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    •  
      CommentAuthordjh
    • CommentTimeNov 10th 2016
     
    Thanks for your comments Will. I'm busy trying to do the calculations you suggested earlier, which will take me a while, but you've pointed out some possible factors I wasn't considering and that's motivated me to do some quantitative thinking.

    I must make a point of recording some of my data. I have a plan to expand my logging to capture it all but at the moment the numbers I'm discussing are just read from the displays of meters. I must at least write them down.

    Yesterday, I reduced the MVHR to 50 m³/hr during the day, when there's only one person at home. Temperature dropped below 19°C, and the decreasing temperature was why I turned down the ventilation. Humidity climbed to 47%. I haven't yet worked out the implications. Last night I turned it back up to 165 m³/hr and I've put it back at 50 m³/hr today.
    • CommentAuthorcjard
    • CommentTimeNov 10th 2016
     
    Does spacial paint make the place seem bigger? ;)
    • CommentAuthorringi
    • CommentTimeNov 10th 2016
     
    I questions if the "mhrv rate" is correctly recorded for most homes....

    It is also well know that people breathing add more water to the air when the RH is lower. Like with plants, showers, cooking, uncovered wc etc.

    Hence using average values for a building with such a low RH is questionable, but djh may not wish to spend a day with a plastic bag over his head….

    The only way to get a good measurement of RH is to use a “wet bulb” and a “dry bulb” thermometer recording the output of both of them. (About £30 without calibration, £200 with NIST traceable calibration) I don’t think any of us have such equipment.
    •  
      CommentAuthordjh
    • CommentTimeNov 10th 2016
     
    Posted By: ringiI questions if the "mhrv rate" is correctly recorded for most homes....

    In our case it's actively controlled by the MVHR and that was checked with a calibrated meter during commissioning.

    It is also well know that people breathing add more water to the air when the RH is lower. Like with plants, showers, cooking, uncovered wc etc.

    Well, we're hygroscopic objects the same as wood, so we behave like a block of wood ...

    Hence using average values for a building with such a low RH is questionable, but djh may not wish to spend a day with a plastic bag over his head….

    I've heard it can be fun :devil: but I haven't wanted to try it yet :shocked:

    I don't think 45% is particularly low; isn't it in the middle of the target range 30%-60%? When we first moved in to our new building at work, they hadn't sorted the air handling properly and the RH was below 30%. That caused a lot of symptoms and complaints. And I've lived in old, badly ventilated properties with humidities over 60% but I wouldn't want to repeat the experience.

    The only way to get a good measurement of RH is to use a “wet bulb” and a “dry bulb” thermometer recording the output of both of them. (About £30 without calibration, £200 with NIST traceable calibration) I don’t think any of us have such equipment.

    Ah, now that's an interesting idea, though I haven't found any reason to mistrust the meters I do have so far.
    • CommentAuthorringi
    • CommentTimeNov 10th 2016
     
    Posted By: djhWell, we're hygroscopic objects the same as wood, so we behave like a block of wood ...


    No our lungs are a lot more active then that and works hard to keep the RH high enough inside of them. Hence dehydration was a big issue when Everest was first being attempted. Live plants with leafs on them are also more active the a lump of wood.
    • CommentAuthorEd Davies
    • CommentTimeNov 10th 2016
     
    Posted By: WillInAberdeenDJH is extracting 9kg/d of water from his house…
    That's assuming that the air going into the extract vents is at the measured RH. COâ‚‚ can vary widely across the room so I expect humidity can, too (as is pointed out in the Trouble With Humidity document Steamy linked on the previous page). RH sensor in the extract duct, anybody?
    •  
      CommentAuthordjh
    • CommentTimeNov 11th 2016
     
    Posted By: ringi
    Posted By: djhWell, we're hygroscopic objects the same as wood, so we behave like a block of wood ...

    No our lungs are a lot more active then that and works hard to keep the RH high enough inside of them. Hence dehydration was a big issue when Everest was first being attempted. Live plants with leafs on them are also more active the a lump of wood.

    Thanks for the correction. So where a block of wood is a constant-RH whatever the temeperature device, a person is a super-CRHWT device that also maintains constant internal temperature.
    •  
      CommentAuthordjh
    • CommentTimeNov 11th 2016
     
    Posted By: Ed Davies
    Posted By: WillInAberdeenDJH is extracting 9kg/d of water from his house…
    That's assuming that the air going into the extract vents is at the measured RH. COâ‚‚ can vary widely across the room so I expect humidity can, too (as is pointed out in the Trouble With Humidity document Steamy linked on the previous page). RH sensor in the extract duct, anybody?

