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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorgoodevans
    • CommentTimeAug 11th 2020 edited
     
    Yep I know what you are thinking - well insulated homes don't need active cooling in the UK. But unfortunately my home isn't perfect and it is hot. I have actively cooled my ground slab in the evenings before V. Hot days. Here is my thinking and actual numbers/costs.

    During the night, when it is coolest we choose to put the windows downstairs on night latch - on heatwave nights not enough air comes in to cool the walls and floor.

    The MVHR can run on boost - the marginal energy usage litre of the extra air pumped is approx 1 watt per litre (edit - that's 1 JOULE per litre of air) - at normal ventilation rates the MVHR is much more efficient (and in any case is required). Boost isn't free - and if outside is 18 deg C and inside is 22 deg C we get around 4 watts for cooling for 1 watt of power. (1 litre of air takes 1 joule of energy per degree C of cooling/heating).

    On a hot (30 deg C +), sunny day my house heats up by around 2 degrees during the hot part of the day when the windows are closed (say 12 hours). I estimate during this period the total gain is something like 5kWh solar gain, 12kWh conduction through the insulation, 7kWh electrical waste heat and DHW tank loss and 2kWh heat from us - a total of 26kWh - which can all be lost on a typical summer night/evening by leaving the windows open. But on nights like we are having at the moment I need more help - otherwise from day 2 or 3 the house gets too warm for 'perfect' - but still much better than almost any other house.

    We have an air source heat pump and under floor heating and ceramic tiles throughout downstairs. If outside is the same temperature as inside then the cooling COP from the heat pump is around 6.0 (it delivers 9 litres/min of water with a delta T of 3.1 deg C - just shy of 2kW of cooling at 1.3 amps or 0.31kW). As the temperature drops outside the COP improves (but is counteracted by the slab getting colder) but overall I can get 16kWh of cooling for around 2 to 2.3kWh of electricity at most (costing 30 pence). By morning my slab is down to 19 deg C.

    During the day the slab cools the house - today the slab has warmed up to 21.3 degrees C and has kept the temperature of the house to a maximum of 23.5 deg C.

    This only works because the slab acts as a massive heat/cool store, the house insulation and thermal mass is adequate to allow the slab to retain enough cooleth for the whole day, which means I need only cool the slab at night which means the COP is no worse than 6 (during the day it costs twice as much).

    This is a heatwave solution, when the night minimum temp is nearer 16 deg C - open windows upstairs and night latch downstairs cools the house ready for the next day with ease. So far it has been used for around 7 nights this year (and looking at the forecast for a couple of further nights) It is a little extravagant on energy - but not excessively so I think.
    • CommentAuthortony
    • CommentTimeAug 11th 2020
     
    Nice 👍
    • CommentAuthorgoodevans
    • CommentTimeAug 11th 2020
     
    Hmm - of all people I thought you would be most critical (in a nice way). I suspect you are in a better position for passive cooling with your external shutters and excellent insulation. Out of interest what temp can you keep your house down to, with what additional expenditure on your MVHR?
    • CommentAuthorEd Davies
    • CommentTimeAug 12th 2020
     
    Posted By: goodevansIt is a little extravagant on energy - but not excessively so I think.
    About £3/year (say 10 nights at £0.30/night) at the time of year (summer nights) when energy is cheapest and nearest to carbon-free. I'd say that was an excellent result. Shows just how bonkers this RHI thing about not being able to use the system for cooling is.

    And to think that the other morning I was wearing a woolly hat working on my house. I didn't absolutely need it but it was more comfortable with than without under a grey sky with a chilly breeze blowing off the North Sea.

