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    • CommentAuthordelprado
    • CommentTimeOct 19th 2017
     
    An explanation much needed!

    Thanks
    • CommentAuthorsnyggapa
    • CommentTimeOct 19th 2017
     
    on the basis that it is warmer inside than outside, humidity is relative to temperature so relative humidity is decreased when temperature is increased by the heat exchanger. I think :)

    also water condenses out somewhere (hence the need for a drain) but I don't know if it is condensed out of the incoming air on the way in, or the outgoing air on the way out. I suspect the latter
    • CommentAuthordelprado
    • CommentTimeOct 19th 2017
     
    How does mechanical ventilation keep the humidity lower than the outside air it brings in?
    • CommentAuthorgoodevans
    • CommentTimeOct 19th 2017
     
    It depends how you measure humidity...

    The typical measure is relative humidity which is the percentage of water in the air compared to the maximum that it can take - 100% = fog. A useful measure because wood and other materials expand and contact and rot based mainly on the relative humidity and humidity sensors typically measure relative humidity for the same reason. However if you warm up that air the relative humidity drops because warm air has a higher capacity to take moisture.

    So even on a foggy day you can fill up the house with 100% humid air - warm it up and have way less than 100% humidity inside the house. Cold foggy air has hardly any moisture in it.

    In a sealed house however cooking, showers, breathing can raise relative humidity levels to fairly high levels.

    MVHR allows the outside air in with reduced heat loss, fans and trickle vents do the same job but with heat loss.
    •  
      CommentAuthorfostertom
    • CommentTimeOct 19th 2017
     
    Absolute humidity is the amount of water in the outside air in gm/m3 or whatever. Your MHRV doesn't change that at all as it draws it in.

    Relative humidity, for the same sample of water-containing air, varies dependent on the air's temp. Warm the air up and its RH falls, even while its absolute humidity remains unchanged.

    Humans subjectively sense both absolute and relative humidity, but RH to a much greater degree - the general RH in a room dominates perception of clamminess or not. Very local, detailed RH also determines whether the water content will condense out or not on any given piece of surface.
    •  
      CommentAuthordjh
    • CommentTimeOct 19th 2017
     
    Exactly, RH is what decides whether or not condensation occurs and RH is the important measure for all biological processes. Specific humidity (or absolute) is what determines physical processes like vapour diffusion etc. The picture is complicated further by 'sorption curves' and 'water activity' and suchlike that connect the RH to the real world.
    • CommentAuthordelprado
    • CommentTimeOct 19th 2017
     
    Thanks all. So what I dont understand then, if one of the benefits of ventilation system is to maintain humidity in the sweet spot, then whe we say this we are only referring to relative humidity, yet surely things like spores or other harmfuls, are, in fact operating within the absolute paradigm?
    •  
      CommentAuthordjh
    • CommentTimeOct 19th 2017
     
    No, spores etc are biological - they operate according to the RH.
    •  
      CommentAuthorfostertom
    • CommentTimeOct 19th 2017 edited
     
    Posted By: delpradobenefits of ventilation system is to maintain humidity in the sweet spot
    Sort of - but the more the ventilation rate, the more effectively internal absolute humidity simply follows whatever external is currently. Sometimes there's a case for reducing ventilation, when external absolute humidity is very high and you don't want to import it - but that barely figures in practice.

    Things like internal vapour generation on the one hand, and air conditioning/dehumidification on the other hand, modify that generality about internal absolute humidity - and heating the inside air greatly modifies RH from whatever the absolute humidity figure is.

    MHRV acts no differently from ordinary extract ventilation, as far as importing outside air at its current absolute humidity level. MHRV does not do any air conditioning/dehumidification-type tricks on the absolute humidity. However MHRV does begin the process of heating the incoming air, thus begining to modify its RH.
  1.  
    For MVHR in PassivHauss's the general feeling is that there is a reasonable risk that humidity ends up at the dry end of the optimum range for humidity. I suspect this is more of a risk in mainland Europe with colder drier winter weather. The internal relative humidity can be adjusted by modifying the flow rate.
    •  
      CommentAuthorSteamyTea
    • CommentTimeOct 20th 2017
     
    It helps me, when I am thinking of these things, to create an image in my mind.

    Imagine a small box with sand evenly covering the base. Now jiggle the box up and down a little bit. You can imaging that most of the sand will move into the air a little bit and then settle on the base in roughly the same position. Now jiggle it a bit more and some of the sand will clump in some places, and be thinner in others.
    Give it a few really big shakes, and keep shaking, and the majority of the sand will stay in the air within the box.

    Now imagine that the sand is water molecules and the shaking is the temperature.
    You can see that when there is little temperature (small shakes), all the water molecules (sand) is condensed on the base. Lots of shaking and none of it is, it is all in the air.
    Between the two, when clumping occurs, is when you get condensation in places (cold spots).

