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    •  
      CommentAuthorSteamyTea
    • CommentTimeJan 16th 2014 edited
     
    Sorry to labour this old chestnut, but it would be interesting to get to the bottom of this.

    Does thermal mass in a building really help stabilise and raise the overall internal temperature or is it just an illusion?

    I did my BSc Dissertation in a related area, mainly the effects of solar inputs on thermal mass. I wanted to show that you could heat a house with relatively low solar inputs (wintertime in Cornwall) by storing the energy in varying amounts of thermal mass. In the small model (physical) I created it matched the computer model I made. It basically showed that Newton's Law of Cooling holds true (probably why it is a Law and not a wish). I did manage to show that it was possible to store a small amount of excess energy for a short period of time (till the next solar cycle started), but it was a small amount in a highly insulated structure and needed fans to transfer the energy into the thermal mass. So not exactly a passive system.
    In hindsight I think I proved nothing but learnt some interesting mathematical modelling and experiment design/testing.

    Unfortunately real life is not like a laboratory and a house is not like an expanded polystyrene box with a window and a fan, the sun is obscured by cloud, temperatures vary, the wind blows and if you miss a rare event (a thunderstorm) you cannot go back and repeat the experiment.
    But having said that I have over the years since thought about it a lot, and collected real life data. Disaggregating the variables is pretty tricky, but since a bit of prompting on another thread I looked at some of last winters data. I have limited it to variations in internal and ambient temperatures, windspeed and solar gain (the charts are below).
    It is a bit hard to compare temperature with windspeed and solar gain as they units are different (though may be able to break the units down into SI ones i.e. J/m^2).

    I have lots of ideas as to why it may not work, just as others have lots of reasons as to why it does. I am willing when time and cash permit to create 'Silly Sunday Experiments' to test ideas, but I may need online help in the design and execution.

    So I hope that we can get to the bottom of this difficult and complicated problem.

    Now some charts:
    • CommentAuthorMikel
    • CommentTimeJan 16th 2014
     
    ST,

    We have started this trial of control system for heat pumps. One of the objectives is to test whether running the heat pump using a time of use tarrif will save money and part of the rationale is that the building will retain heat.

    I'll let you know when it is all installed and we have some data. Our house is a hybrid, part 80s timber frame, part 2006 modern block. There is temp sensor in the timber frame part and two sensors in the modern block extension.

    Mike
    •  
      CommentAuthorSteamyTea
    • CommentTimeJan 16th 2014
     
    That's good, any idea what they are using for weather data? Bit hard to tell from my data but seems that windspeed has the biggest affect on temperature. But windspeed is related to wind direction which is related to temperature and solar power.
    • CommentAuthorMikel
    • CommentTimeJan 16th 2014
     
    Bit too early for that. You have to hold off with the questions for the time being. I think there was a idea to use the weather forecast as part of the control parameter set but at the moment they are part way with the installation and are not planning to complete until early next month.

    Do you have air permeability data for your house?
    •  
      CommentAuthorSteamyTea
    • CommentTimeJan 16th 2014
     
    No, but it is a lot better than it used to be. I will go to the scrapyard and get a big fan one day and make my own, we discussed ways to to it on here a while back. I usually have the windows open a crack though, has to be very cold not to.
    • CommentAuthortony
    • CommentTimeJan 16th 2014
     
    "Does thermal mass in a building really help stabilise and raise the overall internal temperature or is it just an illusion?"

    It definitely helps stabilise internal temperatures makes it more comfortable, it cannot raise temperatures! Even temperatures feel warmer :)
    • CommentAuthorfinny
    • CommentTimeJan 16th 2014
     
    Does it not depend on the insulative envelope surrounding it?
    Then, comparing two buildings with the same external insulative properties, high and low thermal mass, does it not depend on how good the airflow/change management in the building is?
    I am tempted to believe that all other things being optimal, and well-designed, thermal mass sort of equates to standing losses??
  1.  
    ST are you really going to put you models up against at least 70 years of research that says thermal mass is beneficial??

    Have you crawled the web for the research that has been done on the subject?
    IIRCC Los Alamos National Research Labs carried out quite a bit already in the 1940s and 1970s.


    or are you talking about just in your home?
    • CommentAuthordaserra
    • CommentTimeJan 17th 2014
     
    Surely you get the average temperature, over a period related to the amount of mass. So a say 2pm on a sunny day it will actually feel cooler, but at 2am the same day it will feel warmer. It enables you to provide the heat input it a manner that suits the source, eg a short sharp efficient burning fire, or daytime parabolic solar gain and use that heat when it isn't being created without need of thermal stores pumps sensors etc.
    •  
      CommentAuthorSteamyTea
    • CommentTimeJan 17th 2014 edited
     
    Posted By: tonyIt definitely helps stabilise internal temperatures makes it more comfortable
    I think it should, but have only seen poor research methodology into it (similar to what I did). There seems to be at best a very small stabilisation effect.