    One tiny quibble: “the marginal energy usage litre of extra air pumped is approx 1 watt per litre”. Pretty sure you mean 1 watt per litre per second or 1 joule per litre.
    • CommentAuthorgoodevans
    • CommentTimeAug 12th 2020
     
    yep - you are correct 1 joule per litre. I was being careful as well - even used the word energy - then went and said watts.
    • CommentAuthorgoodevans
    • CommentTimeAug 12th 2020
     
    The thing with RHI is they they only want to pay for the energy that has been displaced from fossil fuels (and cooling typically uses this technology so it wouldn't displace any usage). The perverse thing for this incentive is it doesn't encourage insulation - the less energy you use - the lower your incentive. For our house our annual spend is around £90 on heating, £90 on DHW and £3 on cooling. It simply wasn't worth the paperwork to apply - and if we had we would be breaking the rules to keep cool.
  1.  
    Interesting numbers, thanks!, and personally I don't see why active cooling is any 'worse' or better than active heating, and as your house is well insulated you will have minimised both.

    Given the choice during design, it might even be good to maximize solar gains, so the house needs less heating in winter and more cooling in summer, as the CoP for cooling is better than heating, is better matched to solar generation days, and solar gain can be temporarily reduced by shading and shutters etc.

    I think people in the UK maybe suspicious of active cooling, having seen countries where the aircon is wastefully set to shivering cold indoor temperature, but setting a sensible summer indoors temperature is no different thought process than setting a sensible winter temperature. Nobody blinks an eyelid at having aircon in a car which is much less energy efficient than an ashp.

    Don't know much about your floor, but if there are say 10tonnes of screed/slab (50m2 area * 100mm of thickness actively exchanging heat with the room) it will take about 10MJ (3kWh) to heat/cool by 1degC. Does that match with your temperature swing of 19-21.3 = -2.3degC?

    What is the main source of 7kWh of electrical waste heat? If it's cooking then the extractor fan will dump that straightaway. If it's not too noisy you could perhaps run it overnight to draw cool air through the upstairs windows and down to cool the downstairs, if you cannot safely leave the downstairs windows open.

    The south facing surfaces of the roof and walls are likely to be much warmer than air temperature, mine get too hot to touch, so conduction will be more than might be expected.

    Edit: where does the motor heat from the mhrv go, into or out of the house? If in, then each 1J of motor power causes 1J of heating, offset against the 4J of cooling you mentioned, so net 3J of cooling : CoP=3. So it's better to use the ashp for active cooling instead, CoP =7
    •  
      CommentAuthordjh
    • CommentTimeAug 12th 2020
     
    Posted By: WillInAberdeenGiven the choice during design, it might even be good to maximize solar gains, so the house needs less heating in winter and more cooling in summer

    I think the point is that it should be possible to design for both, in that solar gains in winter can be maximised whilst those in summer are minimised through the use use passive techniques such as brise soleil that exploit the difference in solar altitude in summer and winter, as well as active techniques such as shutters.

    But I don't disagree with the rest of your comments.
    • CommentAuthorgoodevans
    • CommentTimeAug 16th 2020
     
    Posted By: WillInAberdeenWhat is the main source of 7kWh of electrical waste heat? If it's cooking then the extractor fan will dump that straightaway.
    I have no extractor fan in the kitchen - just MVHR - in heat wave conditions there is no way to loose the kitchen heat until the evening - the kitchen never gets more than a degree or two hotter then the rest of the house and during the heat of the day the summer bypass is off to keep the cool. In short I can't dump any heat because it's all cooler than outside. The main source of the electrical heat is fridges, freezers and computers - and we drink lots of tea. Cooking accounts for 1 to 3 KW depending on the day.
    • CommentAuthorgoodevans
    • CommentTimeAug 16th 2020 edited
     
    Posted By: WillInAberdeenEdit: where does the motor heat from the mhrv go, into or out of the house? If in, then each 1J of motor power causes 1J of heating, offset against the 4J of cooling you mentioned, so net 3J of cooling : CoP=3
    Well spotted MVHR is inside the envelope so I was too generous to the MVHR.
    • CommentAuthorgoodevans
    • CommentTimeAug 16th 2020
     
