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Green Building Bible, Fourth Edition
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    • CommentAuthorTriassic
    • CommentTimeFeb 24th 2014 edited
     
    The architects have said they are concerned about over air tightening our proposed new build. I think this was as a result of us saying we'd rather not have MVHR, favouring opening a window if we need fresh air and the fact we live in the country, so door and windows do tend to get opened (how else do you hear the bird song and smell the country smells if you're hermetically sealed in a box?).

    So being a complete novice at air tightness and having Googles the subject and been baffled by the huge amount of information, I was wondering if someone here could synthesise the requirements. We realise there will be a spread, starting with minimum building regs at one end and passive house at the other, but where is that happy mid point - you know that point just before the requirement for MVHR kicks in?

    Thanks in advance.
    • CommentAuthorEd Davies
    • CommentTimeFeb 24th 2014 edited
     
    Scottish regs recommend something more than trickle vents (i.e., passive stack or mechanical ventilation with or without heat recovery) when the designed airtightness used for SAP calculations is better than 5 m³/(m²·h). May not apply to you directly but maybe useful guidance.

    Domestic 2010, 3.14.2:

    Recommendations for trickle ventilation in the table below are made on the basis that infiltrating air rates of 5 to 10 m /h/m @ 50 Pa will be achieved as a matter of course in modern dwellings. However where the designer intends to use low fabric infiltration air rates of less than 5 m /h/m @ 50 Pa in the SAP calculations (see section 6 Energy) the areas of trickle ventilation shown may not suffice to maintain air quality and therefore an alternative ventilation solution should be adopted (see clause 3.14.10).
    Deliberately setting yourself up to over-ventilate the house and throw away heat does seem like conspicuous consumption, though. Just because you have MVHR doesn't mean you can't open the windows.
    •  
      CommentAuthorfostertom
    • CommentTimeFeb 24th 2014 edited
     
    Go for max airtightness, then open windows as nec - you won't suffocate! Deliberately leaving things leaky will just leave you with draughts esp when the wind blows, and still won't provide enough ventilation when it's still and heavy outside.

    Also, leaving well insulated structures with small all-over leakiness (as distinct from major leaks around windows etc) is a major cause of moisture transport leading to interstitial condensation. This latter is a new finding of Fraunhofer research (the authors of WUFI) - water vapour transport by air leakage turns out to be at least as big an interstitial condensation risk, as is water vapour diffusion, which has been the traditional focus.
    • CommentAuthorTriassic
    • CommentTimeFeb 25th 2014 edited
     
    So the architect says "We will also need to install MVHR if we use the U-value ranges of between 0.12 - 0.15 and the air permeability of the envelope is at or below 1.5 air changes per hour, which again is another item to consider. "

    So should we be going for a tight building considering the moisture transport issue raised by Tom and install a simple MVHR system?
    •  
      CommentAuthorfostertom
    • CommentTimeFeb 25th 2014
     
    The point is, it's only worth having MHRV once U-values are down to the 0.12 -0.15 range - doesn't mean you have to. The other necessity is to be airtight.

    Why should low U-values and/or airtightness make MHRV necessary? If no MHRV you just open windows - not going to die gasping! It's true that you'll use more energy that way, because none of the heat in the outgoing air is recovered.

    It's also true that without MHRV you won't experience the delightful always-outdoor-fresh feeling that everyone reports, with MHRV. It continually removes all the many indoor toxins that we all live with and make us feel less than 100%.

    An airtight building does avoid the condensation caused by air-exfiltration-transported moisture, that I mentioned. But you'll get that benefit from airtightness, with or without MHRV.
    • CommentAuthorJonti
    • CommentTimeFeb 25th 2014
     
    What would be the energy requirements for say 80m2 floor area or say 280m3 volume 0.15 U-value range building and how could you run an MHRV if you were off grid and did not want a diesel generator. Wind or small hydro maybe?

    Jonti
    • CommentAuthorRobL
    • CommentTimeFeb 25th 2014
     
    Our 300m^3 house with 2 adults 2 kids, all out during school hours, has a sentinel kinetic plus with humidity & CO2 control. It uses an average of 13W throughout the day according to one of those plug in power meters, left averaging over a few days.
    The lowest power it uses is 9W(nobody there - this number really should be less), then 30W(excessive CO2),the highest power is 50W(lots of humidity due to a shower).

    Best advice is get an efficient unit, and use CO2/humidity control - that way it won't keep going at excessive power when it's not needed because there's nobody there.

    I think it needs to respond to CO2 quickly(<30mins), to keep everybody happy. The humidity feedback could be slower(days!), as most houses have significant humidity buffering - generally many days worth - so perhaps off grid you could only activate the humidity sensor when you have wind/pv/other renewable resource ?
    •  
      CommentAuthorfostertom
    • CommentTimeFeb 25th 2014 edited
     
    Posted By: Jontihow could you run an MHRV if you were off grid
    Electricity for anything 24/7 has to allow for great intermittency of all 'renewable' sources, even hydro. So batteries! - then take your pick of source.

    If off-grid, you'll have had to solve this one already, and adding a low power MHRV is neither here nor there - in other words being off grid does not affect whether MHRV is gd idea for you, or not.
    • CommentAuthorEd Davies
    • CommentTimeFeb 25th 2014
     
    My rule of thumb would be that the capital cost of off-grid production in winter would be about £40/W or perhaps a bit more so, taking RobL's average of 13 W, that would be about £520 worth of panels, etc. Bit more wear on the batteries, as well, of course.

    You need to balance the cost of running the MVHR vs the cost of replacing the heat which would otherwise be lost from the building. If you're a pyromaniac with cheap or free wood and think you'll drop dead before you're ever too infirm to deal with moving and using the wood then the calculation is different from if you're trying to run the house purely on nuclear fusion.

    Similar sort of calculation for the sewage treatment plant. Was going for Bio-rock but my house designer specced one with an air pump. Started writing an email saying to change and including the calculations of why and realized that actually generating the electricity would only cost a bit more than upgrading to a fanless plant. And that's over the winter months - spending the extra on PV to run the plant would then give more spare power in the brighter parts of the year.
    • CommentAuthorborpin
    • CommentTimeFeb 25th 2014
     
    Posted By: RobLBest advice is get an efficient unit, and use CO2/humidity control
    Do you really find that the CO2 level matters? I have neither run it at slightly below what the calcs say and the humidity is fine (actually tends on the low side). Not measured the CO2 as the controller was very expensive (Helios).
    • CommentAuthorJonti
    • CommentTimeFeb 26th 2014
     
    Thanks for the answers and suggestions. This is just theoretical at the moment but that may change in the near future.

    Jonti
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