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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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      CommentAuthordjh
    • CommentTimeMar 5th 2018
     
    I know some on here are interested in degree days and there has been some discussion of base temperatures. I haven't been especially interested and I don't remember exactly what was discussed. But I subscribe to the Vilnis Vesma Energy update newsletter and today it was all about degree-day base temperatures. It refers back to the website which includes details of what base temperature to use and how to calculate them.

    http://www.enmanreg.org/degree-day-base-temperature/

    There's also a bunch of other information about degree days and other topics on the site.

    In the newsletter, it mentions a spreadsheet: "I'd like to thank regular reader David Bridger who sent me an Excel spreadsheet which, when given monthly consumption and degree-day figures to base 15.5C, will estimate the correct base temperature to use. I have his permission to distribute this with my training courses"
    • CommentAuthorgoodevans
    • CommentTimeMar 6th 2018
     
    Hmm - I'm usually quite good at data representation - but I'm struggling to see the point here - it seems that one non zero intercept has been exchange for another. Has anyone else got a handle on what is trying to be said here?
    • CommentAuthorgyrogear
    • CommentTimeMar 6th 2018
     
    I'm no maths person, but I more-or-less went happily along with the reasoning, as the negative non-zero intercept looked somewhat illogical to me... and the positive one seemed reasonable given lower thermal transfer gradient at lower outside temperature etc.

    However, where I get confused is the actual level of 15.5°C, because in France the habitual reference is set at 18°C, based on a "recommended national internal temperature" of 19°C", plus a factor of 1°C added for occupancy, lighting, other heat sources).

    Does this mean that the average outside temp in France is 2.5°C higher than in UK ?
    Maybe it is (there seems to be an awful lot of brits living here LOL).

    gg
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      CommentAuthordjh
    • CommentTimeMar 6th 2018 edited
     
    Posted By: goodevansHmm - I'm usually quite good at data representation - but I'm struggling to see the point here - it seems that one non zero intercept has been exchange for another. Has anyone else got a handle on what is trying to be said here?

    It seemed fairly straightforward to me. There is exactly one temperature that is correct for use as the base temperature for each building, unique to that building. The base temperature is said to be the lowest at which the building 'balances' with no need for heating. The base temperature can be determined by plotting actual heating consumption against that predicted using degree days, and adjusting the base temperature used to calculate the degree days until the graph is a straight line. Any other base temperature results in a graph with a curved section near the origin.

    Posted By: gyrogearDoes this mean that the average outside temp in France is 2.5°C higher than in UK ?

    Given that France is entirely to the south of the UK, I'd be surprised if it wasn't warmer in France. :bigsmile:
    • CommentAuthorgoodevans
    • CommentTimeMar 7th 2018 edited
     
    Posted By: djhThere is exactly one temperature that is correct for use as the base temperature for each building, unique to that building.
    That is my thought also so if you get the right base temperature the intercept would be 0kWh for 0 degrees.

    However the article writer appears to be striving for a linear relationship as a priority. I think the problem with the data set is that the degree day count and/or the energy usage is averaged over a week - during a week that is both above and below the base temperature there will still be heating but the degree days will average to zero perhaps.

    Or it may be that on those load heat days solar gain is more/less likely and that results in a non linear relationship. Either way an intercept that does not go through the origin is 'wrong' in some way.
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      CommentAuthorSteamyTea
    • CommentTimeMar 7th 2018
     
    Without going into too much detail, and based on what we found out on here a few years back, that report is just showing the weakness of HDD.
    I did a chart somewhere showing that, at my location, windspeed had a greater affect on energy usage.

    After a lot of digging about, I found that the 'average' temperature was calculated on the median, not the mean. This is for historic reasons when only 2 temperature readings were taken every day, but can cause some skewing of the data at the extremes i.e. at the lower readings (not bothered by the higher ones as no heating needed).

    HDDs are only a rough calculations and cannot be applied to all and every building, no matter how much you tinker with the base temperature.
    •  
      CommentAuthordjh
    • CommentTimeMar 7th 2018
     
    Posted By: goodevansThat is my thought also so if you get the right base temperature the intercept would be 0kWh for 0 degrees.

    However the article writer appears to be striving for a linear relationship as a priority. I think the problem with the data set is that the degree day count and/or the energy usage is averaged over a week - during a week that is both above and below the base temperature there will still be heating but the degree days will average to zero perhaps.

    I didn't pay that much attention, but it's clear in the articles that it's the straight line that matters and that a non-zero intercept is normal. As you and ST suggest, it may be to do with the bucketing of the data and the way the stats are calculated.

    My purpose in posting was for those who are interested in using HDD to be able to calculate their base temeperature.
  1.  
    Here in cold (and, in the summer, hot) Montreal, 18C is used as the basis for heating and cooling degree-days. If the temperature is below 18C, you need some heating to keep your internal temperature at 21C and, if it averages 18C or more, you will need some cooling to maintain your internal temperature at 21C (21C is the temperature used by the energy audit programs here and is used by such tools as hot2000 to figure out the heating and cooling load - of course, the source of that heat can (and often is) purely internal gains from appliances, lighting etc. - but these are all factored in anyway). 15.5C is used in the UK because people seem to like cold houses!!

    Paul in Montreal.
    • CommentAuthorgoodevans
    • CommentTimeMar 7th 2018
     
    We also like small houses (well not so much like but usually have). The effect of people and unintentional gains from electrical equipment may affect our houses to a grater extent. I was horrified to find my average electrical usage was around 800W (no electric DHW or Space heating) most of which ends up as heat. Together with better insulation and a small envelope in the new place that translates to a base temperature some 6.5 DegC below internal temp without the usual solar gains etc. But my consumption should come down with better lights, fridges, etc - most of which will be upgraded with the new house.
  2.  
    The base temperature is irrespective of what your interior temperature is - it's just an apples-to-apples comparison of different locations. All houses have a negative energy requirement when the exterior temperature is at the HDD base and the interior is at the external temperature. Even if your internal temperature is higher than the HDD base, there still could be a negative energy requirement (due to solar and other incidental gains). However, you can plot some graphs of the HDD number versus the supplied heating energy to find out how efficient your building is (assuming you can hold everything else constant). One challenge is that you might have two days with identical HDD numbers, but one of them is sunny and the other is dull - so the amount of heating energy you have to supply on those days to maintain your constant interior temperature (and we use 21C here in Canada) could be quite different. HDD numbers are more away to compare climates at different locations. FWIW Montreal has about 10x more HDDs per year than CDDs.

    Paul in Montreal.
    • CommentAuthorgoodevans
    • CommentTimeMar 7th 2018
     
    Posted By: Paul in Montreal15.5C is used in the UK because people seem to like cold houses!!
    I think you said it here - the base temperature is based on what the target temperature is.
    If you need an internal temp of 10 DegC then your base temp will be say 7 DegC. However I suspect my base temperature will be below 15 degrees even if I keep the house to Canadian 21 DegC standards.
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