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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    • CommentAuthorwholaa
    • CommentTime5 days ago
     
    Given that ventilation is shown to have a major impact on the transmission of covid19, how is this seemingly never ending corona crisis going to impact domestic ventilation? Could the crisis undermine confidence in this near essential greentech? Can domestic MHRV filter covid 19?
    •  
      CommentAuthordjh
    • CommentTime5 days ago
     
    Posted By: wholaaGiven that ventilation is shown to have a major impact on the transmission of covid19

    Sorry, but what major impact is it shown to have? Does it make things worse or better? Please give an independent reference for any such claims.

    how is this seemingly never ending corona crisis going to impact domestic ventilation?

    Specifically, what evidence have you found that impacts this claim whatsoever?

    Could the crisis undermine confidence in this near essential greentech?

    Only if people treat unsubstantiated claims as gospel IMHO.

    Can domestic MHRV filter covid 19?

    I don't see what relevance the nature of the filtering mechanism has to do with anything? Please explain. Also please explain why you might expect to find SARS-Cov-2 virus in the external air around a dwelling? (I assume you meant the infectious agent SARS-Cov-2 rather than any gross, disgusting by-products of a Covid-19 infection?)
  1.  
    https://www.hse.gov.uk/coronavirus/equipment-and-machinery/air-conditioning-and-ventilation.htm

    "Good ventilation can help reduce the risk of spreading coronavirus, so focus on improving general ventilation, preferably through fresh air or mechanical systems. Where possible, consider ways to maintain and increase the supply of fresh air, for example, by opening windows and doors (unless fire doors). Also consider if you can improve the circulation of outside air and prevent pockets of stagnant air in occupied spaces. "

    Most of the guidance on ventilation systems is for workplaces, because that's where most ventilation systems are found. For domestic situations this may be slightly relevant if you have visitors. If you share your home with other people AIUI you have a higher likelihood of infecting each other, which ventilation isn't going to make much difference to. It doesn't sound like Covid would undermine the case for having sufficient ventilation in a home, either in the form of MHRV or open windows. Shh, not a doctor.
    • CommentAuthortony
    • CommentTime4 days ago
     
    Basically you can’t catch it outdoors, virus is short lived and blown away / diluted outdoors

    Very crowded and no wind, slightly different as akin to indoors

    MVHR is very unlikely to suck in virus, mine would filter it out anyway apart from aerosols but they won’t get near in the first place.

    Indoors in offices/shops/theatres/ restaurants a different matter

    HK only recently closed restaurants in the evenings and third wave is passing https://covid19.sph.hku.hk/dashboard

    No impact on domestic ventilation (cruise ships are changing the way their ventilation systems work)

    Good piece on aerosols here with loads of good refs
    https://www.nytimes.com/2020/07/30/opinion/coronavirus-aerosols.html
    •  
      CommentAuthorfostertom
    • CommentTime4 days ago edited
     
    Assuming it's true that the virus spreads person to person via exhaled droplets large/heavy enough to drop to the floor, rather than in free-floating aerosols which remain airborne therefore pervasive;
    and that coughed-out droplets pretty much loose their momentum through still air within about 2m, then slowly sink to the floor;

    this means
    a) that the floor is a dangerous place, especially if you're a small child, and even more so if you're a child you wants to crawl around or investigate things at floor level;
    b) shoes and trousers need as much care to not touch as a mask, use only once, remove with same care and put straight in the wash, or hang up outdoors for 12hrs.

    What about someone walking? His/her 2m radius is going to be tear-drop shaped i.e thin forwards, long tail rearwards which will tend to keep droplets airborne longer while it remains turbulent. I avoid people's 'slipstreams'.

    And breezes - much the same. Standing in a queue for the supermarket, it can be hard to be not-downwind from someone.

    And ventilation - the 2m rule applies to still air; ventilation at least increases turbulence, if not clear airsteams; will greatly increase the radius and duration of risk of encountering others' droplets, spread the droplets all around the room. Against that effect, the only benefit is dilution, but ventilation dilution rates are pretty marginal, not remotely like for example quickly reducing particle concentration to one-third. I'd have thought that risk of 'catching' will not be linear with particle concentration; real risk reduction must be only after massive dilution.
  2.  
    An obvious analogy would be mechanical dust extraction in workshops. Overhead ducts provide a a continuous suction, and smaller branches at a lower level suck air from closer proximity to the machines, the creators of the dust.
    With a continuous vigorous upward airdraught dust, or coronavirus germs, or human breath, would be instantly sucked away harmlessly. You could breathe directly into someone's face with impunity if the extractor fan was powerful enough.