    The 9 kg seems a bit high to me, but I haven't checked my calcs. I make the ventilation loss

    net loss (50 m³/hr) = 2.58 kg/day
    net loss (125 m³/hr) = 6.45 kg/day
    net loss (165 m³/hr) = 8.52 kg/day

    I haven't observed any great differences in RH in different parts of the building, except for spikes with showers, but I haven't been looking for differences.

    Putting an RH+T sensor in all four MVHR ducts is one of the things I plan on doing in December. Should let me measure external conditions and determine when the duct heater is on as well as double-checking the internal conditions and the MVHR performance.
    • CommentAuthorringi
    • CommentTimeNov 11th 2016
     
    This is all making me questions if duct heaters are a good option, as it is hard enough to set the HRMV rate without having to think about the effect on heating as well.
    •  
      CommentAuthordjh
    • CommentTimeNov 11th 2016
     
    Posted By: ringiThis is all making me questions if duct heaters are a good option, as it is hard enough to set the HRMV rate without having to think about the effect on heating as well.

    I still don't have enough data to make my mind up, to be honest. But the possible heat output (the heat load) is limited by the air temperature and the ventilation rate, so you do need to run the ventilation at the design rate to stand any chance of meeting the design heat load. What complicates my system is the desire to run the heating at night on E7, so if it gets properly cold I'll need to use some other heaters as well or use the duct heater during the day.
    • CommentAuthorringi
    • CommentTimeNov 11th 2016
     
    Posted By: djhWhat complicates my system is the desire to run the heating at night on E7,


    Small E7 storage heaters can be plug into normal sockets (but check the total load).
    • CommentAuthorEd Davies
    • CommentTimeNov 11th 2016
     
    What would the advantage to DJH of storage heaters? His house seems to already provide enough storage in that it he can get away with heating the air just on E7 anyway - just with a bit much ventilation.

    Am I right to assume that the Passivhaus goal of heating with ventilation air only assumes that heating runs 24 hours?
    • CommentAuthorringi
    • CommentTimeNov 11th 2016
     
    One modem fan controlled Small E7 storage heaters in say the hallway may remove the need to have too much ventilation.
    • CommentAuthorEd Davies
    • CommentTimeNov 11th 2016 edited
     
    Right, but why wouldn't an ordinary convector heater do the same thing? Could still be run off E7, just using the structure of the building rather than special bricks to store the heat.
    • CommentAuthorringi
    • CommentTimeNov 11th 2016
     
    Posted By: Ed DaviesRight, but why wouldn't an ordinary convector heater do the same thing? Could still be run off E7, just using the structure of the building rather than special bricks to store the heat.


    Controlling it will be harder. Personally I would be happy to use a oil filled electric heater on full price, as the cost would be so low with the level of insulation DJH has.
    •  
      CommentAuthorSteamyTea
    • CommentTimeNov 12th 2016
     
    Posted By: djhWhat complicates my system is the desire to run the heating at night on E7
    Can you get a couple of large storage heater, say 35 kWh ones, put them in an insulated box, plumb them into the MVHR system so that they are after the heat exchanger i.e. on the pipework to the rooms.
    Be just like a duct heater, but you can store up to 70 kWh of energy.
    When I measured the air coming out of my storage heaters, it was at 80°C, so by the time there is a bit of air flow around them, that should drop around 50°C which will stop the burning smell.

    You can get old storage heaters for nothing.
    •  
      CommentAuthordjh
    • CommentTimeNov 12th 2016
     
    Posted By: Ed DaviesAm I right to assume that the Passivhaus goal of heating with ventilation air only assumes that heating runs 24 hours?

    I believe so. At the designed minimum external temperature the heat losses are such that the design heat load (10 W/m²) can just keep the inside temperature at 20°C. And I believe the 10 W/m² was chosen to be the maximum heat that can be delivered through a fresh-air-only ventilation system (i.e. no recirculation) whilst limiting the air temperature to, I believe, 50°C. But I also think the design minimum temperature is -10°C, so most of the time it shouldn't be necessary to use the heating continuously. And if actual external temp goes below design temp then additional heaters or jumpers are required.

    Right, but why wouldn't an ordinary convector heater do the same thing?

    Coincidentally, I tried that last night to see what would happen. It didn't seem to make a lot of difference, but then it has its own thermostat and it may have caused the duct heater to reduce its output as well. I plan to try a 'radiant' quartz heater tonight.
    • CommentAuthorEd Davies
    • CommentTimeNov 12th 2016
     
    DJH, what temperature does your duct heater heat the air to? 50 °C or less?
    •  
      CommentAuthordjh
    • CommentTimeNov 12th 2016
     
    Posted By: Ed DaviesDJH, what temperature does your duct heater heat the air to? 50 °C or less?

    I think I put the set point at 45°C, from an abundance of caution (or nervousness!)

