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    • CommentAuthorlineweight
    • CommentTimeMar 19th 2017
     
    I think the idea is you can put the whole heat exchanger bit in the dishwasher?

    Not having to regularly purchase new filters is definitely a selling point. Not just cost wise - one less thing to remember to buy/replace can only be good
    • CommentAuthorMikC
    • CommentTimeMar 19th 2017
     
    Posted By: lineweightI think the idea is you can put the whole heat exchanger bit in the dishwasher?

    Not having to regularly purchase new filters is definitely a selling point. Not just cost wise - one less thing to remember to buy/replace can only be good


    If that is true then I like it. Agreed, no filters is advantageous.

    Someone on here must have one of these installed and can comment.
    • CommentAuthorlineweight
    • CommentTimeMar 21st 2017 edited
     
    A couple of presentation slide shows downloadable from here ... frustrating not to get the full content of the talks they accompany but maybe useful as an overview of options...

    http://www.aecb.net/publications/ventilation-experts-have-an-indepth-look-at-mvhr/
  1.  
    Posted By: lineweightI think the idea is you can put the whole heat exchanger bit in the dishwasher? Not having to regularly purchase new filters is definitely a selling point. Not just cost wise - one less thing to remember to buy/replace can only be good
    I've mine fitted 3 years and only took out the heat exchanger once to clean it. Even if it was full of dust each FreshR is programmed to speed up one fan and slow the other to maintain optimum heat exchange efficiency. So instead of one unit costing £3/annum to run it would cost £3.30/annum. There's one in the workshop in Holland that has been running for the last 5 years without cleaning, there's a layer of dust on the wires and it uses 10% more electricity but it still maintains perfect heat exchange efficiency.
  2.  
    Posted By: lineweightAny reason that I can't take one of those units, not connect any ducts (or just very short ones) to it, put it in the same location as I'd put the Fresh-R, set it to run at the same flow rate as the Fresh-R, and rely on the gas diffusion thing to to do its magic just like with the Fresh-R unit?
    The heat exchanger in a ducted system is about 400mm wide, one side is cold the other side is warm, the best place to fit it is at the center line of the wall, if you put a regular heat exchanger there you'd have part of it sticking out side the wall and part inside the room, then where would you fit the fans?
    The beauty of the Fine Wire heat exchanger is that its only 20mm thick allowing it to be fitted at the center line of the wall with plenty room for fans.

    Posted By: lineweightIs the fundamental difference that a ducted system has to work harder to push air through the pipes?
    FreshR ventilates as much as any other system when the house is occupied but sleeps when nobody's home. Its not recommended to do this with a ducted system, they need a constant airflow to keep ducts warm so condensation doesn't form reducing fungus and mould growth risk.
    With FreshR the only temperature drop is at the heat exchanger but that happens much quicker than the speed of condensation so it mostly happens outside the unit.
    • CommentAuthorEd Davies
    • CommentTimeMar 24th 2017
     
    Posted By: Viking HouseWith FreshR the only temperature drop is at the heat exchanger but that happens much quicker than the speed of condensation so it mostly happens outside the unit.
    Does it recover the latent heat?
    • CommentAuthorMikC
    • CommentTimeMar 24th 2017 edited
     
    Posted By: Ed Davies
    Posted By: Viking HouseWith FreshR the only temperature drop is at the heat exchanger but that happens much quicker than the speed of condensation so it mostly happens outside the unit.
    Does it recover the latent heat?


    Exactly what I wondered.

    Thanks for clarify g the cleaning question VH.
  3.  
    Posted By: Ed Davies
    Posted By: Viking HouseWith FreshR the only temperature drop is at the heat exchanger but that happens much quicker than the speed of condensation so it mostly happens outside the unit.
    Does it recover the latent heat?
    Probably not Ed, do you mean similar to the ERV units that are common in the States using a paper thin membrane between the 2 air flows where moisture is transferred? How much extra energy could be harvested using this method?
    •  
      CommentAuthorfostertom
    • CommentTimeMar 25th 2017 edited
     
    AFAIK it means allowing condensation to happen on the heat exchanger, and drain away reliably, so that (some of?) the latent heat of condensation is given up and passed to the incoming stream.

    US/Can MHRVs did/do this but European ones didn't, but now better ones do.

    Because it saves much complication to make sure that all potential condensation is exhausted even if its latent heat content is thrown away.

    It's a matter of original design, adjusting flow rates relative to heat exchanger area - incoming air lingering longer on the heat exchanger surface will cool it more, to dew point of outgoing air.
    • CommentAuthorEd Davies
    • CommentTimeMar 25th 2017 edited
     
    Posted By: Viking House: “Probably not Ed, do you mean similar to the ERV units that are common in the States using a paper thin membrane between the 2 air flows where moisture is transferred?”

