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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    If you ventilate a house with outside air at 5degC, and heat it up to indoor temperature, it can then absorb more than double the amount of water vapour than the same ventilation with outside air at 15degC. So ventilation in the winter is much more effective at controlling humidity than ventilation in the summer.

    This suggests you should increase MHRV flow rates in summer to control humidity and decrease them in winter to avoid wasting heat.

    But does anyone here actually do that? Do you use humidistat to do this or manumatically? Or do people stick at one flowrate all year, and supplement by opening windows in the summer?

    Interested as am looking at a MHRV project and deciding how big to size the system.
    • CommentAuthortony
    • CommentTimeJun 4th 2019
    If you ventilate your house with MHRV you won’t have any condensation problems.

    I slow mine down in the daytime in excessively hot weather and at night when it is generally cooler than my house I increase the flow rates to increase night time cooling. I also take out my heat exchanger, some may have summer bypass mode that has a similar effect.
    • CommentAuthortychwarel
    • CommentTimeJun 4th 2019
    I turn my off through the summer, as I go for natural ventilation then ( windows open) no condensation problems
    • CommentTimeJun 4th 2019
    I do generally increase my ventilation rate in summer to help cool the house, yes. It does make a difference but nowhere near as much as opening windows. My wife doesn't like flies and other insects in the house, especially midges, so I'm restricted in when we open them. Incidentally, and a slight diversion from topic, I'm currently looking for insect screens to overcome that problem, so if anybody's got any recommendations ...

    Opening windows also rapidly reduces the humidity. When I did it yesterday it went to 38%. As soon as I close the windows it climbs back up - to 41% yesterday - and sits there.

    We have an automatic summer bypass so I don't have to be religious with the MVHR speed, but if I remember I will turn it down to speed 0 (50 m³/hr) if it's hotter outside than inside. Cooling is more effective at night anyway when the external temp is generally a lot less.

    In winter we keep the ventilation on speed 0 during the day and set it at speed 1 (125 m³/hr) or speed 2 (165 m³/hr) overnight because we have a post-heater and it needs the air speed to transfer the heat. In the shoulder seasons we generally run at some mix of speed 0 and speed 1, unless we have lots of visitors.

    Our system is nominally 300 m³/hr capacity, but the max speed 3 is still set at the factory default of 225 m³/hr and we never use it. Speeds 1 & 2 were set for commissioning - I don't remember whether it's PH or BR rules - and haven't been changed since. Speed 0 is the default 'trickle' setting but is very nearly enough for two people's air supply.

    In our house a humidistat would not help. I expect I could build some kind of temperature-based controller if I cared, but I actually enjoy the manual intervention.
    Thanks all!

    Noted the summer overheat issue some of you have down South... it's not such a consideration here!

    I'm interested if anyone adjusts their ventilation specifically because of getting better drying effects with dry winter air than with muggy summer air?
    • CommentTimeJun 5th 2019
    No, although I was concerned about too-dry air when we first moved in, following experiences in the first winter in our new offices at work. But my house seems to have some as yet unexplained humidity buffering properties.
    I just leave ours alone the whole year. Auto summer bypass.

    Cooling impact is limited - in summer we need open windows (and room fans at night when it's really hot)

    Our MVHR goes to boost based on bathroom humidity sensors (amongst other things) - there are a few high humidity atmospheric considitions when it goes into boost permanently and i have a setting to over-ride for those few days a year.
    Will, in my experience outside walls and windows are colder in winter and so more likely to attract condensation. Preventing that requires lower indoor humidity in winter and so the improved drying effects of cold winter air are needed.
    • CommentTimeJun 13th 2019
    Luckily, cold winter air is available exactly in winter :)

    The dew point of air at 20°C and even 50% RH is only 9°C so it would require extraordinarily poor windows to suffer from internal condensation if there is a ventilation system running.

    In our house, internal RH stays much the same throughout the year.
    Mmm, I couldn't get the numbers to work on this approach. The family do the same amount of cooking showering and breathing in summer as in winter, so generate the same number of kg of water vapour every day. If we ventilate the same number of m3 of air every day, then each m3 must carry away the exact same number of g of water, summer and winter. Even if we allow the inside humidity to rise to 60-70% in summer, it doesn't compensate for the ventilation air from outside arriving carrying much more water in summer than it does in winter, so in summer we can't carry away enough kg of vapour unless the unit is well oversized.

    The picture we are getting is that most MHRV units are way oversized to cope with this (per DJH's 300m3 unit that runs at 50m3), and many people (except DJH) don't turn them down in winter, so ventilate their heat away. We'll probably go the other way, start with a small unit for winter, and open windows in summer, not being driven by BRegs.

