Go to Forum Home Building Services Purge Ventilation in Basements

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    • #31947

      Hi all,

      It is my understanding that the Building Regs require 4ach of 'purge' ventilation of habitable rooms, including those in basements with no windows. I am working on a project with two such rooms:

      Bedroom (with en-suite): Volume = 28m3, 4ach = 112m3/h
      Study: Volume = 40.5m3, 4ach = 162m3/h

      I cannot ventilate 'through another habitable room' because there is no permanent opening. Immediately adjacent to these rooms is a central atrium which goes to the top of the mostly open plan two-storey building.

      These are massive extract rates which do not fit nicely with the normal supply rates to these rooms.

      I would be very grateful for any ideas or experiences which might help me. I can't find anything online!

      Other thoughts are:

      1) Can I extract to the atrium rather than outside?
      2) Would mechanical purge extraction need to meet the usual Passivhaus requirements for noise, balance and draughts?

      Many thanks in anticipation.

    • #39216
      Mark Siddall

      Hi Andy,

      Achieving the supply/extract volume you require is indeed onerous. Strictly speaking 'purge' ventilation relates to the winter condition and removing moisture/smells. For the summer you should also be able to address overheating.

      I would suggest that if you were trying to keep cool in the summer, but could not sleep due to fan noise, there would be a failing in the design.

      In this respect, if the building is to satisfy the Passivhaus Standard, then all criteria (noise etc.) must be met. …arguably the need to minimise noise should be met in all cases no matter what standard.

      Perhaps there is a common sense solution to the AD-F aspects of this question.

      May be the question needs to be “why do the rooms need purge vent rates?”

      …Cooling and/or fresh air?

      Can cooling be provided by another means? If so why do you need a purge vent rate?

      …Maybe due to moisture? As there is no bathroom (just bedroom and office) humidity/moisture loads are not onerous.

    • #39217

      Hi Mark. Thank you for taking the time to reply.

      I don't expect to need to use 'purge' (as defined in ADF) ventilation in any normal situations. Indeed, as you explain, there would be a problem with the design if this were not the case. The normal ventilation provided by the MVHR more than covers the moisture and fresh air requirements. Overheating won't be an issue in this basement either for a variety of reasons. The only forseable need for purge ventilation is for occasional ADF wet paint or burnt toast situation.

      I have since checked ADF again, and the purge ventilation needs to be to the outside.

      So my problem remains. I need to be able to extract at a total rate of 68.5m3/h from these two rooms directly to the outside for smell type emergencies. There's no way the MVHR can do this I believe. I can't even beef up the extract in the en-suite because there's a door on it (obviously).

      I'm presuming that the only solution is a dedicated extract solution which remains airtight when not in use.

      I wonder if you've even had to solve this problem or know of anyone else solving it?

    • #39218
      Tom Foster

      The current re-write of AD F makes a covert necessity of 24/7 Mechanical ventilation or MHRV – the hole-in-wall-type vent rates are eye-watering. In a current change-of-use 1 bed single aspect flat conversion 90,000mm2 'background ventilation; is required – equiv to a 12″ square hole! That's in addition to 'purge' (opening windows) and Kit/Bath 'extract'. Insane. By contrast. MHRV looks convincing – but client's not ready for that yet!

    • #39219

      I can't even beef up the extract in the en-suite because there's a door on it (obviously).

      You can open a window to achieve purge ventilation. Is that any different to opening a door?

      Or, some people are happy without a door on the ensuite … (just grasping at straws for you)

    • #39220

      Hi Dave,

      I tried to argue that opening a door (to the atrium) was easier than opening a window (which would be just as effective as a permanent opening to the atrium which has lots of external windows). However, the Building Control Officer wouldn't have it! It is so tempting not to have a door to the bedroom or office (to make them permanent openings) and then put doors in when the BCO has gone away.

      Unfortunately, the ensuite is for a guest bedroom, so we might not have many visitors with no ensuite door! Thanks for the suggestion though.

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