Green Building Forum - Use of Heating Degree Days Tue, 19 Dec 2023 05:34:12 +0000 http://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 Use of Heating Degree Days http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214472#Comment_214472 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214472#Comment_214472 Mon, 01 Dec 2014 10:12:28 +0000 andyman99 Use of Heating Degree Days http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214474#Comment_214474 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214474#Comment_214474 Mon, 01 Dec 2014 10:28:52 +0000 SteamyTea
The usual method is to plot the kWh against HDDs (kWh on the y-axis, HDDs on the x-axis).
Then add a Trend line and extend it backwards until it crosses the y-axis (easy enough in Excel).
The point where is crosses the y-axis is your fixed loads, in your case DHW. This will give you your daily kWh usage.
You can also just separate out the non space heating period and average that (total kWh used divided by days). That will give you your kWh/day.
As you know the energy delivery rate (3.1 litre/hour or 31 kWh.hour) you should be able to estimate your burn time.

So if you use 8 litres of oil a day. 8 litre / 3.1 litre per hour = 2.58 hours.]]> Use of Heating Degree Days http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214479#Comment_214479 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214479#Comment_214479 Mon, 01 Dec 2014 11:35:39 +0000 andyman99 Use of Heating Degree Days http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214480#Comment_214480 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214480#Comment_214480 Mon, 01 Dec 2014 11:45:00 +0000 SteveZ Use of Heating Degree Days http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214481#Comment_214481 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214481#Comment_214481 Mon, 01 Dec 2014 11:57:21 +0000 atomicbisf
My usage seems to be tracking downwards over the past three years but it would be good to try to separate out the weather element. Also I can see with November being the first full month of switching from an electric shower to DHW from a gas heated cylinder, we're down more than 100 kWh of electricity and 20 kWh of gas on last November and 162 kWh elec and 551 kWh gas on Nov 2012.

Ed]]> Use of Heating Degree Days http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214483#Comment_214483 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214483#Comment_214483 Mon, 01 Dec 2014 12:20:31 +0000 rhamdu
Carbon Trust uses 15.5C on the assumption that when external temperature is above 15.5C most buildings need no heating. http://www.carbontrust.com/resources/guides/energy-efficiency/degree-days

However, it can get a lot colder than that, before a well-insulated building needs any heat additional to that provided by bodies, pets, lights and appliances.

I am not arguing for a lower base temperature. On the contrary, it might be better to use a base temperature of, say, 20C, and explicitly subtract the 'other' sources from the calculated heat demand.

I'm sure you Passivhaus peeps are across this one :bigsmile:]]> Use of Heating Degree Days http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214490#Comment_214490 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214490#Comment_214490 Mon, 01 Dec 2014 14:09:54 +0000 Ed Davies
http://www.degreedays.net/]]> Use of Heating Degree Days http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214491#Comment_214491 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214491#Comment_214491 Mon, 01 Dec 2014 14:16:53 +0000 Ed Davies Posted By: rhamduI am not arguing for a lower base temperature. On the contrary, it might be better to use a base temperature of, say, 20C, and explicitly subtract the 'other' sources from the calculated heat demand.I'd agree with that - particularly if you can make some estimate of the solar gain appropriate to the times of year in question.

But - you then really need to do things at higher time resolution than per month down to of the order of the heating timeconstant of your building. A month with the temperature at 15.5 °C where you just about don't need heating is different from a fortnight at 19 °C when it's plenty warm enough and a fortnight at 12 °C when you do need some heating. They're different HDDs at 15.5 °C base but the same at 20 °C base.]]> Use of Heating Degree Days http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214492#Comment_214492 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214492#Comment_214492 Mon, 01 Dec 2014 14:18:10 +0000 andyman99 http://www.degreedays.net/desktop/
It downloads a csv file, best opened with excel or other spread sheet app.