    It's notable that quite a lot of the heat goes into the duct walls and thence into the first floor structure. I can roughly track the duct layout using my IR thermometer in the mornings. The temperature at the supply terminals is noticeably warmer for those with shorter duct lengths. I'm quite content as long as the heat goes into the house somewhere though.
    • CommentAuthorringi
    • CommentTimeNov 12th 2016
     
    Posted By: djh
    1. I think I put the set point at 45°C, from an abundance of caution (or nervousness!)

      It's notable that quite a lot of the heat goes into the duct walls and thence into the first floor structure. I can roughly track the duct layout using my IR thermometer in the mornings. The temperature at the supply terminals is noticeably warmer for those with shorter duct lengths. I'm quite content as long as the heat goes into the house somewhere though.


    So you are "kilm drying" all the wood in the first floor structure, what else are you kilm drying at assuming at least 30c?
    •  
      CommentAuthordjh
    • CommentTimeNov 12th 2016
     
    Posted By: ringiSo you are "kilm drying" all the wood in the first floor structure, what else are you kilm drying at assuming at least 30c?

    I assume I kiln-dried it all last winter :bigsmile:

    All the usual: insulation, plumbing, electrics etc. I doubt there's much more stress than from central heating, DHW distribution and hot summers in more normal houses.
    •  
      CommentAuthordjh
    • CommentTimeNov 12th 2016
     
    I've done a bit more of my humidity calcs. I don't have a lot of confidence in having correctly interpreted the sources, nor in having done the arithmetic properly, so do feel free to correct me.

    First another simple bit: internal vapour generation.

    From http://www.egt.bme.hu/szikra/w_english/building_physics/BF_LN_L03.pdf

    Main moisture sources:
    * Occupancy – 40 g/hour, per person
    * Drying, plants, cleaning, washing, showering – 25 L, per person/week

    40 g/hour => 960 ml/day/per person
    25 L/week => 3571 ml/day/per person
    Total = 4.5 L/day/per person

    Total for two people = 9 L/day = 9 kg/day

    So a bit more than the ventilation loss.
    •  
      CommentAuthordjh
    • CommentTimeNov 12th 2016
     
    And here's another bit: the vapour diffusion through the walls.

    I've ignored diffusion through the roof because it has a vapour retarder.

    Indoor
    * temperature 20°C - 18.5°C
    * RH 45% - 50%
    * vapour pressure 1.05 kPa (http://www.csgnetwork.com/vaporpressurecalc.html)

    Outdoor
    * temperature 2°C - 4°C
    * RH assumed 100% (maybe as low as 80%?)
    * vapour pressure 0.706 kPa

    Wall structure, inside out
    * Clay paint (thickness say 100 µm)
    * Lime render (thickness say 75 mm)
    * Straw bale (thickness 457 mm)
    * Lime plaster (thickness say 75 mm)
    * Silicate paint (thickness say 100 µm)


    vapour resistivity figures

    from Neil May's breathability paper
    * Lime plaster 45-200 typical 75 GN.s/kg.m
    * Mineral wool, flax, sheepswool 5-7 typical 6 GN.s/kg.m
    * Woodfibre insulation boards, 25 typical 25 GN.s/kg.m
    * Cellulose insulation (blown) 40-50 typical 45 GN.s/kg.m
    * Silicate paints 250-350 typical 300 GN.s/kg.m thickness 100 μm
    * Clay plaster 30-50 typical 40
    * clay boards 90 typical 90

    * Beeckosil fine Sd < 0.02 m (W24 < 0.08 kg/m²h¹/²)
    ** [data sheet http://www.beeck.com/bmf-pdfs/produkt-downloads/bmf--id59-535.pdf]

    from https://buildingscience.com/documents/digests/bsd-112-building-science-for-strawbale-buildings
    * straw bale 18" 200-400 metric perms


    Vapour resistance calculation

    flow through clay paint
    # resistance = guess 40 * 0.0001 = 0.004 GNs/kg

    flow through lime
    # resistance = 75 GN.s/kg.m * 0.075 m = 5.625 GNs/kg

    flow through straw - what about moisture content? does it matter?
    # permeance = 400 metric perms
    # so permeance = 400 * 0.0968 g/MNs = 38.72 g/MNs
    # so resistance = 1/ 38.72 = 0.026 GNs/kg

    flow through lime
    # resistance = 75 GN.s/kg.m * 0.075 m = 5.625 GNs/kg

    flow through silicate paint
    # Sd < 0.02 m
    # so µ < 0.02 / 0.0001 = 200
    # so resistance < 200 * 0.0001 / 0.2 = 0.02 / 0.2 = 0.1

    Total resistance = 0.004 + 5.625 + 0.026 + 5.625 + 0.1 = 11.38 GNs/kg


    Vapour flow calculation

    Vapour flux = Mass Flow/area = (Pin - Pout) / Rt
    = (1.05 kPa - 0.706 kPa) / 11.38 GNs/kg
    = 30 µg/s.m²

    Wall area (approx) = 44*6 = 264 m²

    Mass flow = 13.92 mg/s = 1.2 kg/day

    So not comparable with the ventilation or internal generation, but not insignificant either.

