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Author Topic: Degree Days and Base temperature  (Read 1382 times)
Countrypaul
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« on: October 04, 2019, 08:41:24 PM »

Has anyone on here used degree days and actually worked out what the base temperature for there house normally is?

We have been in the house for just over 1 year and I now have our energy use for the start of October to the start of October which turns out to be around 14,000kWh - everything electric and heating at the moment is direct electric.  I have a few tasks still underway to try and reduce useage (recently found some air leaks for example), have now been fitting blinds/curtains (whic we didn't bother with last winter!, and at last we have room thermostats working for the UFH rather than it being either on or off. Have had a PV diverter fitted at last, so that should also make a slight impact esecially in autumn/spring for DHW and some (albeit a small amount) of background heat.

Based on the degree days information along with energy use, and the calculated heat loss for the house, I have been trying to figure out what the actual base temperature of our house for degree days should be. I have also got the average daily temperature for each day and know for example we needed no heating on until October this year.

Based on the simple view that heating did not go on until October (significant temperature drop overnight from 30-Sep to 1-Oct and then remained low with daily temperature averaging 7.5C) whilat the average daily temperature in late September here was around 12C (and the temperature in the house was acceptable) is it reasonable to use 12C as the base temperature?

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JohnS
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« Reply #1 on: October 04, 2019, 09:28:26 PM »

It is norm to use around 15 or 16 degrees C as the base temperature.  There are various websites that give info on this.  Just google.

What you need to do is to get info on heating degree days for your location on, say, a weekly basis and to compare these with the electrical usage each week.  Plot on against the other and the graph, or a trend line of the graph will show the extra heat needed when the temperature is colder.  Outliers were usually explainable eg hose warmer when aged parents visiting, or effect of children going off to uni and rooms unheated, less long hot showers etc.

I did this for around five years as various improvements I made helped lower the slope.  Better insulation better controls, better draught proofing etc. 

There was a time when a research group at Oxford Uni collected this info and provided me with analysis but then they wanted me to pay them to give them info about energy usage and I stopped.

I could look out some of the old graphs etc if you need any more info.
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2.1kWp solar PV
Countrypaul
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« Reply #2 on: October 04, 2019, 11:35:05 PM »

Thanks John,

The use of a figure of 15 or 16 seems to be standard but have little basis for use other than it works in a "bog standard" built as cheap as possible developer home. The base temperature for a passiv house will be a lot lower than an old tin hut, but the same principle will apply, there is a whole section on the DegreeDay site about the base temperature and using linear regression to determine it if you have lots of data.

I don't have the detailed daily/weekly information that I would like and it might be tricky for me to record that - time will tell.

I know how much was power was used as standard and how much was E7. From over summer when there was no heating on, I can get a base line for the amount of E7 is normally used for things like the dishwasher etc. the remaining being heating. I thought I could get a baseline of what is used as standard from the average figures over summer, and from that assume the higher figure over winter was due to heating - however I have overlooked the PV. I guess that over summer the base usage may appear lower due to the PV supplementing the grid power and my reading are only from the grid meter. The result being that I may be underestimating the amount of base power used for things like cooking, Xbox, PCs, TV, etc. and as a result overestimating the amount used for direct heating (I know that the power used for Xbox, PCs TVs etc. will largely result in heat contribution anyway.

Ignoring the PV, I had assumed that the heating was about 9000 kWh /year (not sure if 2018/19 winter was average overall or not yet). Based on that figure, it looks as though we are paying around  750 pa for heating at present. I am looking at putting in an ASHP, but the cost for that is about 4500. If the ASHP was to give a COP of 3, then I might save 500 pa which would give a payback of 9 years. If on the otherhand the COP drops to say 2, then the saving would be 375 pa and a payback of around 13 yrs - all assuming it never goes wrong.

I am trying to use the degree day data to get a clearer idea of when the heat is likely to be needed and how much - but getting the right base temperature seems to be very significant especially when the house appears to be better insulated than average.
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titan
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« Reply #3 on: October 05, 2019, 08:28:25 PM »

I used degree days from my local weather station to build into my spreadsheet to estimate heating requirements for a house then yet to be built. You have enough real data for what you want it is not an exact science as you point out, have you considered solar gain. Could you put a meter on the heating, if you have your PV metered and an immersun or similar what more do you need to know how much power you consume, or am I missing the point completely
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