I do have an idea about burner run times, but of course they vary greatly with external temperatures, so its a bit of guess work to extrapolate them too much. Its not possible AFIK to use a meter on the boiler itself. The boiler run signal doesn't mean the burner is necessarily running too, i.e. once up to temp it will cut out the burner until more heat is called for, but the boiler is still "ON"]]> Use of Heating Degree Days http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214497#Comment_214497 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214497#Comment_214497 Mon, 01 Dec 2014 15:31:18 +0000 SteamyTea Posted By: atomicbisfI downloaded some but they appeared to be nonsense, but it was probably just because I didn't have an appropriate program to open them with.I had a quick look and used 'ISOMERSE15', which seems to be Bath (there are other local ones). They seemed normal to me.

The 'Base Temperature' is always a hard one to decide on. 15.5°C is used as a standard as this allows you to compare two different buildings, does not tell you much about your particular building. As an example on iMeasure I used the lowest of 12°C and still got nonsense numbers. This may be because I don't turn my heating on till the outside temperature is at least 10°C or below. This is not really a refection of iMeasure, more a case that I live somewhere very mild, my house is well insulated and I am as tough, or as tight, as a Yorkshireman or a Cornishman (take your pick, a Cornishman is a Scott with all the generosity squeezed out of him).
Excel allows you to copy and paste text data in and then convert it to columns. It is under the Data tab and called Text to Column. You choose the 'delimiter' as 'comma' for a *.csv file. Make sure no other options are selected (like a full stop) as this may confuse the data.


Posted By: Ed DaviesBut - you then really need to do things at higher time resolution than per month down to of the order of the heating timeconstant of your building.
Yes, with any statistical based analysis, the higher the resolution the better the estimate. This is because the standard error of the mean (HDDs used to be set on the mean of two temperatures in the UK) needs a large number of datapoints to make up for the lack of daily ones.

As Ed Davies said earlier, other things can affect your space heating, solar gain is one but the biggest is windspeed (especially if you have a leaky house).
DHW is affected (or is if effected) by ground temperature, but this tends to be a gradual change over a week or two and can often seem out of step with air temperature. Ground temperature is affected by rainfall as much as anything else.]]> Use of Heating Degree Days http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214498#Comment_214498 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214498#Comment_214498 Mon, 01 Dec 2014 15:35:41 +0000 skyewright Posted By: rhamduAnyone have any thoughts on an appropriate base temperature for well-insulated buildings?
What ever is appropriate for your building.

Ideally you'll have data & can use that to plot graphs of heating energy against various HDD bases. If you find one where the plot passes through 0 HDD & 0 Wh you have your base. :smile:

For us ('improved' 1970's, block - insulated cavity - block, bungalow), HDD12 works pretty well. For tony it's probably about HDD-2, at which point he turns on a light for heating. :wink: :bigsmile:]]> Use of Heating Degree Days http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214503#Comment_214503 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214503#Comment_214503 Mon, 01 Dec 2014 16:11:13 +0000 SteamyTea Posted By: skyewrightWhat ever is appropriate foryourbuilding.Yes, but this then gives meaningless results when comparing against other places and other buildings.

I think the reason that the EST uses 15.5°C is because that is the mean air temperature that an average house in an average area needs to start heating. That in itself is pretty meaningless to us on here, but if this was stated on an EPC along side the HDDs, then it would become meaningful.]]> Use of Heating Degree Days http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214504#Comment_214504 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214504#Comment_214504 Mon, 01 Dec 2014 16:17:28 +0000 Paul in Montreal
Paul in Montreal.]]> Use of Heating Degree Days http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214506#Comment_214506 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214506#Comment_214506 Mon, 01 Dec 2014 16:28:45 +0000 SteamyTea That may account for Canada's high energy usage then, or more likely the on/off nature of your weather. How many days does spring last, 3 isn't it :bigsmile:

One half of Buffalo needed emergency aid while the other half needed a shovel last week didn't it?]]> Use of Heating Degree Days http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214514#Comment_214514 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214514#Comment_214514 Mon, 01 Dec 2014 17:15:22 +0000 Ed Davies Posted By: skyewrightWhat ever is appropriate foryourbuilding.
Posted By: SteamyTeaYes, but this then gives meaningless results when comparing against other places and other buildings.
But comparing buildings isn't the only use.