    Now I need to think about any possible pumping via hygroscopic materials the other way.

    And then think about how changing conditions affect the various rates to see whether it exhibits any stability of interior RH without allowing for buffering.
    • CommentAuthorEd Davies
    • CommentTimeNov 12th 2016
     
    For this sort of thing you might find my little calculator handy: https://edavies.me.uk/2014/05/cra/

    Checking your numbers (other than the conversions from Sd, etc, which I'd have to look up again…calculator does µ values but not the other non-SI units) I put in:

    Indoor air: 19 °C, RH 47%.
    Layer 1: Clay paint, thickness 100e-3 mm, resistivity 40 GN·s/(kg·m).
    Layer 2: Lime render (inner), thickness 75 mm, resistivity 75 GN·s/(kg·m)
    Layer 3: Straw bale: thickness 457 mm, permeance 38.72 µg/(N·s).
    Layer 4: Lime render (outer), thickness 75 mm, resistivity 75 GN·s/(kg·m)
    Layer 5: Silicate paint, thickness 100e-3 mm, resistance 0.1 GN·s/kg
    Outside air: 2 °C, RH 100%

    and got substantially the same results:

    Vapour resistance: 11.379826446280992 GN·s/kg
    Vapour permeance: 0.08787480237247441 µg/(N·s)
    Vapour pressure: 326.826894426334 Pa
    Vapour flow: 28.719848757723657 µg/(m²·s)

    Yeah, it could round the results a bit but maybe it's better not to.
    • CommentAuthorringi
    • CommentTimeNov 12th 2016
     
    Posted By: djh
    Posted By: ringiSo you are "kilm drying" all the wood in the first floor structure, what else are you kilm drying at assuming at least 30c?

    I assume I kiln-dried it all last winterhttp:///newforum/extensions/Vanillacons/smilies/standard/bigsmile.gif" alt=":bigsmile:" title=":bigsmile:" >

    All the usual: insulation, plumbing, electrics etc. I doubt there's much more stress than from central heating, DHW distribution and hot summers in more normal houses.


    How much vapour would have been stored by the above assumeing it was at 20c and 75% RH for a few months since it was last "kilm dried"? (Unless the insulation is a good buffer I expect not enough to explain what is going on.)
  1.  
    Interesting numbers DJH, thanks for posting them!

    Didn't quite follow 30ug/s/m2 being 1.2kg/d

    The outside render is almost half of the total vapour resistance and is outboard of the insulation. A conventional simplistic analysis would say that would mean condensation on the inside face of the render. As the temp on this inside face is virtually external, and the external RH =100%, the RH on the inside of the render is also 100%. As the vapour pressure gradient across the render is virtually nil, no vapour can diffuse through the outside render. In contrast the plaster layer inside the wall, experiences almost the entire vapour pressure delta (300Pa inside to outside), so vapour flow through it will be about twice the figure calculated here (so 300/5.6) most of which will condense in the straw. To be honest I think this shows up the flaws in the conventional CRA methods, not in your wall!
  2.  
    Not sure about 3.5kg/d/person from cooking/washing - that's 6 pints! Sure I'd notice if that much disappeared from my saucepan, or was left in my shower tray to evaporate...?

    To be fair, I don't have a better figure!
  3.  
    Interesting that the vapour creation rate almost balances the extraction rate (mystery solved?). What happens in the summer?

    Thanks again for posting the calcs!
    • CommentAuthorEd Davies
    • CommentTimeNov 12th 2016
     
    Assuming the thermal conductivity of straw is 0.040 W/m·K and render is 0.12 W/m·K [¹] and the paints have zero thermal resistance my calculator says:

    Interface Straw bale / Lime render (outer)
    Temperature 2.838264299802759 °C
    Vapour pressure 870.3599103637765 Pa
    Equilibrium VP 749.3619036351089 Pa

    So, yes, the calculated vapour pressure is higher than the equilibrium (“saturated”) pressure (so RH = 870/749 = 116%). So on this simple static theory there's a condensation risk at this interface though not as apocalyptically so as Will suggests.

    [¹] Test result PDF linked from http://www.eden-lime-mortar.co.uk/html/eden_lime_insulation.html rounded up from 0.113 to 0.12. Just the first search result for “lime render thermal conductiivty”.
    • CommentAuthorEd Davies
    • CommentTimeNov 12th 2016
     
    Posted By: WillInAberdeenInteresting that the vapour creation rate almost balances the extraction rate (mystery solved?). What happens in the summer?
    Yes - well worth watching what happens through the year.
   
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