    Obviously this would be uncomfortable and noisy and not very relaxing in a house, but you could turn it off when only people belonging to the same bubble were present.
    • CommentAuthorEd Davies
    • CommentTime4 days ago
     
    It seems to me there are two reasonable objectives to making a house airtight and adding mechanical ventilation:

    1) energy saving (through reduction of ventilation heat losses due to unrecoverable heat and over-ventilation in some circumstances) and

    2) improved indoor air quality due to elimination of under-ventilation in other circumstances and through application of ventilation and airflows in appropriate places and directions (e.g., not having outdoor air coming in to wet rooms and blowing from there to the rest of the house).

    People will argue ferociously that one or the other is the “real” reason. That's silly; different people have different priorities. Whatever, the first reason has little to do with airborne diseases one way or the other, The second can only make the situation better, I'd think, so I fail to understand why anybody would have their confidence undermined in any way but rather would be encouraged to fit an appropriate ventilation system by any threat from airborne diseases.

    A bacterial infection which could breed in the ducts or heat exchanger might be a cause for concern but I don't think viruses work like that. Legionella is the only thing close which could potentially be a concern but AFAIK it doesn't seem to be a problem at all with domestic ventilation, particularly without air conditioning.
    • CommentAuthortony
    • CommentTime4 days ago edited
     
    Tom please don’t assume

    https://www.nytimes.com/2020/07/30/opinion/coronavirus-aerosols.html

    It has now been proven that it can spread via aerosols especially indoors
    •  
      CommentAuthorfostertom
    • CommentTime4 days ago edited
     
    By "assuming that", I meant that it's hitherto conventional wisdom - but now under debate.

    Posted By: Ed DaviesThe second can only make the situation better, I'd think
    What about my point that
    Posted By: fostertomthe 2m rule applies to still air; ventilation at least increases turbulence, if not clear airsteams; will greatly increase the radius and duration of risk of encountering others' droplets, spread the droplets all around the room
    I'm not one to knock ventilation, but pointing out why it's not an unquestionable positive with this virus - it deserves some 'scientific' thought. Cliff Pope has made a start, in defining how ventilation might need to be, to not actually be counter productive.
    • CommentAuthorEd Davies
    • CommentTime4 days ago
     
    Posted By: fostertomWhat about my point that … the 2m rule applies to still air …
    I'd suggest the flow rates caused by ventilation are very small compared with other natural air movements, e.g., convection caused by a “radiator”.

    E.g., imagine an extract terminal in the top corner of a room extracting from a net area of 40 m² with a room height of 2.4 m. The volume is ~100 m³ which at 0.45 AC/h would be 45 m³/h. If the extract terminal is, bizarrely, right in the corner of the ceiling then the area at a given distance from the terminal is 4πr²/8 ~= 3/2r².

    The flow speed of the airflow 1 metre from the terminal is then (45 / 3600) / (3/2) ~= 8.3 mm/s. Not a lot even quite close to the terminal and completely negligible further away compared with the speed bits of spit fly around in the videos I've seen.

    It'd be ridiculous to think it really matters if you're 1.9 or 2.1 metres away from somebody - everybody having a keep-out ellipse 1.9 metres towards supply terminals and 2.1 metres towards extract terminals - 2 m is just a vague guideline as to what seems likely to reduce the probability of transmission significantly. That's not to say that talking to my neighbour the other day from about 2 metres away I didn't casually move sideways when I realised I was directly downwind from him but that was in a wind speed of multiple metres per second.
    •  
      CommentAuthorfostertom
    • CommentTime4 days ago
     
    OK - that deals with that point - turbulence negligible.

    But still, dilution also negligible or v slow. 3a/c/hr means it takes an hour before only 25% of the original air is still left.
    •  
      CommentAuthordjh
    • CommentTime4 days ago
     
    Posted By: fostertomBut still, dilution also negligible or v slow. 3a/c/hr means it takes an hour before only 25% of the original air is still left.

    I'm not sure what your point is, Tom? Quite apart from being irritated by your introduction of special 'Tom units' of a/c/hr instead of the normal ACH (with a space before, please).