Isn't it more like that the HDD value is the result. Or, at least it's a big part of the result. Effectively, the regression line for a house is defined by its HDD base (the intercept) and energy required per degree day (the slope). Both matter.

E.g., a house with relatively poor insulation but high solar gain would have a lower HDD base but a steeper energy/hdd than a better insulated house with less solar gain.

Also, if you use the wrong base you'll get the wrong slope (as you'll effectively be incorrectly counting the periods with and without energy use).]]> Use of Heating Degree Days http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214517#Comment_214517 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214517#Comment_214517 Mon, 01 Dec 2014 17:35:00 +0000 SteamyTea What it should do is show how effective any changes you make are.

It is really 'the curse of the mean' that is the problem. This tells you nothing about the distribution of temperature.
Generally temperature is evenly distributed with a relatively small standard distribution in the UK (have to be careful where you put the lowest and highest temperatures as they look different in the °C or K scale).
Then it all goes to pot when you take windspeed, direction and temperatures into account. I have higher temperatures the higher the windspeed, but I suspect that the NE coast of Scotland don't.]]> Use of Heating Degree Days http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214518#Comment_214518 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214518#Comment_214518 Mon, 01 Dec 2014 18:04:53 +0000 skyewright Posted By: SteamyTeaWhat it should do is show how effective any changes you make are.
Which is how & why we use it.
Currently we are at ~5kWh/HDD12.
The aim is to further reduce both the '5' and the '12'.

Posted By: SteamyTeaI have higher temperatures the higher the windspeed, but I suspect that the NE coast of Scotland don't.

Often the same here on the coast of NW Scotland. A high energy Atlantic storm often brings a body of significantly warm air with it. NW winds can be strong too, and typically colder, but also typically brighter.

HDD isn't a perfect tool. but it's a very handy one, especially if you also have other weather data available too. Trying to combine the whole lot into one formula is way beyond anything we could attempt, but having the other data available does at least mean you can see if there are possible reasons for anomalies in energy v HDD data.

Ed's point about solar gain is a good one and applies to us to some extent. There's still scope for plenty more insulation...]]> Use of Heating Degree Days http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214519#Comment_214519 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214519#Comment_214519 Mon, 01 Dec 2014 18:05:43 +0000 skyewright Posted By: Ed DaviesBut comparing buildings isn't the only use.

Isn't it more like that the HDD valueisthe result. Or, at least it's a big part of the result. Effectively, the regression line for a house is defined by its HDD base (the intercept) and energy required per degree day (the slope). Both matter.
Spot on. That's how we use it. It gives us a marker against which to measure changes in the house's performance as we make improvements.

How that compares to other houses isn't a particular concern for us. A body looking at things nationally, such as a government may have different priorities.

** Edited to reduce duplication. I wrote this one before the other (above) but it seemed to get lost when first posted. Now it has turned up!]]> Use of Heating Degree Days http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214532#Comment_214532 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214532#Comment_214532 Mon, 01 Dec 2014 19:17:08 +0000 fostertom Posted By: rhamduCarbon Trust uses 15.5C on the assumption that when external temperature is above 15.5C most buildings need no heatingThat is absolute nonsense because that 15.5o depends on how insulated/airtight the building is (amongst several other things) which is exactly what you're trying to assess - totally circular - starting point determines 'result'.

People warn that degree-days methods give poor results - no wonder. Big consultancies collect elaborate data bases so they can plug in a credible base temperature for any given set of building characteristics - but there again it's only as good as a manual assessment, which would probably deliver a fair estimate of energy consumption too - so why bother with putting it through a spuurious degree-day calc?