    Posted By: WillInAberdeenThe south facing surfaces of the roof and walls are likely to be much warmer than air temperature, mine get too hot to touch, so conduction will be more than might be expected.
    Walls are thin coat white and never seem to get hot - the roof on the other hand is dark grey concrete tiles - I've not felt it but I bet it's toasty. Heat getting through here is part of the solar gain of 5kWh. I calculated/guestimated the effects of solar gain in the early spring based on anecdotal evidence that sunny days can be 2 degrees colder than overcast days and require similar heating requirements over 24 hours. The summer sun effects may be radically different - but it is the best I can do at the moment.
    • CommentAuthorgoodevans
    • CommentTimeAug 16th 2020 edited
     
    Posted By: WillInAberdeenDon't know much about your floor, but if there are say 10tonnes of screed/slab (50m2 area * 100mm of thickness actively exchanging heat with the room) it will take about 10MJ (3kWh) to heat/cool by 1degC. Does that match with your temperature swing of 19-21.3 = -2.3degC?
    My slab is approx 100m2 and 80mm thick - 8m3 assuming 2.4 tonnes per m3 and a SHC of 888 J/(kg.K) I make that about 4.7 kWh per degree C. During the early stages of cooling when the room and slab temperature arn't too far adrift I calculate I deliver 2kW of cooling into the slab and measured a 0.5 degree drop per hour. Less later on into the night as the now cool slab looses cooleth to the room. But there is alot of hand waving here - the slab thermometer may not get a good average slab temp, the flow rate meter is just a plug on a spring behind a glass window, not every square meter of slab has pipes in it etc etc. but the numbers seem to be reasonable. (It was definitely cheap to cool the house for those few days)

    And for the record It looks like 1 m2 of ceramic floor will release around 2 to 3 watts of cool for each degree below room temperature in still air - I did notice that once the doors and windows were opened it appeared that the air movement allowed the slab to reach the room temperature within an hour or two.
    • CommentAuthorEd Davies
    • CommentTimeAug 16th 2020
     
    Posted By: goodevansAnd for the record It looks like 1 m2 of ceramic floor will release around 2 to 3 watts of cool for each degree below room temperature in still air…
    That's surprising. I'd expect the dominant resistance to be the boundary between the air and the ceramic so for it to be more like 5 W/m²·K or a little more. Just using Stefan-Boltzmann (i.e., just looking at heat transfer by radiation) implies that amount assuming usual household emissivities.

    There'll also be some heat transfer by convection/conduction which are a bit harder to quantify. It's said BS EN ISO 6946:1997 gives an indoor downwards thermal resistance for an air/maternal interface of 0.17 m²·K/W so a conductivity of 5.88 W/m²·K whereas BRE Digest 108 is quoted as saying 0.14 m²·K/W so 7.14 W/m²·K.

    So, does your ceramic have an emissivity significantly less than one (well, less than 0.9 or so)? Or is there another significant amount of resistance in the path? Am I wrong that the interface resistance is dominant and the time it takes for the heat to flow through the concrete matters more than I think?
  2.  
    Thanks for the info GE, interesting numbers!

    Coolth transferred by radiation from the slab will not cool the air, but rather it will cool the walls and ceilings and furniture. Depending on their material, they will absorb the coolth and buffer it like the slab does, or more likely their surfaces will cool down (so they become similar to slab temperature) and that will stop the net radiative heat transfer from the slab. Thereafter, the coolth transfer will continue only by convection to the air, from the floor walls and ceiling, so in the order of ~1W/m2K.

    The numbers in the BRE digest etc are for steady state heat transfer to the external environment (considered infinite) so don't account for this.
    • CommentAuthorgoodevans
    • CommentTimeAug 16th 2020
     
    Ed - I don't think this is the reason for the black body radiation being lower than the slab temp would indicate.

    What happens here is - the body of the slab may be 2 degrees lower than ambient - but the surface of the slab is not - the energy gain (or loss in this case) occurs at the boundary and may only be around 1 degree colder or less. Convection doesn't help take away the cooleth here either - in the still air a thin layer of cold air sits over the floor.

    For a warm floor - convection allows the layer of the warmer hair to be moved away from the surface so heat transfer is better for UFH in heating mode.