    But guaranteed slow air change is surely better than no guaranteed air changes at all? And by all means open lots of windows as well to increase air changes, as long as you've bought a constant volume MVHR system.

    As has already been pointed out, almost exclusively everything written about ventilation systems in this connection has been to do with commercial or industrial systems.

    PS where do you get your 3 ACH => 25% original air after an hour figure from? Naively it would imply completely new air after 20 minutes, although I accept there will be some mixing.
  3.  
    There's a lot of mixing, because you don't get a 'front' of clean air sweeping linearly across the room. Some will flow straight from inlet to extract, whereas some will linger in corners, behind furniture, etc.

    Tom's approximation is that you take the original air volume in the room and mix it with three times that volume of clean air, over the course of an hour. You now have the original volume x4 of air. Obviously, three-quarters of the mix have now spilled out of the house and one-quarter remains. There are more mathematically-correct approaches with an exponential decay of concentration, but as this is all so approximate, it's as good a method as any.

    More typical mhrv rates maybe ~0.5ach (AC/h? h^-1?), so an hour after an infected person leaves the room, ~67% of the air they breathed remains in the room using Tom's approach, or exp(-0.5)=61% if you'd prefer. So as we all said, some ventilation is better than none, but don't rely solely on it to protect you from infection.

    My kids' schools are planning to hold classes with all their windows open for the foreseeable future (to dramatically increase the AC.h-1) so unfortunate to think about the heating usage through winter - unavoidable I s'pose.
    • CommentAuthortony
    • CommentTime3 days ago
     
    Time exposed is an important factor, in our own homes we should be safe,

    http://news.tvb.com/programmes/straighttalk20160001/5f16b6d5335d19732ebc2a4c

    Leading expert makes interviewer wear a mask indoors with 2m !
    •  
      CommentAuthorfostertom
    • CommentTime3 days ago
     
    Posted By: tonyTime exposed is an important factor
    I guess it's time exposed x concentration, but neither quantity linear.

    My guess is that the first minute of exposure (at constant concentration) is much more significant than the tenth, i.e. exposure for 10mins is far less than 10x as dangerous than exposure for 1min. If you've been exposed for 10mins then exposure for an hour is hardly any worse. Or am I wrong?

    My guess is also that concentration (droplet volume per m3 of air) at 10ppm (or whatever) is far less than 10x as dangerous than concentration at 1ppm. If concentration is 100ppm it's hardly any worse than 10ppm.

    Or am I wrong - maybe it's all strictly linear, or even non-linear the other way? I'd like to have it clear.
    • CommentAuthorJonti
    • CommentTime3 days ago
     
    Tom,

    the 2 meter rule is quite effective in preventing the spread though the further apart the better. What doesn't work so well are masks which don't stop aerosol. When the masks were first mooted it was emphasised that it was in addition. In reality businesses just saw it as a way to do away with social distancing.

    Get ready to ride wave two :cry:
    •  
      CommentAuthorfostertom
    • CommentTime3 days ago edited
     
    It's inconsistent to say that "the 2 meter rule is quite effective" despite being no protection against aerosols;
    and "masks ... doesn't work so well" because they don't stop aerosols.

    Both the 2m rule and masks are only supposed to help with droplets, can't help with aerosols. Maybe one is "quite effective" and the other not - I'd like to understand why.

    The 2m rule protects both parties but masks only marginally (if at all) protect the wearer but are to protect others. Maybe that's it?

    It's incredible that after all this time we're still really unclear about these things - it's not rocket science, and not subject to changing scientific discovery.
    •  
      CommentAuthorfostertom
    • CommentTime3 days ago edited
     
    As confirmed by https://www.theguardian.com/world/2020/aug/02/secrecy-has-harmed-uk-governments-response-to-covid-19-crisis-says-top-scientist .

    Blaming Cummings (why not? everyone else does), who rightly believes govt shd be guided more by science, unfortunately that's definitely not so that the public will be better informed - quite the reverse, it's so the current govt can use the info to devise better tactics to retain power (meaning Tories forever, if he has anything to do with it) utilising whatever disasters may happen.
    Elites of all colours think they know that the public can't be trusted with 'the reasons why' but must be 'nudged' or otherwise manoevred into the desired behaviours. Even worse, giving 'the reasons why' simply gives the media something to tear to shreds. So scientists urging more, and more scientific, transparency won't be heeded. But the rest of us can see the arrogance and contempt of our elites in it.
    • CommentAuthorbarney
    • CommentTime3 days ago
     
    Tom - you need to look at viral load - long exposure to small quantities allows the natural immune system to stay effective

    A massive viral load for even short durations overwhelms the immune response and you'll end up unwell (and in this scenario, reasonably seriously unwell).

