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General: Decentralised MVHR

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  1.  
    • CommentAuthorArtiglio
    • CommentTimeOct 14th 2023
     
    Good day all

    A friend has moved into a new home ( to them) and is experiencing issues with mould. It would appear that there is a bit of an issue of cold bridging associated with an rsj used as part of an extension built about 20 years ago ( in as much as the mould pretty much follows the line of the steel )
    Use of a dehumidifier has much reduced the problem and they are taking some temperature and humidity readings in the room. Previous owners had ( they say) no problems , however they were a couple that were out most of the day, whereas the new occupants work from home and have 3 dogs.
    They are starting to lean towards installing a decentralised mvhr innthe affected room.
    Has anyone on here any experience with such units and if so have any recommendations or advice?

    Many thanks.
    •  
      CommentAuthordjh
    • CommentTimeOct 14th 2023
     
    I can't speak to any particular brand or model, but I think that ventilation will definitely help. Also of help of course would be if any way can be found to reduce the thermal bridge. Insulation outside the RSJ to raise its temperature, or insulation inside which will reduce the temperature of the RSJ but increase the surface temperature might be possible? I hope and presume that there isn't an externally ventilated air path around the RSJ?
    • CommentAuthorArtiglio
    • CommentTimeOct 15th 2023
     
    Thanks Djh, the owners are not keen on stripping anything back to see what has or hasn’t been done during the original renovation/conversion, they have been handed a few drawings for the structural side of things but little else . To me it looks as though the steel inserted into the flank wall as part of the extension has the stone work sitting on it that then extends up past the pitched roof. The roof is ventilated at soffit and ridge level and it wouldn’t surprise me if there is little more beneath the steel than a layer of plasterboard. Looking at the way the whole thing has been done i’d not be surprised if the insulation was little more than the depth of the ceiling joists , so leaving the steel to some degree exposed to the air in the ventilated roof space. But obviously in the absence of pictures from when it was done or cutting an access panel you won’t know.
    They’re friends or friends and i’m not a building professional and i don’t want to get too involved. They need a surveyor and some invasive inspection undertaken to my mind, but not surprisingly it’s not what they want to hear.
  4.  
    Haven't understood the construction, but driven rain can permeate into stone walls and drain down through them (often the interior of the wall is filled with stone rubble to provide a drainage path, equivalent to a cavity wall). If it drains down to where a RSJ has been inserted and there's nowhere else for it to go, then it has to escape somehow, IE into the room.

    A neighbour had this effect where large window openings had been cut into their barn conversion, they needed containers to catch drips during heavy driving rain. Was cured by rendering the outside of the stone wall.


    On the dMHRV, we have basic vent axia Tempra in our utility which is effective for drying clothes. Perhaps a bit noisy for a lounge, though quieter than a dehumidifier. There are a new generation of high performance dMHRV coming onto the market, often German, some are very high end with prices to match.


    Worth remembering that any MHRV dries the house, only when the outside air is drier than the inside - when it's cool outside (cold air carries less moisture). This last summer and autumn have been warm but humid and difficult for drying. A dehumidifier works all year round.
    • CommentAuthoran02ew
    • CommentTimeOct 15th 2023
     
    Is this a high humidity room: kitchen, bathroom or lounge, dining room?
    •  
      CommentAuthorfostertom
    • CommentTimeOct 15th 2023 edited
     
    Posted By: WillInAberdeenWorth remembering that any MHRV dries the house, only when the outside air is drier than the inside - when it's cool outside (cold air carries less moisture). This last summer and autumn have been warm but humid and difficult for drying.
    That's right - more ventilation doesn't necessarily cure condensation - at times (esp spring/autumn) can make it worse and then it's a matter of stopping up all entry of moisture-importing outside air. Dealing with condensation by ventilation can be succesfui but requires experience (check out Peter Rickaby) and calculation - just like avoiding interstitial condensation does, in modern high-insulation wall-sandwiches. Easier to cure the cold-bridge at source.
    • CommentAuthorArtiglio
    • CommentTimeOct 15th 2023
     
    Wia/ FT thanks for the reminder, i had a perfect reminder of this in my current home renovation, a large vent as a result of a removed internal fan allowed expernal air to condense on an internal wall when the wind blew from the right direction, enough to give very obvious staining to the wall.

    An02ew- the affected room is above (by 5 steps) a lower area that forms a kitchen diner, it looks to me as though the air from the kitchen diner ( which is in use pretty much from morning till evening) is rising up through the house to the sitting room where the problem is occurring.

    Way too many possiblities and variables for me to work my way through ( especially as everything is covered up) hence my suggestion they engage a surveyor. I’ll pass on the comment about the dehumdifier versus mhvr . They can make their own mind up about investigating the possibility of any likely cold bridging if and when they get a professional imvolved.

    Many thanks all.
    • CommentAuthorrevor
    • CommentTimeOct 15th 2023
     
    As coincidence has it, got an email yesterday about this product from Venti who have just developed and are marketing a single room MVHR. It works by expelling the moist air recovering the heat in the unit and then reverses the flow bring external air in taking the heat from that recovered in the unit. Without relooking at the video it expels for 7 mins and brings in fresh for 7 mins on a continuous cycle. If this works as claimed think it is a clever bit of alternative thinking on Venti's part.

    https://www.youtube.com/watch?v=ulzcDMJ5Fno
    •  
      CommentAuthordjh
    • CommentTimeOct 15th 2023
     
    Posted By: WillInAberdeenIf it drains down to where a RSJ has been inserted and there's nowhere else for it to go, then it has to escape somehow, IE into the room.
    It would likely have shown up as a problem for the previous owners if that were the case, I think. But of course they could be lying.

    Artiglio, I'd second your suggestion that they engage a surveyor and accept some limited disruption to find out what's going on.
  10.  
    Hi Revor, there are many that work like that now, some are PH certified IIRC. Search for 'push pull heat recovery'.

    They seem to be taking over from the ducted kind for some applications, cheaper and easier to retrofit.
    •  
      CommentAuthorfostertom
    • CommentTimeOct 15th 2023
     
    Don't put one in a bathroom. By definition the incoming air is colder (even by a little bit) than the outgoing air and is blown in - aka a cold draught, which you certainly feel when stepping wet out of the shower.
    •  
      CommentAuthordjh
    • CommentTimeOct 15th 2023
     
    Second what Tom said. I turn our MVHR down when I shower, rather than up, for that reason. But hopefully in this case, they're not planning to do that.
    • CommentAuthoran02ew
    • CommentTimeOct 15th 2023
     
    Is it possible to retro-fit whole house MVHR and benifit form all its advantages throughtout the home? not an easy option but possible and it work alot better than SRHRV.
    • CommentAuthorArtiglio
    • CommentTimeOct 15th 2023 edited
     
    The ensuite ( which is the shower that’s used most) has just a standard extractor linked to the light , there had been issues there but they’d increased the overun time to the max and that sorted things out.

    The property was originally a small stone cottage that had 2 extensions ( to provide a kitchen diner and a sitting room) , it’s been looked after ( current issues aside) and given the layout and there being no need for a renovation , i would think a centralised whole house system would not be a straightforward installation and i’d almost guarantee that the cost and disruption would make it a complete non starter. To my mind to go for a full system without a decent survey and investigation of the current issue would be foolhardy in the extreme.
    • CommentAuthoran02ew
    • CommentTimeOct 15th 2023
     
    MVHR will conciderable reduce the moisture held in the air which will cure the mould issue, insulating the beam will not remove any moisture from the air and its possible the problem will find the next nearest cold bridge.

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