In practice snouts air will leak in to solve your problem.
    •  
      CommentAuthordjh
    • CommentTimeOct 28th 2021
     
    Posted By: tonyas internal air pressure is reduced so extract rate declines

    Unless it's a constant volume dMEV :)

    Posted By: luz13827I know trickle vents aren't ideal, but would we need to add one or two, to allow for incoming air to be sucked in when the fans are running? Or a wall vent?

    If you work out what rate you will set the constant extraction at then you need to make sure you have sufficient openings to provide that amount of incoming air. Sorry, I don't know how much that will be or where exactly to find the information. I would start by looking in the building regs and also by asking the fan manufacturer for advice.
    • CommentAuthorluz13827
    • CommentTimeOct 28th 2021
     
    Thanks both! I didnt realise they could exhaust less when there's less input air, that's interesting.

    The building regs state 2500mm2 background ventilation in living room/bedrooms when using continuous extract. I'm not quite sure how to work out how that balances with the exhaust air, or when the kitchen extractor fan is on, but I'll do some more digging here.
  1.  
    Posted By: luz13827My question is... with a lot of air potentially being extracted continuously, and intermittently (with the kitchen hood), how do we ensure that there is enough incoming air to balance out the air pressure...

    You don't say what sort of windows you have. Unless they are new with rubber seals then air will come in through them. If the cooker hood is not running you will probably get air through that vent even if it has a non return flap of some sort. Is there a chimney in the flat? if so unless it is properly closed off then there will be another source of infiltration. Unless you have new doors and windows you will probably have enough air coming in through leaks to be OK. If you read the issues on this forum about the problems of getting good air tightness figures - well unless you have v. good air tightness you will probably have enough leaks to be OK.
    • CommentAuthorluz13827
    • CommentTimeOct 29th 2021
     
    Thanks Peter. We are replacing all of the doors and windows (full renovation) so I think they will be pretty solid. Some of the walls in the property will be new, and the majority are party walls, but there will be some old original Victorian solid brick external walls ones left, so I imagine these could be a weaker point, but we will be improving these too so don't expect a huge amount of air leakage. That's interesting re: cooker hood - good to know that that could also be a source of air! The chimneys breasts are being entirely removed.
    •  
      CommentAuthordjh
    • CommentTimeOct 29th 2021
     
    If you're replacing all the windows, I can see why you'd want to avoid putting trickle vents in. You might do better with a separate air vent in that case they can be simple circular ducts through a wall, or rectangular openings like an airbrick. But they can all be controlled and closed if required to control drafts.

    2500 mm² is (50 mm)² I think. Which is a square opening 2" across, or equivalent as a circle or rectangle. So not very much at all.

    Something like an Envirovent ECO dMEV HT seems like it would meet your needs in both the bathroom and kitchen.
    • CommentAuthorluz13827
    • CommentTimeOct 29th 2021
     
    Thanks djh! The Envirovent ECO dMEV HT looks like it'd be exactly what we're after, thank you.
    •  
      CommentAuthordjh
    • CommentTimeOct 31st 2021
     
    BTW, it might be worth looking at the Swegon Casa FreeAir 100 thread, since that's an MVHR unit with similar abilities I think (although I haven't looked in any detail)
    • CommentAuthorMike1
    • CommentTimeNov 3rd 2021 edited
     
    Posted By: luz13827We are looking at installing either continuous dMEV or a humidity-sensor extractor fan in the bathroom, and possibility also the kitchen.
    If you install two then, unless they're on the same elevation, there will be a tendency for them to create an airflow (draft) in one and out of the other due to the difference in wind pressure. A central unit would normally be preferable.
    • CommentAuthorluz13827
    • CommentTimeNov 7th 2021
     
    @mike1 ah so better not to install two... they wouldn't be on the same elevation. When you say central, do you mean central within the house? The bathroom is positioned centrally, so having one there only?
    • CommentAuthorMike1
    • CommentTimeNov 7th 2021 edited
     
    Central ideally means the MVHR system that you'd rather avoid - though anything with a single extract would avoid the problem.

    BTW, it may be possible to fit MVHR in a small conversion, using compact ducting such as the Zehnder ComfoPipe Plus twin duct - https://products.zehnder-systems.com/en/product/zehnder-cpp-double-channel
    • CommentAuthorluz13827
    • CommentTimeNov 7th 2021
     
    @Mike1, thanks - I did look quite a bit into MVHR as that would have been ideal, but aside from the ducting, the problem is also a space to install the actual unit. We just don't have the space. So you think two continuous exhaust units would be quite problematic - we may have to stick to one then
    • CommentAuthortony
    • CommentTimeNov 7th 2021 edited
     
    Often in a flat there is space for the unit in the hallway ceiling

    Re ducting, flat rectangular extractor fan ducts can work well and will be less obtrusive.
    • CommentAuthorluz13827
    • CommentTimeNov 7th 2021
     
    @tony, I believe the unit needs to be fairly close to an external wall? The main hallway is a communal hallway, so then our own private hallway is more 'internal'/not close to an external wall. And I don't think we'd want the unit visible in a bedroom or kitchen.

    So I am trying to ensure adequate ventilation through good monitoring, dMEV and some type of supply ventilation (whether that's wall vents, or something else). I know there will be some heat loss using this method.
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