Posted By: rhamduit might be better to use a base temperature of, say, 20C, and explicitly subtract the 'other' sources from the calculated heat demand
Yes, the only way to use degree-days AFAIC, is to set base temp in fact equal to maintained internal air temp - 18C, 20C, 21C, whatever's desired - which then gives accurate result for heat demand resulting from conduction-only through the fabric, assuming you know its U-values, to be supplied by the heating system (incl boiler casing). Then knock off from that values for all the gains - electrical (incl hot tank), cooking, bodies, solar gain, and possibly effects, if any, resulting from massive fabric.

Degree-days is not a magic wand relieving you of calculating these. All it can help with is conduction-only through the fabric.]]> Use of Heating Degree Days http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214533#Comment_214533 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214533#Comment_214533 Mon, 01 Dec 2014 19:25:39 +0000 Paul in Montreal Posted By: SteamyTeaThat may account for Canada's high energy usage then, or more likely the on/off nature of your weather. How many days does spring last, 3 isn't it:bigsmile:" alt=":bigsmile:" src="http:///forum114/extensions/Vanillacons/smilies/standard/bigsmile.gif" >

The high energy usage is because it's cold, not because of the basis temperature of HDD :)

What the basis temperature says is that if the external temperature is below the basis, the building will lose heat, if above, the building will gain heat. Now, your building could be losing heat when it's +18C outside, but the internal and incidental gains (insolation) could be greater than the heatloss at that temperature. HDD doesn't know or care about that - this is can only be computer by a proper heat load program that takes all these factors into account. The HDD basis temperature has to be the same for comparisons between two buildings at any two arbitrary locations to be made, but, of course, knowing the incidental gains also has to be taken into account as a location that's very sunny will lead to lower energy requirements than one that's dismal, everything else being equal - but only your heat load program will tell you whether your building will actually need heating (or cooling) at any particular external temperature.

For cooling, it's the same thing. A day with an 24 hour average of 18C is pretty warm - especially during the day time and is exacerbated by incidental gains so, again, the CDD figure won't tell you your cooling load.

As for Buffalo, that's an example of what a warm lake will do on a cold day. As the climate changes and the lakes get warmer, such lake-effect snows can only get worse - but more than six feet in a couple of days is pretty extreme!

Paul in Montreal.]]> Use of Heating Degree Days http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214555#Comment_214555 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214555#Comment_214555 Tue, 02 Dec 2014 08:41:19 +0000 Seret Posted By: SteamyTeaHDDs will give you a good first estimate, but as we have discussed before, they are not brilliant.

I've actually found they're pretty strongly predictive of usage over a fairly long timescale (ie: annual). They're a useful tool, as they're simple to use and widely available in pre-processed form.


The usual method is to plot the kWh against HDDs (kWh on the y-axis, HDDs on the x-axis).
Then add a Trend line and extend it backwards until it crosses the y-axis (easy enough in Excel).
The point where is crosses the y-axis is your fixed loads, in your case DHW. This will give you your daily kWh usage.


I've actually found that this isn't a good way to determine baseload if you're using the same source for space heating and DHW. The baseline usage (which will be DHW) forms a cluster of data points at one end of the series, the trend line will intersect the y-axis at a point somewhat below that. The effect will be greater the steeper the line.

This for example is some of my heating data:

https://www.dropbox.com/s/os21u3vcysp3y7p/chart.png?dl=0

It suggests my baseload is 32kWh per week, but I know it's not. It's more like 60kWh.

Best way to determine baseload is just to look where that cluster of summer data points is. If you want to get technical, sort all the heating-on data from the heating off and take the average (median maybe?) of those.]]> Use of Heating Degree Days http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214559#Comment_214559 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214559#Comment_214559 Tue, 02 Dec 2014 09:09:04 +0000 SteamyTea You have just highlighted the things I don't like about them :wink:

They do need to be used with care, why I think they are not brilliant, but good enough for a first approximation.