    Edit to add: we are not so concerned about occasional overnight condensation on window glass, more about long-term raised RH that causes mustiness in corners and cupboards.
    • CommentTimeJun 13th 2019
    Posted By: WillInAberdeenThe picture we are getting is that most MHRV units are way oversized to cope with this (per DJH's 300m3 unit that runs at 50m3)

    Sizing is determined by building regs required extract and supply rates. I don't remember ours offhand, but we nominally have four bedrooms of which one is normally occupied, so our normal ventilation rate is a long way below what we would use in a fully-occupied house. There is also a desire to somewhat oversize units, since that results not only in quieter operation but also more efficiency at lower fan speeds I think (I'm open to contrary information on the last point, but the powers at the various rates I have measured are 50 m³/hr -> 9 W, 125 m³/hr -> 29 W, 165 m³/hr -> 49 W, IIRC)
    Sorry, that's what I meant by us 'not being driven by BRegs' - we are in Scotland.

    Quite a few folk have mentioned on GBF that their ventilation was sized to an arbitrary number of 0.5ac/h from the English building regs, that takes no account of occupancy, lifestyle, local climate or time-of-year. Like you, many folks report they can actually run comfortably at a small fraction of their installed capacity for most of the year, if not all of it.

    I'm interested what capacity we would actually utilise for our family's lifestyle, which raises the question whether to size it for winter, summer (ie much bigger), or something in between..

    Power loss in a laminar flow duct is theoretically proportional to the square of the flowrate, but yours seems to be doing better than that, at higher flows :-)
    • CommentTimeJun 14th 2019
    Posted By: WillInAberdeenSorry, that's what I meant by us 'not being driven by BRegs' - we are in Scotland.

    I don't understand that. You have building regs, no? And they say to follow CIBSE Guide B and that is much like the English regs?

    The system generally needs to be sized for the maximum designed occupancy - either for your own use at Christmas or perhaps a midsummer BBQ when it turns out to pour down, or for whoever owns the property after you. Clearly the same system needs to work all year round, so I'm not sure why the regs should take any account of the season? Extract and fresh air supply rates don't depend on the climate either.

    I think the power requirements are a blend of increasing fan efficiency at higher speeds and increased duct losses at higher speed. But I'm not sure I've remembered my numbers correctly, now. In fact, I found my records and the numbers are 50 m³/hr -> 9 W, 125 m³/hr -> 21 W, 165 m³/hr -> 29 W
    No, in Scotland there are Building Standards (not Regs). The Technical handbook (similar to ADs) allows for different ventilation strategies depending how airtight the building is, but no prescribed continuous ventilation rate.

    (Edit to add: the English AD F seems to specify a minimum ventilation rate of 0.3l/s per m2 floor area, which is roughly equivalent to 0.5ac/h.
    Whereas CIBSE guide B2 says "Formerly, much use was made of expressing ventilation rate in terms of air changes per hour (ACH). This is now falling out of favour because ... it fails to reflect the physical need to provide fresh air...")

    The house we are considering buying, has trickle vents that I expect will already satisfy the technical handbook, given its likely level of airtightness.

    We are thinking to install MHRV of sufficient size that we could keep the trickle vents closed in winter. As mentioned we will open the windows in Summer.

    Ventilation rates (should) depend on the external humidity, you need more ventilation to dry a house in a warm damp climate like Arran than a cold dry area like Aberdeen.

    Midsummer BBQ parties around here are traditionally held in the garage, as it is inevitably going to pour down. The garage door is half-closed to keep the rain off the chef, with the grill just outside and the guests inside. We don't have midges here so don't need smoke to keep them out.
    • CommentTimeJun 14th 2019 edited
    Posted By: WillInAberdeenNo, in Scotland there are Building Standards (not Regs).

    Notice I didn't capitalise. I was not talking about the names of particular pieces of legislation. You still have regulations that govern building. Call them what you will.

    I was responding to your claim that people oversize ventilation systems. You are now saying that in your case, you are not planning to provide ventilation by means of a mechanical ventilation system, but by a more complicated method of which a mechanical system forms but one portion. In that circumstance, clearly the sizing calculations appropriate fore a whole house ventilation system are not appropriate.
    Hmm, think I mentioned yesterday that I am indeed looking for a MHRV system, sized 'for winter, and will open the windows in summer, as we are not being driven by BRegs.' Nothing more complicated than that!

    You kindly provided your actual vs design flowrates vs occupancy, and mentioned 'a desire to somewhat oversize units' for noise and power, which is helpful in confirming our plan to go for a smaller unit.

    You also mentioned that "Sizing is determined by building regs required extract and supply rates" but as I responded to both your questions, this isn't a requirement on us under Scottish standards.

    Moving onwards, for 2adults/3kids I am conservatively looking at water vapour generation:
    5*1.25kg/d (respiration) + 1kg/d (showers) + 2kg/d (kitchen) + 1kg/d (laundry) = 10kg/d.

    Some of this will diffuse away through breathable surfaces, but conservatively ignoring this, and taking winter air conditions of 5degC/90%rh/6gm-3 outside and 21degC/60%Rh/11gm-3 inside, the ventilation required is

    10kg/d / (11- 6)g/m3 *1000/24 = 80m3/h

    So we will look for 80m3/h of ventilation, from MHRV and leakage.