    Posted By: finnydoes it not depend on how good the airflow/change management in the building is?
    Possibly, but if they are using different regimes then it is not the same building. Ventilation often features in the literature, to me it is another variable that is not properly accounted for.

    Posted By: bot de pailleor are you talking about just in your home?
    Not so much my house, but UK in general. I have data for my house though, rather hoping others have data for theirs so can compare. There is a lot of research from the USA, have read some of it, but does seem to be a lot about keeping places cool.

    Posted By: daserraSurely you get the average temperature, over a period related to the amount of mass
    I think it is much more complicated than that, why I put up charts showing temperatures, solar gain and windspeeds.

    Has anyone been keeping internal temperature records of their houses I could look at? Going to dig my old ones out and set up some more. I am lucking in that I can get access to weather data, so can compare what is going on in the real world.
    • CommentAuthorfinny
    • CommentTimeJan 17th 2014
     
    Recently met a retired building services engineer who told of an experiment done 30 odd years ago comparing the energy use of like for like homes, with and without an entirely freestanding wall blocking the prevailing wind. He reckoned it saved half the energy.. will try to chase him up for some evidence.
    He mentioned that in complex commercial buildings internal air pressure is managed very carefully, zone by zone and in that way buildings could be, in effect isolated from the external pressure changes that can wick heat away.
    •  
      CommentAuthorSteamyTea
    • CommentTimeJan 17th 2014
     
    So some thermal mass a few meters from the building and thermally isolated can work better. That is the kind of research I like.:bigsmile:
  2.  
    I think the answer depends upon your heat source and how you manage it.

    If you're relying on passive solar gain, active solar heating, heat dissipated by electrical appliances and/or overnight Economy 7 then thermal mass is very useful for smoothing out the peaks and troughs.

    If you use an oversized gas boiler that is turned on 20 minutes before you get out of bed or arrive home then thermal mass is probably a hindrance.

    David
  3.  
    There's monitoring data from the Y Foel passivhaus here:

    http://passivebuild.co.uk/bronhaul/

    David
    •  
      CommentAuthorSteamyTea
    • CommentTimeJan 17th 2014
     
    Thanks David
    It was looking at that, and soil temperatures that prompted me to consider investigating this again.

    One thing that has struck me (several times) is the size of the building, this affects the thermal mass to air mass ratio. even though my house is low mass, the ratio may well be higher i.e. more mass to air mass than a larger high thermal mass one.
    Maybe this should be part of the comparison, what do people think?
    • CommentAuthorEd Davies
    • CommentTimeJan 17th 2014 edited
     
    If the heat capacity of the air is noticeable compared to that of the structure then it's a stunningly low thermal mass house. Concrete and air have of the order of the same specific heat capacities (around 1 kJ/(kg·K)) but concrete has about 2000 times the density so a 2.4 m room height only needs 1.2 mm of concrete on the floor to match the heat capacity of the air. Even 9 mm plasterboard on the ceiling and walls will beat that.
    •  
      CommentAuthorSteamyTea
    • CommentTimeJan 17th 2014
     
    Yes, I work on 0.9 J.kg^-1.K-1 for brick/stone thermal mass and a density of 1,900 kg.m^-3, wood is 2.5 J.kg^-1.K-1 with a density of 420 kg.m^-3. Do they sound reasonable?
    Air changes would have to be taken into account, but as you say, still swamps it (or should do).
    • CommentAuthordickster
    • CommentTimeJan 17th 2014
     
    Our small new build is timber frame, dry lined plasterboard with one internally exposed brick chimney, 80% of the floor is suspended timber, 20% concrete (lower bedroom). Having previously experienced the unpleasantness of continuing hot weather when, because of high thermal mass it is near impossible to enjoy a cooler environment, I went for a lower thermal mass build.

    Can't say that it's a better way to build, but the control freak in me wanted it that way. Max temp this (hot!) summer 23C, lowest ever around 17C. Averaging 18-21C throughout the year with me in charge of stove, windows (and wife).
  4.  
    Posted By: dicksterAveraging 18-21C throughout the year with me in charge of stove, windows (and wife).
    Even (especially) if you think you're in control of your wife the opposite is probably true.