They also go terribly wrong for people with storage heating (or broken thermostats) as this is a fixed load and is blind to temperature (not many people adjust the input and output for tomorrows temperature).

If people really want to get an idea of what is going on, then they have to start measuring. Then a real analysis can really show what is happening.
The equipment for this does not have to cost a lot (I do find it strange that people spend thousands on a heating system but nothing on monitoring).

When it comes to analysis, it is probably best to move away from HDDs and look at mean energy use and temperature differences (not hard if you have the data). You end up with a probability density for each temperature bin. Much more useful as it tends (a good stats term) to show when the heating is on or off with a step up or down in energy usage.

I am sure that when I first studied (as part of a BSc) HDDs we used Median rather than Mean. I think this goes back to when the Met Office collected data twice a day, 9AM and 9PM. The Median and the Mean is therefore the same (just the middle point of two). I think they then carried this practice on (and still do) as it gives a fair (another stats term) approximation.
Why I think the PDF (Probability Density Function) is easier and more enlightening.]]> Use of Heating Degree Days http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214566#Comment_214566 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214566#Comment_214566 Tue, 02 Dec 2014 09:48:59 +0000 andyman99
1/I don’t have weekly usage figures. I have an irregular shaped oil tank and only wild guestimates are possible on the site gauge - oil meters are too expensive unfortunately to install for just this purpose. However I’ve just had my winter Oil supply and the guy kindly filled it in 250L increments so I may be able to make some slightly more accurate guesses going forward

2/ The house tends to be heated between 4 and 10 pm to about 19/20 degrees and gradually cools down after that, so no constant temperature. During particularly cold spells or when family visit there might be enforced heating during the day. But this approach is largely consistent over the last 3 years.

I calculated using HDD12 degrees as a baseline. Partly because higher baselines seem to give unrealistic comparison values and also to compare with skyewright as I thought construction was similar, brick and block, epb cavity filled. 2 years of trying to remove cold bridges and leaks. We are also fairly hardy and tough it out until the evening temps are pretty cool.

I have made allowances for oil used for DHW by assuming a 20min burn per hour. This works out at approx. 30L per month. I have used a scaling facture of 1 for summer months (low or zero HDD) a values of 0.2 for Winter months and 0.5 for in between months. This is all a bit of a finger in the air but hopefully it is consistent.

before most improvements: 11/2011 to 08/2013 8.4 kw/HDD12
partial improvements: 08/2013 to 11/2014 5.9 kw/HDD12

The more interesting figure for me will be over this winter.]]> Use of Heating Degree Days http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214567#Comment_214567 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214567#Comment_214567 Tue, 02 Dec 2014 09:52:51 +0000 atomicbisf
How would you go about charting improvement in kWh/HDD? I was thinking of a separate trend line for each year. Or month on the X axis and kWh/HDD on the Y.

Ed]]> Use of Heating Degree Days http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214569#Comment_214569 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214569#Comment_214569 Tue, 02 Dec 2014 10:09:25 +0000 Seret Posted By: atomicbisf
How would you go about charting improvement in kWh/HDD? I was thinking of a separate trend line for each year.

I tend to do a new data set for each heating season. Improvement will show as a change in the slope of the line or as a reduction in baseload. Since I've been working on my place I've got my kWh/HDD line from 8 down to about 5, for example.]]> Use of Heating Degree Days http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214570#Comment_214570 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214570#Comment_214570 Tue, 02 Dec 2014 10:16:15 +0000 SteamyTea Posted By: atomicbisfHow would you go about charting improvement in kWh/HDD? I was thinking of a separate trend line for each year. Or month on the X axis and kWh/HDD on the Y.Will have to think about that as HDDs are not really to do with the time of year, just time.]]> Use of Heating Degree Days http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214571#Comment_214571 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214571#Comment_214571 Tue, 02 Dec 2014 10:26:21 +0000 atomicbisf Posted By: Seret
Posted By: atomicbisf
How would you go about charting improvement in kWh/HDD? I was thinking of a separate trend line for each year.