    I imagine a race condition occurring here - cool escapes from the surface by radiation and a bit of conduction, and is replenished by more cool migrating from the slab body - that isn't instant and it depends on the conductivity of the floor material(s) - a thick solid wood floor would also have an emissivity of 1 but wouldn't release it's heat as fast because the conductivity of wood is lower - even if it's core body temp was kept constant.

    And for those pedants out there - yes I know the actual way to think of the radiation transfer is that the room is radiating it's heat to the floor (or all surfaces radiate and absorb - its the balance that counts) and the floor is conducting that heat down into it's body - but I think are are all more familiar with heat flow coming 'from' the active element - in this case the UFH.
    • CommentAuthorgoodevans
    • CommentTimeAug 16th 2020 edited
     
    I have "under floor heating" installed in the ceiling of the first floor rooms but it's not plumbed in yet - hopefully, if the builders didn't puncture any of the loops when putting in the plasterboard I will be able to transfer some of the cool from the downstairs slab to upstairs to make upstairs as comfortable as downstairs when the sun is beating down on the roof. But that story is probably for next year. It will be interesting to see how effective the cool falls off the ceiling, compared to how it rises off the floor.
    • CommentAuthorEd Davies
    • CommentTimeAug 16th 2020
     
    Posted By: WillInAberdeenCoolth transferred by radiation from the slab will not cool the air, but rather it will cool the walls and ceilings and furniture.
    Good point, it's mostly not the air temperature which matters but the radiant temperature of the room and the interface resistance is competing not just with the resistance within the slab but also the resistances from the walls, ceiling, etc, to the air. A good reminder that the “ambient temperature” is not a simple thing when you're talking about relatively small temperature differences.

    Posted By: goodevansWhat happens here is - the body of the slab may be 2 degrees lower than ambient - but the surface of the slab is not -
    I think you're saying, in effect, that the thermal resistance of the slab is significant compared with the interface resistance.

    For a warm floor - convection allows the layer of the warmer hair to be moved away from the surface so heat transfer is better for UFH in heating mode.
    “hair” :bigsmile: But more seriously, yes, that's why those sources (ISO and BRE) quote higher resistances for downwards heat movement than upwards (or upwards coolth movement rather than downwards).

    And for those pedants out there - yes I know the actual way to think of the radiation transfer is that the room is radiating it's heat to the floor (or all surfaces radiate and absorb - its the balance that counts)
    This pedant is entirely happy to think about moving coolth around but cringes to see “it's” and “its” swapped like that.
    •  
      CommentAuthordjh
    • CommentTimeAug 16th 2020 edited
     
    Posted By: Ed DaviesI think you're saying, in effect, that the thermal resistance of the slab is significant compared with the interface resistance.

    I think there's a parameter called the 'thermal admittance' that addresses this to some extent.

    edit: and also starts to involve 'decrement delay'. :bigsmile:
    • CommentAuthorgoodevans
    • CommentTimeAug 17th 2020 edited
     
    Posted By: Ed DaviesThis pedant is entirely happy to think about moving coolth around but cringes to see “it's” and “its” swapped like that.
    The possessive "its" regularly catches me out - well spotted - it stays unedited as a lesson to me that I simply can't learn during the flow of typing. It did make me laugh given the "pedant" nature of the paragraph. That'll learn me.
  3.  
    >>>a parameter called the 'thermal admittance' that addresses this to some extent.

    No, the thermal admittance describes the ability of the slab to store heat, and so buffer temperature fluctuations which have a daily frequency.

    Ed was talking about something simpler than that, the thermal resistance of the slab, which doesn't depend on frequency.

    Electrical analogy: the thermal admittance is similar to an inductor which damps out any fast fluctuations in the heat flow. Ed was talking about two resistors in series, the slab and the surface.