    No, those are about recovering the moisture (with some of its warmth) in climates where the outside air is very dry. Tom's understanding is much the same as mine though I thought most decent MHRV units had a condensate drain.

    Posted By: Viking House: “How much extra energy could be harvested using this method?”

    Here's a calculation I did earlier: https://edavies.me.uk/2014/03/mhrv-req/#energy-xfer

    That gives 800 watts for the sensible heat recovery and 365 watts for the latent heat recovery. The latent heat amount is a bit optimistic as it assumes all the available heat is recovered (none is carried out in water droplets or air at greater than 100% RH) but it's an indication of the amounts which could be available.
    •  
      CommentAuthordjh
    • CommentTimeMar 25th 2017
     
    Posted By: Ed DaviesPosted By: Viking House: “Probably not Ed, do you mean similar to the ERV units that are common in the States using a paper thin membrane between the 2 air flows where moisture is transferred?”

    No, those are about recovering the moisture (with some of its warmth) in climates where the outside air is very dry. Tom's understanding is much the same as mine though I thought most decent MHRV units had a condensate drain.

    The ones that recover the moisture are called enthalpy recovery systems. There should be a condensate drain on any MVHR unit that is doing its job in this country.
    •  
      CommentAuthorfostertom
    • CommentTimeMar 25th 2017
     
    Posted By: djhThere should be a condensate drain on any MVHR unit that is doing its job in this country
    Do you mean any that's doing 'its job' of latent heat capture - but drain not necessary on one that makes sure condensation doesn't happen - as I believe most UK ones till recently?
    •  
      CommentAuthordjh
    • CommentTimeMar 25th 2017
     
    Posted By: fostertom
    Posted By: djhThere should be a condensate drain on any MVHR unit that is doing its job in this country
    Do you mean any that's doing 'its job' of latent heat capture - but drain not necessary on one that makes sure condensation doesn't happen - as I believe most UK ones till recently?

    No, I mean that any MVHR that is doing its job of heat recovery in this country is going to cause condensation that needs to be drained at some point. Any that aren't aren't doing their job of heat recovery and probably aren't doing their job of ventilation. Calling something an MVHR unit when it doesn't ought to be prosecuted for trade descriptions, IMHO.
    • CommentAuthorringi
    • CommentTimeMar 25th 2017
     
    It not possible for a unit to prevent 100% of the condensation, therefore they all have to have drains. In wall units often drain just be letting the water run out of the air outlet to the outside.
  4.  
    When Physicists Eur and Noor Van Andel were testing the first Fine Wire heat exchanger in Almeer in the mid 90's they took the door off a coffin freezer, sealed a 100mm sheet of EPS on top, cut out a rectangular section and fitted the fine-wire heat-exchanger. They fitted 2 fans on top of the heat exchanger, set them running and left the workshop because they had another appointment. The fans had an over-heating switch-off control mechanism fitted so they expected the heat exchanger to ice up after 1-2 hours, then the fans would get hot and switch off. They were amazed when they returned 24 hours later to find the fans still running, curiously they removed the EPS sheet from the top of the freezer and found the freezer full of snow. They later discovered why: the temperature drop through the 16mm thin heat exchanger was 4 times faster than the speed of condensation so no condensation formed in the heat exchanger but outside on the cold side of it. What they'd developed was actually a snow machine.
    So the FreshR has a condensation drain as a precaution but in cold climates the defrost cycle is just a fraction of the defrost cycle of a plate heat exchanger.
    • CommentAuthorringi
    • CommentTimeMar 25th 2017
     
    VH,

    That is about what I would expect, even my plate heat exchanger type MVHR gets very little condensation as the air is expelled to the outside before it can form under most conditions.
    •  
      CommentAuthorfostertom
    • CommentTimeMar 25th 2017
     
    What physics parameter governs 'speed of condensation'?
    How long it takes to suck the heat out of water vapour molecules?
    So the admissivity (thermal capacity/conductivity - or is it the inverse?) of water?
    • CommentAuthorringi
    • CommentTimeMar 25th 2017
     
    Even a single spec of dust in the air can start the condensation process......
    • CommentAuthorEd Davies
    • CommentTimeMar 26th 2017 edited
     
    Not having condensation is a bug, not a feature. It means you're throwing away something like a third of the available energy in the outgoing air. The actual proportion is very variable depending on conditions.

    Posted By: fostertomWhat physics parameter governs 'speed of condensation'?
    My assumption is that the limiting factor is that heat of condensation needs to be removed (radiated and conducted/convected) from the growing droplet otherwise it'll just evaporate again.
    •  
      CommentAuthorfostertom
    • CommentTimeMar 26th 2017 edited
     
    And that takes time, hence 'speed' of condensation?