I tend to do a new data set for each heating season. Improvement will show as a change in the slope of the line or as a reduction in baseload. Since I've been working on my place I've got my kWh/HDD line from 8 down to about 5, for example.

Sounds like a good plan. The slope should have decreased with improvements to insulation but the base load stayed the same as the DHW system is the same. In fact it has gone up a little as we are using it for bathing rather than an electric shower. So to take account of the increase in gas for DHW and decrease in electricity I have also calculated the total cost of elec + gas each month and the total carbon emissions to see if they're on a downward trend too.

Ed]]> Use of Heating Degree Days http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214575#Comment_214575 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214575#Comment_214575 Tue, 02 Dec 2014 10:58:56 +0000 rhamdu
Posted By: fostertomYes, the only way to use degree-days AFAIC, is to set base temp in fact equal to maintained internal air temp - 18C, 20C, 21C, whatever's desired

Yes that's the only way we will get meaningful national statistics which show the benefit of improving the building stock. TWh heating energy per 20C-HDD would be a meaningful national statistic. On the other hand...
Posted By: Ed DaviesIsn't it more like that the HDD valueisthe result. Or, at least it's a big part of the result. Effectively, the regression line for a house is defined by its HDD base (the intercept) and energy required per degree day (the slope). Both matter.

Ed, I do like this approach to monitoring an individual building, despite its snags.
The slope will be a property of the building, and possibly some local conditions such as wind velocities. The intercept (the HDD base) will also depend on the occupants, their stuff and their behaviour. If you take an empirical approach to measuring the building's thermal properties, you need to take a number of sample periods and plot the heating energy used, against the average temperature* for each period.
How long should the sample periods be? Periods shorter than the time-constant of the building won't tell you much. But long periods (weeks to months) won't give you many data points. And in the UK climate, these longer periods will almost certainly include warmer spells where no heating is needed - which will throw out the calculation.
Another snag is that the varying behaviour of the occupants will introduce uncertainty into the calculation of a value which should be fairly well-defined, i.e. the building's physical thermal properties. How much of the variation is random? You'll probably never fully comb out dependencies such as the greater use of lighting in winter.
We should be doing this for a lot of buildings, not just for the benefit of the occupants but to investigate how well buildings reproduce the thermal performance predicted mathematically from their design. Then we need to investigate whether the discrepancies are down to bad theory, or bad workmanship.

*SteamyTea has already highlighted the difficulties of averaging temperature. As ST indicates, we use should use average temperatures here, not HDD data, since we will be going around in circles if we presume an HDD base temperature.]]> Use of Heating Degree Days http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214621#Comment_214621 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214621#Comment_214621 Tue, 02 Dec 2014 16:42:12 +0000 atomicbisf
For 2012 I have 3.62 kWh/HDD and 2013 2.88 kWh/HDD.

This heating season is a little trickier because of our switch from electric shower to gas DHW for bathing, so I do not have summer months for a baseline. But as our electricity is down by 100 kWh on Nov 2013 perhaps crudely I can estimate gas consumption should be up by 100 kWh for DHW (I know this ignores any changes in volume of water heated, cylinder losses and boiler inefficiency) and that gives 2.73 kWh/HDD.

Ed]]> Use of Heating Degree Days http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214694#Comment_214694 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12847&Focus=214694#Comment_214694 Thu, 04 Dec 2014 18:29:33 +0000 atomicbisf
Imeasure can do various things with the data. It's calculated degree day normalised 60 kWh/m2/year for 2014 (not sure if this accounts for the missing data for Jan and the rest of Dec yet to come), 110 kWh/m2/year for 2013 and 96 kWh/m2/year for 2012.

Ed]]>