    Maybe not the greatest mental model, and could maybe do with some improvements, but it works for me.
    • CommentAuthorGreenfish
    • CommentTimeOct 20th 2017
     
    Posted By: delpradoHow does mechanical ventilation keep the humidity lower than the outside air it brings in?

    It doesn't.

    That is Implicit in some other replies, but I think it needs to be said more clearly. MVHR is not air conditioning or a dehumidifier. When sufficiently ventilated the humidity indoors will track that of outdoors.

    As for risk of dryness (or disappointing persistent damp) it really comes down to your local climate and choice of indoor temperatures. If it is 20C and RH 80% outside then it will soon be 80% inside at 20C too. Likewise in those cold dry climates if it is 60% at 10C then indoors at 20C will feel very dry. Wondering if your house will be dry or damp, just do the maths. Of course some construction materials e.g. hay bails, can absorb moisture smooth out variation (learnt that from DJH).

    I genuinely believe that keeping in the 50% to 60% RH sweet spot is impossible to achieve in Cornwall without air con or over heating the house. I would love to know if someone has done it (and how).

    What ventiltion of any kind (MVHR or windows) does is remove the extra human produced moisture that comes from showering, cooking , drying washing or just breathing. A sealed house (at constant or falling temp) would become increasingly humid (and unpleasent to live in because of CO2 and smells too). That is how MVHR (or any ventilation) is seen to lower humidity.
    •  
      CommentAuthordjh
    • CommentTimeOct 21st 2017
     
    Posted By: GreenfishWhen sufficiently ventilated the humidity indoors will track that of outdoors.

    I agree with that statement, but only with a particular, technical meaning of 'sufficiently'. In this context, it means, 'enough ventilation to make indoor humidity track outdoor humidity'. Normally, that is too much ventilation for any other purpose (fresh air to breathe, removal of smells etc) and what is normally used and sufficient for practical purposes is a lower. rate.

    I think the problem of over-dry houses is something that occurs in much dryer and/or colder places than the UK. I did experience it in our new offices when they were first opened though. A combination of too high a ventilation rate and teething problems with the ventilation equipment, I believe, meant the RH was below 30%.
    • CommentAuthorGreenfish
    • CommentTimeOct 22nd 2017
     
    Posted By: djh
    Posted By: GreenfishWhen sufficiently ventilated the humidity indoors will track that of outdoors.

    I agree with that statement, but only with a particular, technical meaning of 'sufficiently'. In this context, it means, 'enough ventilation to make indoor humidity track outdoor humidity'. Normally, that is too much ventilation for any other purpose (fresh air to breathe, removal of smells etc) and what is normally used and sufficient for practical purposes is a lower. rate.

    Let's look at what having less ventilation will do. Human activity generates moisture, if that is not vented out then the absolute humidity will be higher inside with people than in an empty house. The air inside the house will be from outside at some earlier time. Since there is no source of dry air the humidity inside will track that of outside with a lag plus the uncleared generated moisture.

    I don't see how less ventilation removes the tracking, it just delays it and possibly makes indoors more moist.

    DJH I know from discussion elsewhere that your RH is very stable in the "sweet spot", and I think that must be because your hay bails do some magic :) Or maybe you have sufficiently higher inside temps than outside?

    However I think there is another good discussion to be had about how much ventilation is necessary for healthy happy occupants. In my experience the 0.3 m/s per sqm floor rate of the building regs is totally bonkers, as is the 3 ACH old rule of thumb if you live in a spacious house. Number of occupants and lifestyle (how you cook, how long in the shower, how you dry washing) make a huge difference. Practical purposes generally needs less than the regs specify.

    So I measured CO2 and RH and tried to ventilate according to that. With just 2 of us in a big space the CO2 needs minimal ventilation, use boost for cooking smells and wet bathrooms (and dry washing in the wind). This is how I found that no amount of ventialtion (max or min) could get the RH to stay below 60% even when the house was empty. Only thing to lower RH is to heat up the house, because the natural absolute moisture level of the air is high.
    •  
      CommentAuthorSteamyTea
    • CommentTimeOct 22nd 2017 edited
     
    Greenfish
    I seem to remember that you used lime in your building and you had trouble with it setting/drying.
    I have just looked at my internal RH numbers and my mean between Feb and August this year is 40%, with a standard deviation of only 3% (temps are mean 21°C SD 2°C).
    Now our weather is going to be very similar, so I think you have a problem that is not weather related.
    •  
      CommentAuthordjh
    • CommentTimeOct 22nd 2017
     
    Posted By: GreenfishI don't see how less ventilation removes the tracking, it just delays it and possibly makes indoors more moist.