    Decrement delay is the phase shift caused by the 'inductor' - would need more than 80mm of screed to shift the cooling from overnight to mid-afternoon, tho every little helps.
    •  
      CommentAuthordjh
    • CommentTimeAug 17th 2020
     
    Actually I was commenting on Ed's comment on Paul's observation and saying that I think thermal admittance may be more relevant than thermal resistance to the question at hand. And that question pretty much sums up the description of thermal admittance, I think. "Thermal admittance (Y) is a measure a material's ability to absorb heat from, and release it to, a space over time."

    There's a pretty good description of the admittance method at https://www.cibsejournal.com/cpd/modules/2013-01/ that includes a link to a spreadsheet that allows the calculation of heat flows into a concrete slab including the effect of the air boundary, among more complicated scenarios.

    Heat transfer is much slower than electrical currents. There are no real world materials that are pure 'resistances', nor any that are pure 'inductors'. Almost everything acts as a combination of the two when considering the analogies. It isn't possible to make a sensible model of heat transfer using just the resistance values, when the flows are not steady state for extended periods of time, especially for multi-layer structures.
    • CommentAuthorbhommels
    • CommentTimeAug 17th 2020
     
    Another question regarding cooling using an ASHP - blissfully unaware of the setup you have:
    Most ASHPs have a buffer tank that also serves for DHW, for example to provide hot water for a shower in the morning. How would this be compatible with your overnight active cooling scheme? How to avoid spending money on temperature cycling a buffer tank?
    • CommentAuthorMike1
    • CommentTimeAug 18th 2020 edited
     
    I have an apartment that ideally needs some kind of active cooling, but with no possibility of installing external heat pumps / chillers I've been reviewing the research on traditional cooling techniques, particularly those linked to the cooling effect of evaporating water.

    Evaporating 1 litre of water uses 2264.705 kJ of energy, or 0.629 kWh, which is a useful amount of cooling, and one which commercial aircon units sometimes use by spraying a water mist into the incoming or extract air. I haven't yet found anyone doing this for the domestic MVHR market, and legionella may be an issue, but it might be worth pursuing further.

    Provided you're not in an area of high humidity, options such as indoor - or courtyard - fountains or green walls could also be worth considering. Maybe even a piezoelectric fogging system.

    I hope to investigate the options further at some point (when I get time), but it would be interesting to hear from anyone who might already have considered these areas.
    •  
      CommentAuthordjh
    • CommentTimeAug 18th 2020
     
    Posted By: Mike1Provided you're not in an area of high humidity, options such as indoor - or courtyard - fountains or green walls could also be worth considering. Maybe even a piezoelectric fogging system.

    I like the idea, but the most uncomfortable aspect of the recent weather here has been the high humidity. I wouldn't want to be adding any more to it.

    A large tank of water is a pretty effective means of limiting temperature rises, just by its 'thermal mass'.
    • CommentAuthorEd Davies
    • CommentTimeAug 19th 2020
     
    Posted By: Mike1: “Evaporating 1 litre of water uses 2264.705 kJ of energy…”

    …at 100°C; more like 2450 kJ/kg around room temperature [¹].

    As djh says, when you want cooling in the UK the humidity is likely to already be high. As well as adding to the discomfort this will also discourage evaporation of your water.

    Looking at this psychrometric chart [²] imagine air at 30°C and 80% RH (follow the green 30°C line up to where it intersects the red 80% line) then look what happens if it's cooled till it's at 100% RH (move horizontally left from there to the 100% RH line) then read off the temperature by dropping down to the horizontal axis and we see that, at most, we can get down to about 26°C, i.e., 4°C of cooling.

    In reality you won't get that much cooling because you're increasing the absolute humidity of the air as well as cooling it (so you move up and to the left on the chart) so you'll reach saturation at some higher temperature, around 27°C.

    Evaporative coolers work well in hot dry climates (e.g, US south west), in wet places like the UK less so.

