Degree days in Housebuilder's Bible Energy Ratings example

[Drellingore]

Kind folks - can anyone explain to my feeble brain why in Housebuilder's Bible (13th edition) this example uses different values for degree days across the three houses?

 

 

My understanding was that degree days are the number of days in which the average temperature is below a given value (15.5°C for the Met Office dataset), multiplied by the number of degrees by which it falls short. What I don't grok is why this value should be different for the three example houses, unless they're in different locations? Is this table using different baseline temperatures for each example, perhaps the 'required uplift' number?

 

I'm trying to knock up a rough energy statement for a planning application. We haven't done technical design yet so I can't give SAP calculations, and this is only to show that our chuffing-great solar PV array is supported by a draft local policy. I was hoping to give some rough calculations based on the thermal envelope of the building, say we're aiming for EnerPHit standards, and then work through the numbers assuming we meet that standard.

[S2D2]

Yes, lower heat loss so the base load of 1kW in the example holds the house at a higher temperature, meaning fewer HDD as they're calculated against the "no heating required" baseline.

[Drellingore]

19 minutes ago, S2D2 said:

meaning fewer HDD as they're calculated against the "no heating required" baseline

 

Ah, okay, so in the Passivhaus example the number of degree-days has been calculated using 11.9° as the baseline, because that's the point at which no additional energy is required to hit the target. So presumably the author has found more detailed information than the Met Office degree days that use 15.5° as the baseline, like the actual average temperature on each day?

Edited April 26 by Drellingore
formatting, because this forum doesn't support Markdown

[JohnMo]

1 hour ago, Drellingore said:

understanding was that degree days are the number of days in which the average temperature is below a given value (15.5°C

In the normal sense you are correct. But the lower the house heat losses it moves the target temperature of 15.5 downwards.  Our house doesn't need any heat above 10 degrees, so the degree days would start at 10 not 15.5.

 

So if you get the degree days for you location and use 10 as the baseline you won't be be far off.

[Drellingore]

Thanks folks! The lovely website https://www.degreedays.net/ sorted me with specific data. I haven't put any effort into calculating incidentals and have just taken 1kW as a placeholder figure, and that meant my base temperature was nearly 15°C anyway

[SteamyTea]

5 hours ago, JohnMo said:

Our house doesn't need any heat above 10 degrees,

Same with my house, maybe a bit lower.

 

HDDs are based on a normal distribution around the the mean temperature, with the standard deviation changing for different climate zones (the kurtosis) rather than actual location specific data.

As the UK has a more leptokurtic temperature distribution (flatter in the middle bit), this can skew the number of 'official' HDDs and CDDs.

They are still useful for a first order approximation and when you have a system up and running are probably the best method to fine tune your weather compensation curve.

[Drellingore]

5 minutes ago, SteamyTea said:

leptokurtic

 

 Word of the week!

[SteamyTea]

Just now, Drellingore said:

 

 Word of the week!

Not a word i have used this week till now.

[Drellingore]

1 minute ago, SteamyTea said:

Not a word i have used this week till now.

Not a word I have encountered this lifetime 'til now 

[SteamyTea]

I may be used the wrong word and meant platykurtic anyway.

 

Flat in the middle bit.

If you plot the CET PDF you find that around the mean temperature the probabilities actually reduced a bit.

A 'bit' is a statistical term to mean 'i can't tell you exactly'.

Why the mean is really called the 'central tendency' as that gives some wiggle room on the numbers.

[S2D2]

8 hours ago, Drellingore said:

 

Ah, okay, so in the Passivhaus example the number of degree-days has been calculated using 11.9° as the baseline, because that's the point at which no additional energy is required to hit the target. So presumably the author has found more detailed information than the Met Office degree days that use 15.5° as the baseline, like the actual average temperature on each day?

7 hours ago, Drellingore said:

Thanks folks! The lovely website https://www.degreedays.net/ sorted me with specific data. I haven't put any effort into calculating incidentals and have just taken 1kW as a placeholder figure, and that meant my base temperature was nearly 15°C anyway

 

That's the one, the data is the same (daily average temperature) but that website lets you choose where to clip the area (at the "no heating required" point) which in this case represents HDD.

[SteamyTea]