    Now you're just twisting words. 'Tracking' implies equality, not an arbitrary offset and time difference! Which in turn causes an averaging.

    But the main thing you're ignoring is buffering.

    PHI specifically recommends reducing the ventilation rate to avoid over dryness in cold climates, and I'd rather trust their science.

    So I measured CO2 and RH and tried to ventilate according to that. With just 2 of us in a big space the CO2 needs minimal ventilation, use boost for cooking smells and wet bathrooms (and dry washing in the wind). This is how I found that no amount of ventialtion (max or min) could get the RH to stay below 60% even when the house was empty. Only thing to lower RH is to heat up the house, because the natural absolute moisture level of the air is high.

    Did you measure the external temperature and RH? That would probably explain your difficulty. FWIW, I did some quick calculations using one of the dew point calculators:

    ext T ext RH int RH @ 20

    0 100 26.18
    5 100 37.35
    10 100 52.54
    10 50 27.15
    15 100 72.94
    15 50 36.48
    20 100 100.00
    20 50 50.00
    25 100 100.00
    25 50 67.76
    • CommentAuthorgyrogear
    • CommentTimeOct 22nd 2017 edited
     
    "How does mechanical ventilation keep the humidity lower than the outside air"

    I wonder if this phraseology is not a leading question : it suggests that it is "mechanical ventilation" that is the prime mover in the affair, whereas (as alluded to above) (straw and lime...) there is no doubt also a materials aspect to be taken into account.

    Not to mention a spatial aspect (also as alluded to): How many lofty spacious areas where air can circulate... & how many "dark corners" where air will stagnate...

    And obviously human issues also (occupancy, lifestyle) also as alluded to.

    In my mind, notwithstanding the above (AIANAA) (and I am not an architect...), there must also be a spatial factor, translates to compactness of the house - what is the volume, in relation to the area of the envelope ? Thus a very compact house (cubish) might have a floor-to-envelope ratio of, say 1:6 (aspect factor 0.16), and a very expansive house might have a much larger ratio, say 1: 2.5 (aspect factor 0.4). Et cetera.

    So the materials choices for those walls might just come into play for ventilation, as they might for solar mass storage: as heat transits into and out of MASS at various times of day, there will be a "hidden" effect on air humidity...

    Or something like that.

    As an aside, and to return to my starting point, my extraction-only ventilation has been OFF since June (it failed, just a few days after my neighbour had *his* HRMV replaced...) (now there's a coincidence).

    I replaced the unit last week only (it is still not up and running due to electrical difficulties): the thing is, it has not been missed - internal RH is monitored as 56% upstairs and 57% in the basement. For 77% outside (we live in a dampish area).

    I suspect that evaporative cooling has a lot to do with it - we have EWI and cement-render inside walls.
    Just the two of us these days, with five cats and two dogs and windows generally open, and very solid door hinges operating around 44 times a day...

    gg
    • CommentAuthortony
    • CommentTimeOct 22nd 2017
     
    As soon as you warm air up the relative humidity goes down (warm air can hold more moisture than cooler air)

    So assuming indoors is warmer than outdoors RH will be lower.

    By continuously bringing in cooler air even it is at 100% RH will result in lower than outside RH

    In order get the indoor RH up you would have to do a lot more then normal living and drying washing indoors.
    • CommentAuthorBeau
    • CommentTimeOct 22nd 2017 edited
     
    Posted By: SteamyTeaGreenfish
    I seem to remember that you used lime in your building and you had trouble with it setting/drying.
    I have just looked at my internal RH numbers and my mean between Feb and August this year is 40%, with a standard deviation of only 3% (temps are mean 21°C SD 2°C).
    Now our weather is going to be very similar, so I think you have a problem that is not weather related.


    40% :shocked:
    That sounds exceedingly low for this neck of the woods (Dartmoor). Our experience is much as Greenfish not that we have a super air tight home but probably better than many homes. Tried extra ventilation but made little difference. Dehumidifier or turning up the heating is the only way we can get RH down in a warm wet southwestery.
    • CommentAuthorGreenfish
    • CommentTimeOct 22nd 2017
     
    Posted By: Beau
    Posted By: SteamyTeaGreenfish
    I seem to remember that you used lime in your building and you had trouble with it setting/drying.
    I have just looked at my internal RH numbers and my mean between Feb and August this year is 40%, with a standard deviation of only 3% (temps are mean 21°C SD 2°C).
    Now our weather is going to be very similar, so I think you have a problem that is not weather related.
    40%http:///newforum/extensions/Vanillacons/smilies/standard/shocked.gif" alt=":shocked:" title=":shocked:" >
    That sounds exceedingly low for this neck of the woods (Dartmoor). Our experience is much as Greenfish not that we have a super air tight home but probably better than many homes. Tried extra ventilation but made little difference. Dehumidifier or turning up the heating is the only way we can get RH down in a warm wet southwestery.