    David
    • CommentAuthordickster
    • CommentTimeJan 17th 2014
     
    She told me what to write, of course.
    •  
      CommentAuthorSteamyTea
    • CommentTimeJan 17th 2014
     
    Just looking at my raw data and I have a mean temp of 19.7°C
    No real control, just do what seems necessary when needed.
    I need to filter out some min and max temperature readings (currently 51°C, probably from calibration tests), but the min was 11°C (probably a water temp test).
    The Standard Deviation is 6.4°C around the mean of 19.7°C, but if I work to 0 decimals to take account of the 0.5°C accuracy, then 70% of the time I am between 14°C and 26°C as the distribution is pretty even.
    There is a bias towards the unheated parts of the house (upstairs), until I add my kitchen temperature data (where I work).
    •  
      CommentAuthorjoe90
    • CommentTimeJan 17th 2014
     
    I am very interested in this as I am a fan of thermal mass and currently plan to construct my new build like Tony's, 300mm insulation Cavity wall. I also dislike Plasterboard walls and want something solid to hang things on. I am planning lots of solargain with thermal blinds to avoid excess heat (like Viking House had on his web site but cant find the link now) Also will have a woodburner so thermal mass will absorb and store heat for later.
    • CommentAuthorRobL
    • CommentTimeJan 17th 2014
     
    I am a big fan of high thermal mass buildings, assuming daily occupancy. My experience of low thermal mass is of a scandinavian holiday cottage that I have rented a few times, near Nottingham, in early Nov. It's triple glazed throughout, and lightweight construction.
    Every morning we turn on the electric heaters as it's too chilly - but by mid afternoon the windows have been opened to cool it down again. Gah !
    •  
      CommentAuthorSteamyTea
    • CommentTimeJan 17th 2014
     
    I think some sort of experiment is needed.
    Box with bricks in it and one window maybe, and a sunny day
    • CommentAuthoratomicbisf
    • CommentTimeJan 17th 2014
     
    I think our house has little thermal mass and it does warm up very quickly if the heating is on or the sun is out and shining through the windows.

    I find lightweight construction more 'friendly' than heavy with a light and airy feel, but that is probably just personal preference.

    Ed
    •  
      CommentAuthorSteamyTea
    • CommentTimeJan 17th 2014
     
    I wonder if window area to wall area makes a big difference.
  5.  
    My experience of high thermal mass houses is that they are fine for those buildings that need a large heat source to maintain a given temperature but that very low energy houses such as PH have problems with comfort because getting the house up to temperature is difficult unless you can pump a lot of heat into it quickly. I visited a PH which had high density concrete block walls on the ground floor and timber frame walls on the first floor. The owners complained that it was cold downstairs and warm upstairs. I decided to use a lightweight wall construction with a concrete raft foundation for my PH.
  6.  
    Posted By: PeterStarckMy experience of high thermal mass houses is that they are fine for those buildings that need a large heat source to maintain a given temperature but that very low energy houses such as PH have problems with comfort because getting the house up to temperature is difficult unless you can pump a lot of heat into it quickly.
    The only reason you would need to pump heat into the thermal mass quickly is if you'd let it cool, e.g. while on a winter holiday, & wanted to get it back up to temperature quickly. If you're willing to wait a day or two on the rare occasions you go on holiday during winter then this isn't a major issue.

    Posted By: PeterStarckI visited a PH which had high density concrete block walls on the ground floor and timber frame walls on the first floor. The owners complained that it was cold downstairs and warm upstairs.
    This is common in all houses. Heat rises so you tend to get a natural temperature gradient from the ground floor's sub-floor to the first floor's ceiling. Thermal mass will not change this.

    David
    •  
      CommentAuthorSteamyTea
    • CommentTimeJan 17th 2014 edited
     
    Posted By: PeterStarckHeat rises so you tend to get a natural temperature gradient from the ground floor's sub-floor to the first floor's ceiling
    Not in my house, always colder upstairs than down in the winter, but that may be for other reasons (no heating upstairs and more air movement).
    But agree that thermal mass would not affect the buoyancy of the internal air.

    I think response time is part of the problem of using thermal mass to stabilise the internal temperature. If we lived in the tropics (or close to them) and had little temperature variation and solar input then you can use it effectively. Not the same in the UK with wild swings even by the hour sometimes. Ideally we want to be able to add or subtract the thermal mass (which is what you in effect do with a thermal store connected to your heating system).
    • CommentAuthorfinny
    • CommentTimeJan 17th 2014
     
    </blockquote>This is common in all houses. Heat rises so you tend to get a natural temperature gradient from the ground floor's sub-floor to the first floor's ceiling. Thermal mass will not change this.

    The perception of a cold downstairs and warm upstairs can also be down to the neutral pressure point of the building normally being around halfway up it.. ie leaks downstairs tend to suck air in to the building whilst leaks upstairs tend to push warm air out.

    This effect would be reduced in a PH with better airtightness.
   
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