Green Building Forum - A couple of Qs on continuous extract fans (dMEV) Sat, 02 May 2026 12:59:10 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 A couple of Qs on continuous extract fans (dMEV) https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17189&Focus=291603#Comment_291603 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17189&Focus=291603#Comment_291603 Sat, 11 Sep 2021 16:30:17 +0100 luz13827
- 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?]]> A couple of Qs on continuous extract fans (dMEV) https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17189&Focus=291609#Comment_291609 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17189&Focus=291609#Comment_291609 Sat, 11 Sep 2021 20:32:58 +0100 djh 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.]]> A couple of Qs on continuous extract fans (dMEV) https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17189&Focus=291613#Comment_291613 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17189&Focus=291613#Comment_291613 Sun, 12 Sep 2021 07:10:44 +0100 tony
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]]> A couple of Qs on continuous extract fans (dMEV) https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17189&Focus=291618#Comment_291618 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17189&Focus=291618#Comment_291618 Sun, 12 Sep 2021 09:01:21 +0100 luz13827
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?]]> A couple of Qs on continuous extract fans (dMEV) https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17189&Focus=291638#Comment_291638 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17189&Focus=291638#Comment_291638 Sun, 12 Sep 2021 20:44:07 +0100 tony
Reducing temperature makes everything worse]]> A couple of Qs on continuous extract fans (dMEV) https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17189&Focus=291683#Comment_291683 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17189&Focus=291683#Comment_291683 Tue, 14 Sep 2021 20:10:11 +0100 Mike1
Posted By: luz13827Hello, unfortunately MVHR isn't suitable for our renovation for several reasons
Nothing we can help with?]]> A couple of Qs on continuous extract fans (dMEV) https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17189&Focus=305708#Comment_305708 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17189&Focus=305708#Comment_305708 Sun, 14 Apr 2024 23:16:48 +0100 alexeix
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?]]> A couple of Qs on continuous extract fans (dMEV) https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17189&Focus=305733#Comment_305733 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17189&Focus=305733#Comment_305733 Thu, 18 Apr 2024 20:36:27 +0100 Mike1 Posted By: alexeixcentralised MVHR is not possible due to ceiling heightIf you're tight on space, then 25mm high rectangular ducting may be the solution, unless you have an extremely low ceiling height.]]>