    For comparison, if i had sized using summer outside conditions of 15deg/80%rh/10gm-3 then the required capacity would have been 400m3/h. In England, AD F would require 200m3/h.
    • CommentTimeJun 14th 2019
    Posted By: WillInAberdeenSo we will look for 80m3/h of ventilation, from MHRV and leakage.

    If you look at CIBSE B, you will see that the recommended fresh air supply rate is well above that. If you read other documents, notably Swedish ones, you will appreciate the importance of providing enough fresh air to people, especially children.
    That's interesting, CIBSE B2 refers to CIBSE A which quotes air change rates around 10l/s/person for non-domestic buildings where people are active, based on where "odorous pollutants arise due to human activities, e.g. body odour" or 45l/s/person if smoking is still allowed. For air change rates in houses, it refers to AD F air change rates 'to control moisture', presumably they think we don't mind sweaty smells at home! Our family don't smell too bad yet, though teenage years are ahead... However as I quoted above CIBSE B is backing away from prescribed air rates.

    Have you any references for the other Swedish documents?
    • CommentTimeJun 15th 2019
    search for swedish iaq research

    I think CIBSE are saying the focus should be on per person supply rates rather than per volume, or per floor area rates. That is generally sensible, IMHO. There is an assumption in regulations about the amount of space per person in domestic situations that simplifies domestic calculations (and reduces arguments from people trying to outguess the system). I believe it's more explicit in German law than English.
    Had a quick search round, seems there is one major Swedish study that the rest are citing, with measurements across hundreds of houses, and a couple of smaller followup studies.

    Air change rates were measured 0.3-0.5 ac/h, but up to 0.6 ac/h in newer low-energy buildings with MHRV.

    The pollutants NOx, formaldehyde and VOCs were measured as indicators of air quality, all were well within recommended health limits.

    Interestingly, houses with more ventilation, tended to have higher NOx levels, apparently brought in from traffic outside.



    Also the low-energy houses with MHRV had excessively low RH%. The leading researcher Sarka Langer comments :

    "Whilst in the housing stock and new conventional buildings RH is most of the time within the comfort zone, in passive buildings it lies mostly below the lower limit of 30%. This explains the higher (worse) Indoor Environment Index found in passive buildings and indicates that these buildings may be over-ventilated."


    All the above referred to Swedish winter climate, which is maybe more comparable to eastern Scotland than the south of UK.

    I am getting more comfortable going for an air change rate rather less than 0.5ac/h, maybe 0.3ac/h.

    As we discussed above, ventilation should ideally be based on occupancy, lifestyle, construction and climate (total m3/h basis), rather than on floor area (ac/h or l/s/m2 basis).

    However as a rough benchmark of the ventilation required for a family house, a 100m2 house with 0.3 AC/h would need 100*2.4*0.3 = 72m3/h, which is in the ballpark I was looking at.

    Thanks everyone for the comments, it's helpful to get questions/challenge when learning something new.
    • CommentTimeJun 16th 2019 edited
    I would recommend sizing for an increased exchange rate, even if you don't use it most of the time. I'd also consider trying to increase humidity buffering, wherever possible, to avoid extremely low RH. It's also pretty easy to add humidity if required with a few plants or an upside-down earthenware pot standing in water, or even a humidifier :)

    Excessively low RH is a well-known problem possibility in PH. Reducing ventilation rates can be a good way to ameliorate it. Increasing humidity supply is another: enthalpy exchanger in the ventilation system, humidity injection as mentioned above etc. It's not an insuperable problem.

    Here are some file names from my disk that might point to other info:

    blomqvist 2009 - Distribution of Ventilation Air and Heat by Buoyancy Forces Inside Buildings.pdf
    crump et al 2009 - NHBC - Indoor air quality in highly energy efficient homes - a review.pdf
    feist et al 2006 - Re-inventing air heating - Convenient and comfortable within the frame of the Passive House concept.pdf
    grimsrud 2006 - Ventilation Standards and High Performance Buildings.pdf
    koiv et al 2010 - Indoor climate and energy consumption in residential buildings in Estonian climatic conditions.pdf
    kurnitski et al 2007 - Use of mechanical ventilation in Finnish houses.pdf
    meisenberg & tschiersch 2011 - Thoron in indoor air: modeling for a better exposure estimate.pdf
    nasa 1989 - Interior Landscape Plants for Indoor Air Pollution Abatement.pdf
    siddall 2012 - Passivhaus ventilation: Its not a lot of hot air Indoor-Air-Quality-and-Ventilation-Mar-28_MJS_29-KdeS41.pdf
    wargocki et al 2002 - Ventilation and health in non-industrial indoor environments: report from a European Multidisciplinary Scientific Consensus Meeting - EUROVEN.pdf
    wolkoff & kjaergaard 2007 - The dichotomy of relative humidity on indoor air quality.pdf

    I haven't actually looked at them again to see which might be helpful, I'm afraid. I just selected titles that looked like they might be relevant to you.

    HTH, Dave

    edit: PS I haven't read the paper you cited, but there was research in Sweden a while ago that indicated higher ventilation rates benefited school children, so maybe that has translated into a fashion for too high a ventilation arte in houses?
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