    [¹] https://en.wikipedia.org/wiki/Latent_heat#Specific_latent_heat_for_condensation_of_water_in_clouds

    [²] https://upload.wikimedia.org/wikipedia/commons/9/9d/PsychrometricChart.SeaLevel.SI.svg
    • CommentAuthorMike1
    • CommentTimeAug 19th 2020 edited
     
    Posted By: djhI like the idea, but the most uncomfortable aspect of the recent weather here has been the high humidity. I wouldn't want to be adding any
    That's where adding water mist to a MVHR extract would be the best solution - the extra humidity is pumped outside the building, but the cooled extract air cools the incoming. Though my particular apartment is in France, so less humid than the UK.

    Posted By: Ed DaviesPosted By: Mike1: “Evaporating 1 litre of water uses 2264.705 kJ of energy…”
    …at 100°C; more like 2450 kJ/kg around room temperature [¹].
    Just as well that 1 litre of water weighs 1kg then...

    ...and that water doesn't need to be at 100°C to evaporate - otherwise washing lines would be redundant :)
  4.  
    Posted By: Ed DaviesLooking at this psychrometric chart [²] imagine air at 30°C and 80% RH


    There's no chance you'd ever have that temperature and humidity combination in the UK. None. At 30C, if you're at 65% RH it feels like 40C. Our hottest days in Montreal were around 36C with 40% RH - this gives a humidex of 44C which feels pretty unbearable.

    See https://memory.psych.mun.ca/tech/js/humidex/

    Highest ever dewpoint I saw here in Montreal was around 24C - so high that windows were steaming up on the outside.

    See http://bmcnoldy.rsmas.miami.edu/Humidity.html for a dewpoint and RH calculator.

    Paul in Montreal.
    • CommentAuthorEd Davies
    • CommentTimeAug 20th 2020
     
    Posted By: Paul in MontrealThere's no chance you'd ever have that temperature and humidity combination in the UK. None.
    For outside temperatures, not yet. Maybe by 2100. ;-)

    Average RH at Wick airport so far this month has been 89.7%. Temperatures have been a lot lower than 30°C, of course; highest was 21°C when the RH was 75%. I haven't logged data for any airports further south but they've obviously had higher temperatures. Dunno what RHs, though.

    Remember, though, we're talking about indoor conditions - evaporative cooling of the outdoors would be a bit of a losing game [¹]. OK, it's an extreme case but at lower temperatures the difference in temperature between 80% RH and 100% RH for the same absolute humidity will be about the same because equilibrium vapour pressure rises approximately exponentially with Celsius temperature, roughly doubling for each 10°C increase in temperature.

    [¹] Though that's why the air over the oceans is cooler than over the land and why, in a warming world, with more evaporation and precipitation the differential between the oceans and the land can be expected to increase meaning that a specified increase in global average surface temperature is an underestimate of the amount of warming which people, who mostly live on the land, will likely see.
    •  
      CommentAuthordjh
    • CommentTimeAug 20th 2020 edited
     
    Posted By: Ed DaviesAverage RH at Wick airport so far this month has been 89.7%. Temperatures have been a lot lower than 30°C, of course; highest was 21°C when the RH was 75%.

    Averages aren't much use - RH max occurs at T min and vice-versa (approx). e.g. Wattisham today:

    03:00 95.1% 19.9°C
    19:00 32.9% 23.6°C

    or on the 17th
    03:00 99.3% 14.6°C
    14:00 66.9% 21.9°C

    but as you say, it's the interior conditions that are important here.
  5.  
    Posted By: djhbut as you say, it's the interior conditions that are important here.


    Indeed - but the interior humidity in summer is the same as the outside - unless you're madly boiling things.

    For the Wick example, 21.9C and 66.9% RH is a dewpoint of 12.9C (that's actually not particularly humid for that temperature) - if you raise that air to 30C inside, the RH drops to about 48%. That Wick temperature/RH combination outside has a humidex value of 26C - which is listed as "no discomfort".

    The Wattisham example a6 23.6C has a humidex of 23.6C so there's essentially no humidity ... the dewpoint is at -1.4C so it's very dry!

    Believe me when the temps are in the 30s and the dewpoints are in the 20s, it's very very uncomfortable. We only get conditions like that in Montreal when the airmass is coming from the Gulf of Mexico.

    Paul in Montreal.
   
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