    Thanks Beau for the confirmation.

    Steamy RH 40% at 21C in West Cornwall, really??? I would look at getting a new sensor or recalibrating.

    No lime in my build, but 2 coat wet plastered masonry did take a fair while to dry without heating in a cold wet winter. Well longer than I expected, but I had never built a house before so lots of things were a surprize. It is dry now I promise, and I don't have any condenstaion or damp issues.

    Actually the lack of drafts and cold spots are so comfortable that we tend to run rooms comfortably at 19C and that is significant. The room with the WBS gets hotter when burning, but not started that yet. Outside is %86 at 13C (drying up after the 99% of yesterday's storm) @1008Pa a handly online calculator says that is an absolute humidity of 0.01 kg/m3. If my MVHR brought that air inside and warmed it up to 19C then (without any buffering or absorption of moisture happening) the RH of that air would be 60%.

    As I type I am in a room at 19.4C and 65%, but the air in the house will have been pulled in mostly over yesterday so that RH looks reasonable. I can vent more or less, it is not going to make it drop until it drys up outside or I heat up inside.

    Posted By: djhPHI specifically recommends reducing the ventilation rate to avoid over dryness in cold climates, and I'd rather trust their science.
    I trust their science too. The important point in that advice is cold (and dry) climates. Using MVHR when outdoor absolute humidity is low can result in dry air indoors, so you actually want to keep some of the human generated moisture indoor. True for continental Europe, not our island.

    For me "tracking" can involve an offset (no twisting intended), but you are right djh I have not accounted for buffering. I know that because of it I can use wood moisture measurements to access the average historic RH, but otherwise it is a mystery to me (why I said your hay bails are "magic" :) ).
    • CommentAuthorBeau
    • CommentTimeOct 22nd 2017
     
    Posted By: Greenfish

    Steamy RH 40% at 21C in West Cornwall, really??? I would look at getting a new sensor or recalibrating.



    That was my first thought. Dont think we have ever had that low RH. Gets near 45% on a cracking summers day or freezing cold easterly with the heating cranked up and thats about it. Mind you I dont know how accurate my RH meter is.
    •  
      CommentAuthordjh
    • CommentTimeOct 22nd 2017
     
    FWIW, I've recently discovered CIBSE Guide A, which is an excellent publication on moisture in buildings.
    •  
      CommentAuthorSteamyTea
    • CommentTimeOct 23rd 2017
     
    It may be worth me looking at the sensors, it has been running for a couple of years since I last calibrated it.
    Having said that, when I look at it in detail, it tracks my lifestyle nice and peaks in the 90% when I am cooking (I live in my kitchen), and drops when the fan heater is on (a fast way to warm the place).
    When I get back from the frozen wastes of Canada (actually very nice weather at the moment and 4° further south then my house), I shall try and see when it happening.

    Also, without looking too closely at all places, I am not sure how my RH changes over the country. I may look at that while I am away (I had plans to do all sorts of work related stuff, but wasted 2 days getting ripped off by the local mobile operator).
    •  
      CommentAuthordjh
    • CommentTimeOct 23rd 2017
     
    I have several cheap RH meters and just compare them all from time to time to have some confidence in their accuracy. They were also compared at one time to a calibrated meter I hired for a week.
    •  
      CommentAuthorSteamyTea
    • CommentTimeOct 23rd 2017
     
    You can calibrate them well enough with a jam jar of salt slurry. Or copper sulphate if you have some.
  2.  
    Posted By: SteamyTeaYou can calibrate them well enough with a jam jar of salt slurry. Or copper sulphate if you have some.

    Explain.............please
    • CommentAuthorgoodevans
    • CommentTimeOct 23rd 2017
     
    I just googled it - wet salt keeps air at 75% RH. put a small cup of wet salt in a bag, put in your RH meter - seal the bag for 24 hours and keep it at a steady temperature then read the RH on the meter through the bag - if it says 75% its good (at least at that temp and humidity level).
    • CommentAuthorgoodevans
    • CommentTimeOct 23rd 2017
     
    To be more precise it looks like between 5 deg C and 20 deg C the RH above damp salt is 75.7% give or take 0.1%.

    Baked copper sulphate in a bag will absorb all moisture until it turns blue (it can be re-baked to dry it again) - that would give a zero RH test I believe.
  3.  
    thanks
    •  
      CommentAuthorSteamyTea
    • CommentTimeOct 23rd 2017
     
    So you can reduce the RH levels in a house with a large bag of copper sulphate :wink:
   
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