Green Building Forum - Degree days - base temperature Tue, 19 Dec 2023 05:31:19 +0000 http://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 Degree days - base temperature http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15604&Focus=266392#Comment_266392 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15604&Focus=266392#Comment_266392 Mon, 05 Mar 2018 21:43:13 +0000 djh
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"]]> Degree days - base temperature http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15604&Focus=266440#Comment_266440 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15604&Focus=266440#Comment_266440 Tue, 06 Mar 2018 20:13:16 +0000 goodevans Degree days - base temperature http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15604&Focus=266450#Comment_266450 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15604&Focus=266450#Comment_266450 Tue, 06 Mar 2018 22:16:04 +0000 gyrogear
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]]> Degree days - base temperature http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15604&Focus=266454#Comment_266454 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15604&Focus=266454#Comment_266454 Tue, 06 Mar 2018 23:39:21 +0000 djh 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:]]> Degree days - base temperature http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15604&Focus=266458#Comment_266458 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15604&Focus=266458#Comment_266458 Wed, 07 Mar 2018 00:12:22 +0000 goodevans 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.]]> Degree days - base temperature http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15604&Focus=266460#Comment_266460 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15604&Focus=266460#Comment_266460 Wed, 07 Mar 2018 08:15:42 +0000 SteamyTea 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.]]> Degree days - base temperature http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15604&Focus=266474#Comment_266474 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15604&Focus=266474#Comment_266474 Wed, 07 Mar 2018 13:15:37 +0000 djh 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.]]> Degree days - base temperature http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15604&Focus=266475#Comment_266475 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15604&Focus=266475#Comment_266475 Wed, 07 Mar 2018 15:01:36 +0000 Paul in Montreal
Paul in Montreal.]]> Degree days - base temperature http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15604&Focus=266485#Comment_266485 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15604&Focus=266485#Comment_266485 Wed, 07 Mar 2018 18:01:18 +0000 goodevans Degree days - base temperature http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15604&Focus=266492#Comment_266492 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15604&Focus=266492#Comment_266492 Wed, 07 Mar 2018 19:31:51 +0000 Paul in Montreal
Paul in Montreal.]]> Degree days - base temperature http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15604&Focus=266499#Comment_266499 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15604&Focus=266499#Comment_266499 Wed, 07 Mar 2018 20:21:32 +0000 goodevans 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.]]> Degree days - base temperature http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15604&Focus=273533#Comment_273533 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15604&Focus=273533#Comment_273533 Mon, 29 Apr 2019 13:18:28 +0100 bromley
When using degree days to compare the weather at different locations (e.g. between different cities), it makes sense to choose a sensible base temperature and stick to it for your comparisons. But, when using degree days to analyze energy data from a particular building, it generally makes sense to choose the most appropriate base temperature for that particular building. Buildings vary a lot, and consequently so do their base temperatures. I wrote an article recently that explains the main factors that determine the base temperature of a building:

https://www.degreedays.net/base-temperature

I see the traditional base-temperature defaults such as 15.5 C for the UK and 65 F for the US as a hangover from the days when degree days were disseminated in print. Building base temperatures would have varied back then too, but it just wasn't practical to publish degree days in scores of different base temperatures for scores of different locations to suit everyone optimally. One-size-fits-all base temperatures were the best practical option available at the time. They still linger on today, but times are different now, it's now much easier for people to find degree days in whatever base temperatures they want.

I agree with the process described in the enmanreg.org post that djh started this thread with. It's essentially a simplified version of the process used by the regression tool on the Degree Days.net website (which tests regressions of energy data against HDD and/or CDD in different base temperatures to find the ones with the best statistical fit).

A positive intercept is pretty common as many buildings have non-weather-dependent energy consumption that is metered together with heating/cooling energy consumption. A negative intercept is generally a bad sign, however, indicating that either the base temperature is wrong, or that there are some anomalies in the energy data, or that something odd was happening in the building.

On SteamyTea's comments about how degree days are calculated: actually it depends on the source you get the data from. The original and correct way to calculate degree days is to use the full detail of the temperature data available (which, for most good weather stations, is typically at least hourly). This creates degree days that capture all the known temperature variations within each day. It is, however, a fairly intensive calculation, so many sources use a simpler approximation method instead. Approximating degree days using daily max/min temperatures is common and the accuracy of the figures suffers, particularly for periods over which the temperature was near to the base temperature. Naturally I would recommend using accurately-calculated degree days over approximated degree days whenever possible.]]> Degree days - base temperature http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15604&Focus=273534#Comment_273534 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15604&Focus=273534#Comment_273534 Mon, 29 Apr 2019 15:45:49 +0100 DamonHD
Just as a note, I treat my home's heating base temperature as 12C as that has a good fit to observed behaviour.

Also note that as above base temperature is NOT internal temperature, but the external temperature below which heating is needed to maintain a comfortable internal temperature.

Given an external temperature of ~15.5C right now, and no heating for a month, internal temperatures range from about 18C to 19.5C, see snapshot:

http://www.earth.org.uk/img/20190429-16WWmultisensortempL.png

(Live is at http://www.earth.org.uk/out/hourly/16WWmultisensortempL.png but may be very different by the time you happen to see this post!)

Rgds

Damon]]> Degree days - base temperature http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15604&Focus=273602#Comment_273602 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15604&Focus=273602#Comment_273602 Sat, 04 May 2019 14:21:13 +0100 skyewright Posted By: DamonHD(I'm a big user and fan of DegreeDays.net, please be kind to bromley.)
Just as a note, I treat my home's heating base temperature as 12C as that has a good fit to observed behaviour.

Snap.
I came to that conclusion following analysis of several years of my own data (i.e. heating energy used [easy as we are all electric] and temperatures from my own weather station).

The colder the temp the higher the correlation between heating demand & HDD12. As we get to the change over and the daily average hovers around that figure, things get more complicated (a bit of sunshine can make a lot of difference). In practice we tend to find that we can turn off the heating when the average daily temp is reliably 10C or greater.]]> Degree days - base temperature http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15604&Focus=273604#Comment_273604 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15604&Focus=273604#Comment_273604 Sat, 04 May 2019 15:12:42 +0100 djh Posted By: DamonHD(I'm a big user and fan of DegreeDays.net, please be kind to bromley.)
Just as a note, I treat my home's heating base temperature as 12C as that has a good fit to observed behaviour.
I calculated mine using their regression calculator, with some help from Bromley, and it says 11.5°C. What I discovered during the process is that the base temperature not only depends on your property and where it is but it also depends on:
- how warm you keep your house (we keep ours at 20°C or above, in the spirit of PH)
- whether you have any PV panels (at least unless you [can] separate out all the generated electricity from your data)

So while degree days and base temperatures may be useful for measuring improvements to your house and suchlike, they're not much use for comparing properties. Possibly more use in professional practice with a lot more attention to details than I can give.]]>