    Posted By: Ed Daviesotherwise it'll just evaporate again
    I'd a thought, to condense in the first place - then stay condensed?
    • CommentAuthorringi
    • CommentTimeMar 26th 2017
     
    Remember often the air above at is often at over 100% RH and often does not form clouds, there is a lot more going on with condensation then is admitted to at A level physics....
    • CommentAuthorEd Davies
    • CommentTimeMar 26th 2017
     
    Not quite sure what you're saying Ringi as “above at is often” seems a little mangled but, yes, there's a lot of confusion about relative humidity and air “holding” water. I had a little rant on the subject the other day:

    https://edavies.me.uk/2017/03/vapour/

    Posted By: fostertomI'd a thought, to condense in the first place - then stay condensed?
    In any air, particularly air which is not artificially clean so not having condensation nuclei, water will be condensing and evaporating all the time. In air which is being cooled, such as that going out through a heat exchanger, condensation will happen on droplets at a higher rate than evaporation from them so the droplets will grow but the difference between the two rates will be limited by the rate it can get rid of the heat from condensation. As long as the droplet is warmer than its surroundings it'll tend to evaporate more until it's cooled down to balance. The rate of growth of the droplet is limited by its ability to get rid of the latent heat of condensation needed for that rate of growth.

    Posted By: Ed DaviesNot having condensation is a bug, not a feature. It means you're throwing away something like a third of the available energy in the outgoing air.
    I should add that normally you can't get back all of the sensible and latent heat in the outgoing air using a passive heat exchanger because that would mean that the supply air (air going from the heat exchanger to the rooms) would be warmer than the extract air (from the rooms to the heat exchanger). To actually get all that energy back would require a heat pump. Still, if there isn't at least some condensation happening, then the exchanger is probably wasting some energy.

    If the intake air (from the outside to the exchanger) had droplets in it, e.g., because it was foggy outside, then you could use some of the latent heat from the outgoing air to evaporate those allowing higher heat recovery without warming the supply air further.
  5.  
    Posted By: Ed DaviesIf the intake air (from the outside to the exchanger) had droplets in it, e.g., because it was foggy outside, then you could use some of the latent heat from the outgoing air to evaporate those allowing higher heat recovery without warming the supply air further.
    How do you propose doing that then when both air flows are separated? even the ERV's common in the US have limited water vapour transfer efficiency between air streams.
    • CommentAuthorEd Davies
    • CommentTimeMar 26th 2017
     
    I'm not suggesting any transfer of moisture between the streams.

    Suppose you have 20 °C, 50% RH indoors and outdoors it's 0 °C.

    For a 90% efficient MHRV, just looking simply at temperature change, the outgoing air will be cooled by 18 °C (from 20 °C down to 2 °C) and the incoming air will be warmed by the same amount from 0 °C to 18 °C.

    Using this psychrometric chart: https://en.wikipedia.org/wiki/Psychrometrics#/media/File:PsychrometricChart.SeaLevel.SI.svg

    the outgoing air will also be cooled so that it reaches equilibrium vapour pressure (becomes “saturated”) at a tad under 10 °C and then cools further condensing water until at 2 °C it contains about 2 g/kg of water vapour so, assuming condensation has time to happen fully, it'll condense 5.5 g/kg of water.

    The specific heat capacity of air is about 1000 J/kg·K (close enough to justify easy arithmetic). Similarly the latent heat of vaporization of water at around these temperatures is about 2500 J/g. Therefore the cooling of this kg of air will release 18 °C × 1000 J/K = 18'000 joules. The condensation of the water vapour would release 5.5 g × 2500 J/g = 13'750 J so we've got a total of 31'750 J available.

    If we just used that to heat an incoming kg of air we'd heat it from 0 °C to 31.75 °C. Obviously that can't happen because the incoming air has always got to be a bit cooler than the outgoing air at any point in the flow path otherwise heat exchange isn't going to happen.

    What I think (guess?) actually happens in practice is that the outgoing stream is cooled less than it otherwise would be if it was dry and consequently some heat is lost but still the efficiency of the exchanger is a bit higher than it would be with dry air.

    However, if the incoming air contains water droplets then these can be evaporated (the inverse of the condensation happening in the outgoing stream) and therefore the incoming air can absorb more energy without its temperature exceeding that of the outgoing stream which would stop heat transfer.
    • CommentAuthorlineweight
    • CommentTimeMar 27th 2017
     
    Posted By: Ed DaviesNot quite sure what you're saying Ringi as “above at is often” seems a little mangled but, yes, there's a lot of confusion about relative humidity and air “holding” water. I had a little rant on the subject the other day:

    https://edavies.me.uk/2017/03/vapour/
    .


