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Heating and cooling: Small, reliable MVHR unit

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    Hi all,

    I am after a small, reliable, air-to-air heat recovery unit. Actually, this would be a cold-recovery unit, as this is for a walk-in fridge room which needs a dessicator dehumidifier (I get black mould otherwise, humidity is constantly at >94%, tested with a probe). The room is only 10 square meters, ceiling 2 meters high.

    I looked for heat-recovery units, but they seem quite expensive, considering that they are merely interconnected pockets of foil, 4 ducts, and (at best) two fans. However, I also found these:

    https://www.ebay.co.uk/itm/SAHRU150-Passive-Heat-Recovery-Unit-Low-Cost-Whole-House-Ventilation/

    Is there anything wrong with Sahru units? I intend to keep it always-on 24-7-365. If not a good idea, can you recommend a small, but robust, ducted MVHR (say, <150 cubic meters per hr). Fans are optional, as I can add on my own.
    • CommentAuthortony
    • CommentTimeJun 26th 2018
     
    How about just a dehumidifier
    • CommentAuthorRobL
    • CommentTimeJun 27th 2018
     
    I don't think an MVHR unit would be useful - it'd normally used to exchange stale humidified hot air, replacing it with external cooler air that when heated (via the exchanger) becomes dry warm air.

    I think if you use it on an internal cooled room with no humidity source (ie no people/clothes/showers etc) then best case it will not change the humidity at all. The mvhr can't change absolute humidity; it doesn't take or add moisture to the airflow. You will probably find the absolute humidity of the chilled room is the same as the rest of you house (see wiki for absolute - relative humidty)

    I agree with Tony - a dehumidier. Some chillers do this as part of normal operation, as they have a very cold element that water condenses on.
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      CommentAuthorfostertom
    • CommentTimeJun 27th 2018
     
    Dehumidifiers put out considerable heat.
    • CommentAuthorBeau
    • CommentTimeJun 27th 2018
     
    Posted By: fostertomDehumidifiers put out considerable heat.


    Compressor based ones do but what about the desiccant sort? They are also much better for working at low temps.
    •  
      CommentAuthordjh
    • CommentTimeJun 27th 2018
     
    I think we need more detail. What cools the room? Is the dessicant dehumidifier already in place and what is its spec? What is the purpose of the MVHR?
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      CommentAuthorfostertom
    • CommentTimeJun 27th 2018
     
    Dessicant ones too - not just fan motor but also periodically use heat to dry the dessicant wheel - water ends up in the tank but I'm not sure how.
    • CommentAuthorskyewright
    • CommentTimeJun 27th 2018
     
    Posted By: Beau
    Compressor based ones do but what about the desiccant sort?

    Think about the energy balance of the system, regardless of the type.

    If the room were air tight and perfectly insulated (& effort will have been taked to get close to that), you 'd have a closed system with an energy input (the electricity powering the dehumidifier), in addition to which you are changing water vapour to liquid water and thus releasing latent heat.

    Even if the water was arranged to run away outside the room, you'd still have the input electrical energy.
    • CommentAuthorTimSmall
    • CommentTimeJul 1st 2018 edited
     
    .
  10.  
    The dessicant-type dehumidifier outputs 300-600W of heat. I can keep it at 300W. The room's cooler is rated at 2.5 kW. The room gets human traffic when they bring goods in/out every 1 hr or so. Occasionally, 2-3 people work inside for several ours at a time processing goods. I believe a human outputs about 100 W at rest, thus, the 300W load has been demonstrated not to be a problem.

    The point of the air to air exchanger is to dissipate the heat output from the dehumidifier over a more even area, before it starts mixing in with the cold air in the room, as to avoid a build up of a hot spot in the room.
  11.  
    >>>If the room were air tight and perfectly insulated (& effort will have been taked to get close to that)

    It was, but whoever engineered it 10+ years ago it did not provide for any ways to ventilate it, thus, there is humidity building up inside. I am trying to get rid of the humidity, and to distribute the heat output from the dehumidifier over a larger area, to smoothen fluctuations of the temperature, so that the controller in the room does not get triggerred each time there is a burst of hot air from the dehumidifier.

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