    We've learned this from the way it affects fit and healthy people in hospital environments when they aren't following the PPE requirements (by choice or circumstances).

    Humidity control is as important as ventilation - no one is talking about it right now as we are a maritime climate and humidity never gets low in the summer - not so in the winter where pretty well all indoor spaces are much lower RH - this causes the droplet to evaporate and desiccate quickly - leaving viable virus as minute particles easily suspended in the air for many hours.

    Regards

    Barney
    •  
      CommentAuthorfostertom
    • CommentTime3 days ago
     
    Good info barney - any more like this?
    •  
      CommentAuthorfostertom
    • CommentTime3 days ago edited
     
    Those tactics of retaining power are themselves increasingly being developed as a science. They can, as we've seen, include deliberately spreading public confusion, rather than good information.

    Remember, for a short period back in April, Boris got much cred, in 'progressive' circles, for an initial 'scientific' response, his chief scientists standing behind him at the rostrum, which was to rely on 'herd immunity'
    https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7201134/
    This was short-lived, my guess on Cummings' advice as being exactly the wrong tactics for power; and a reversal of that policy, confusion, ambiguity, 'Nudge', and fake-science talk of 'Behavioual Fatigue' is what we've had since.
  4.  
    I was once advised that in polite conversation, one should avoid discussing politics, religion or sex. Any chance we could stick to talking about ventilation?
    • CommentAuthorbarney
    • CommentTime3 days ago
     
    More Info Tom, ?

    Plenty - my day job is ventilation design (amongst other systems) - I've designed systems to keep everything inside a building or room to keeping everything outside a room or building. This being based on combinations of pressure, flow direction, filtration and absorption - so everything from HEPA filters, activated carbon cells, UV and ozone sterilization at various pressure regimes right up to gas tight arrangements when what's out there cannot be dealt with by any other means other than exclusion (radioactive nuclides of noble gases as one example)

    For the ventilation system to be effective, you first need to understand what it is you are dealing with (size, toxicity)

    I don't disagree that in a maritime UK climate in summer, plenty of fresh air isn't a bad thing, and reducing recirculation is also a good thing - but we need to keep in mind that all that's achieving is an idea something akin to a low "R" number - ie in itself, it's probably helping a bit in controlling spread, it isn't reducing spread nor is it a silver bullet that you couldn't catch a coronavirus in a well ventilated space - anymore than you can say outdoors good - indoors bad- it's all about a balance of probabilities (or actually about specific dilution factors in specific circumstance. For example, you could say that a ventilation rate that gives a DF of a 1000, is good, and one which gives a DF of 500 is less good - but these aren't absolutes, more they are simple metrics on which to base probability

    Personally speaking, good hand hygiene and a bit of space will do more than anything to keep the transmission rate low, ventilation only has a relatively small part to play (except in a few specific medical locations)

    All government is concerned with is not getting to a point where it's totally out of control - we've tested the NHS and it survived the previous peak (albeit with about 45,000 deaths which seems acceptable) - we can deal with the next 200,000 deaths as long as they occur over a reasonable time scale - hence the focus now is getting the economy working and still keeping below the operational peak - which is the difficult bit for the hard of thinking to understand - particularly what "herd immunity" actually means to epidemiologists

    Regards

    Barney
    •  
      CommentAuthorfostertom
    • CommentTime3 days ago
     
    Hi Will. I said
    Posted By: fostertomIt's incredible that after all this time we're still really unclear about these things - it's not rocket science, and not subject to changing scientific discovery.
    Isn't that because of these politics? Why else are we so confused? Even the Grenfell Inquiry has reached the point where it can't help mentioning the politics behind it.
    • CommentAuthorJonti
    • CommentTime3 days ago edited
     
    Posted By: fostertomIt's inconsistent to say that "the 2 meter rule is quite effective" despite being no protection against aerosols;
    and "masks ... doesn't work so well" because they don't stop aerosols.

    Both the 2m rule and masks are only supposed to help with droplets, can't help with aerosols. Maybe one is "quite effective" and the other not - I'd like to understand why.