    Thanks for posting this, has clarified a few things for me.
    •  
      CommentAuthorfostertom
    • CommentTimeMar 27th 2017 edited
     
    Me too - thanks. I experienced a couple of jumps (missing bits) in the explanation, which challenged me and could baffle many -

    The opportunity to say a word about partial vapour pressures (further putting all the constituent gasses of air on a par, incl water vapour) before saying 'If the vapour pressure is lower there's less condensation but ...'

    And in 'water molecules close to the surface have slightly fewer neighbours attracting them' I'm still not sure whether you mean molecules in the liquid, or vapour molecules.

    Great to see the house actually happening, after all those 3D diagrams. Looks very Wild West - the Little House on the Prairie! Finally reclaiming a foothold after the Highland Clearances? Or perhaps you're Welsh, with a name like that.
    • CommentAuthorEd Davies
    • CommentTimeMar 27th 2017
     
    Thanks Tom - useful to see it through somebody else's eyes. I've added a new paragraph:

    Each gas operates independently as a reasonably close approximation to an ideal gas with its own density and pressure determined by the number of molecules present (usually counted in moles), its molecular mass and the temperature. Each gas's pressure is called its partial pressure and the sum of these for all the gasses present somewhere is the atmospheric pressure there. The partial pressure of water vapour is often just called the vapour pressure.
    and rejigged the concave/convex paragraph:

    My, perhaps simplistic, understanding of the effect of curvature of the surface is that water molecules in the liquid close to the inside of a convex surface have slightly fewer neighbouring liquid water molecules attracting them and therefore can evaporate more easily. Conversely, those in the liquid close to a concave surface have more neighbours and so can evaporate less easily.
    I describe myself as a quarter-Irish (maternal grandmother moved to London from a little croft in the west of Ireland in 1922 or so) Englishman with a Welsh surname living in Scotland - hence my lack of patience with people who can't tell the difference between England and the UK, etc. I have no known Scottish ancestry and, though my father came from Chester so just over the border, the distance back to Welsh is also unknown. My main reason for moving to Scotland was being able to get land I could do what I wanted to on at an affordable price.

    It's all down to the weather this summer, really. Last year was not good.
    • CommentAuthorlineweight
    • CommentTimeMar 30th 2017 edited
     
    Returning to the original subject matter...

    Alongside the Fresh-R and Blumartin units mentioned earlier, I realise that Lunos now have something similar ie. a single unit rather than their system of pairs of alternating intake/extract units also mentined earlier.

    http://www.partel.co.uk/product-details.php?ID=232

    This is quite a bit less expensive (approx £1250 inc VAT) than either the Fresh-R (£2500) and Blumartin (£2100). Appears to do something similar. Main differences as far as I can understand:

    - Minimum flow rate is 15m3/hr compared to 8m3/hr on the Blumartin and zero on the Fresh-R
    - It unfortunately doesn't measure CO2, so relies only on humidity measurements to decide when to operate.
    •  
      CommentAuthordjh
    • CommentTimeMar 30th 2017 edited
     
    The Nexxt-G presumably costs a bit more than the Nexxt-K, but seems like it might compare with my Brink unit except that it has one-third of the capacity (edit 90 not 110 m³/hr vs 300 m³/hr). My unit costs just over £2,100. Three of the Nexxt-K cost £3,600. They do say the Nexxt range includes an optional CO2 sensor.

    Further edit: the brochure says it uses 20 W at 70 % of the maximum airflow volume, which I make 63 m³/hr. At 50 m³/hr my Brink consumes 9 W.
    • CommentAuthorlineweight
    • CommentTimeMar 30th 2017 edited
     
    Posted By: djhThe Nexxt-G presumably costs a bit more than the Nexxt-K, but seems like it might compare with my Brink unit except that it has one-third of the capacity (edit 90 not 110 m³/hr vs 300 m³/hr). My unit costs just over £2,100. Three of the Nexxt-K cost £3,600. They do say the Nexxt range includes an optional CO2 sensor.

    Further edit: the brochure says it uses 20 W at 70 % of the maximum airflow volume, which I make 63 m³/hr. At 50 m³/hr my Brink consumes 9 W.


    Is your Brink unit a whole house / ducted system?

    I enquired about the "optional" CO2 sensor, however it's unfortunately not actually available yet and they weren't able to tell me when it will be.

    Obviously beyond 100m3/hr these units wouldn't be so cost effective but that seems to be around the rate that I'd want for my needs - a small 2 bed flat. Is similar to the Fresh-R and Blumartin in this regard.
   
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