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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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General: A couple of Qs on continuous extract fans (dMEV)

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    • CommentAuthorluz13827
    • CommentTimeSep 11th 2021
     
    Hello, unfortunately MVHR isn't suitable for our renovation for several reasons, so we're looking at monitoring + manual ventilation and fans in the kitchen/bathroom. I had a couple of Qs about the continuous extract fans

    - I know that usually the problem is too much humidity, but could continuous extract fans, in theory, make the air TOO dry? How likely is this a problem in reality? We are planning to use clay board instead of plasterboard, which should help to buffer humidity already.
    - What sort of heat loss is there with continuous extract fans?

    Assuming we do still go with continuous extract, any recommendations for best model?
    •  
      CommentAuthordjh
    • CommentTimeSep 11th 2021
     
    Posted By: luz13827I know that usually the problem is too much humidity, but could continuous extract fans, in theory, make the air TOO dry? How likely is this a problem in reality?

    In theory, introducing too much cold air into a warmer space and heating it up can cause the relative humidity to drop too much. In practice it's not likely to be a problem since you can always reduce the ventilation rate if it is a problem, or introduce a humidifier (i.e. some pot plants). The theoretical problem is the same with any continuous ventilation scheme. I have encountered it just once in reality, in a place I worked when they hadn't set up the ventilation system properly (it was a new office).

    We are planning to use clay board instead of plasterboard, which should help to buffer humidity already.
    - What sort of heat loss is there with continuous extract fans?

    Not a huge amount. Sorry, I'll leave it to you or somebody else to calculate actual values. But our MVHR is actively trying to cool our house in the summer (because the summer bypass opens when appropriate) and although it is useful, it generally doesn't win the game when left to itself.

    Assuming we do still go with continuous extract, any recommendations for best model?

    Sorry, can't help with this.
    • CommentAuthortony
    • CommentTimeSep 12th 2021
     
    Can you afford to suck heat out (and cold in) continuously?

    Smart fans might be a better way to go and have you considered using single room heat recovery fans?

    I like Svara fans close to smart and they think for you programmable via an app.

    Never rely on humidity buffering, it won’t work due to temperature swings
    • CommentAuthorluz13827
    • CommentTimeSep 12th 2021
     
    Thanks both - good to know it's not likely to cause too dry air.

    Reluctant to use single heat recovery as most of them I looked into have Wifi modules which can't be turned off (we try and reduce Wifi and plan to have ethernet set up).

    @Tony can you expand a bit more on: "Never rely on humidity buffering, it won’t work due to temperature swings" I understand RH changes depending on temperature, but I'm not sure what impact this has - do you mean temperature swings within a single day, e.g. day vs night?
    • CommentAuthortony
    • CommentTimeSep 12th 2021
     
    All buildings do humidity buffering by absorbing moisture into the fittings. Furniture and fabric. If condensation is a problem then it will still be there with buffering, indeed it could be worse as moisture stored can be ‘pumped’ to the point of condensation until it can no longer move from the wall say into the air. As condensation takes place at a presumed cold point the air in the house dries pulling moisture out of the fabric until an equilibrium is reached.

    Reducing temperature makes everything worse
    • CommentAuthorMike1
    • CommentTimeSep 14th 2021
     
    Multiple units of whatever type = multiple holes through your walls. Not always a huge issue if on the same elevation, but if not then you tend to get a through draft if there's any wind. A central MVHR unit - or, failing that, any central unit - would be preferable.

    Posted By: luz13827Hello, unfortunately MVHR isn't suitable for our renovation for several reasons
    Nothing we can help with?
    • CommentAuthoralexeix
    • CommentTimeApr 14th 2024
     
    Hi,

    What was the outcome of this?

    I’m in a similar position right now, needing to decide on a ventilation system, but centralised MVHR is not possible due to ceiling height and decentralised MVHR is too expensive and has too many downsides for me.

    Did you run with DMEV and if so, how did it work out?
    • CommentAuthorMike1
    • CommentTimeApr 18th 2024
     
    Posted By: alexeixcentralised MVHR is not possible due to ceiling height
    If you're tight on space, then 25mm high rectangular ducting may be the solution, unless you have an extremely low ceiling height.

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