    The 2m rule protects both parties but masks only marginally (if at all) protect the wearer but are to protect others. Maybe that's it?

    it's incredible that after all this time we're still really unclear about these things - it's not rocket science, and not subject to fchanging scientific discovery.


    The two meter rule may have been designed to deal with the droplet issue and not the aerosol one but that does not mean it is a good way to deal with that. two meters is better than one because of the extra dilution of the aerosol. Of course 3 meters is even better but not as good as 4 meters.

    Posted By: fostertomIt's inconsistent to say that "the 2 meter rule is quite effective" despite being no protection against aerosols;
    and "masks ... doesn't work so well" because they don't stop aerosols.

    Both the 2m rule and masks are only supposed to help with droplets, can't help with aerosols. Maybe one is "quite effective" and the other not - I'd like to understand why.

    The 2m rule protects both parties but masks only marginally (if at all) protect the wearer but are to protect others. Maybe that's it?

    It's incredible that after all this time we're still really unclear about these things - it's not rocket science, and not subject to changing scientific discovery.


    Tom,

    the two meter rule is effective because the aerosol load is more diluted than with one meter. Three would of course be better than two but not as good as four because the further you are away from the source the less viral load is in the air.

    I think that the facts on social distancing are actually really very clear but it seems that they are also inconvenient to many.
    •  
      CommentAuthorfostertom
    • CommentTime3 days ago edited
     
    Barney, I said
    Posted By: fostertomMy guess is that the first minute of exposure (at constant concentration) is much more significant than the tenth, i.e. exposure for 10mins is far less than 10x as dangerous than exposure for 1min. If you've been exposed for 10mins then exposure for an hour is hardly any worse. Or am I wrong?
    but I think you're saying the opposite is true - there is non-linearity but in the opposite sense.

    I thought that inhaling just one or two viruses would be the danger, and that inhaling ten times more would make little difference - just one or two would be enough to multiply exponentially. Now I see that you're saying that an immune system has a likelihood of neutralising any number of viruses, up to a point, but liable to lose that ability with prolonged exposure, regardless of concentration? But 'prolonged' means a great deal longer than 10mins or an hour. I'd love to see someone venture to express this as a curve.

    Very clarifying - thanks.
    •  
      CommentAuthorfostertom
    • CommentTime3 days ago edited
     
    Posted By: JontiI think that the facts on social distancing are actually really very clear but it seems that they are also inconvenient to many.
    The rule is clear, and not resisted or 'inconvenient' to me. But the thinking or science behind it bears a lot more exploration.

    For example, their 2m zone is a vertical cylinder of slowly falling droplets, if they are still, and in still air. Just keep out of thieir 2m radius.

    But in say a supermaket where people are walking slowly, prob not activating turbulent mixing concerns, their 2m zone is an extrusion of recently-occupied space trailing behind them, just as full of slowly falling droplets. How long does it take the droplets to fall; how long should I wait before moving into space that someone has recently occupied?
    • CommentAuthorEd Davies
    • CommentTime3 days ago
     
    Posted By: fostertomBut still, dilution also negligible or v slow.
    Perhaps, but people do say that with mechanical ventilation the air feels better so I'd think that indicates dilution works better than with just “natural” ventilation.
    •  
      CommentAuthordjh
    • CommentTime3 days ago
     
    Posted By: fostertomI thought that inhaling just one or two viruses would be the danger, and that inhaling ten times more would make little difference

    By pure chance, I think you're exactly wrong. I read somewhere at some point that the 'critical load' for this virus was fairly low, specifically about 20 particles (virions). That's the number of particles you need to breathe in to cause an infection. But there's a lot of evidence that exposure to a lot more particles is much more likely to lead to a severe case.

    I don't think there's much point in trying to be too precise in deciding what to do in particular circumstances. Try to stay as far away from other people as possible, and try to stay close to them for as little time as possible.
    • CommentAuthorMike1
    • CommentTime3 days ago edited
     
    Posted By: fostertomI thought that inhaling just one or two viruses would be the danger

    Maybe - for Norovirus, 18 viral particles can cause transmission in 50% of people. Not sure if Coronavirus is thought to be that infections or not, but according to the WHO, the infectious dose isn't known. I guess running a trial to find out isn't going to be very ethical...

    https://www.who.int/news-room/commentaries/detail/transmission-of-sars-cov-2-implications-for-infection-prevention-precautions
   
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