[Greenbuilding] HDD and how they are calculated (daily avg. vs hourly avg)

[Michael O'Brien]

Hi, fellas--

At least where I live, HDD varies quite a bit within local microclimates. For example, a building out by the airport will experience more HDD because of cold air flows from the Columbia Gorge.

We don't have as many weather stations as we used to either, so fewer places that publish HDDs, making it harder to get a sense of microclimates within the metro area.

If you look back through NWS records from 1941 to now, our HDDs fluctuate quite a bit from year to year. For example 2003-4 was 3811 HDD, while 1949-50 was 5268.

So it makes sense to me to go with a representative long-term average when modeling, and try out some extremes just to see what happens. In our case, we decided to design our house with the idea that HDD are dropping over the years, while CDDs are going up, that is, climate change is happening and will continue to affect future weather so the house design includes some cooling features.

Best,

Mike

Michael O'Brien
obrien at hevanet.com



On Nov 20, 2011, at 8:31 AM, Reuben Deumling wrote:

> My understanding is that NOAA simply averages the high and low temperature in a 24 hour period and subtracts that average from 65F. But if one were to say use hourly data, the resulting HDD figure would be different. How much different I don't know. I suspect there are efforts to calculate the error (for a day or month) but haven't found it. Anyone?
> 
> http://knol.google.com/k/degree-days#
> 
> "The calculation method that I explained above is essentially the correct one for calculating heating degree days: for each period over which the outside air temperature was constant, you multiply the degrees below the base temperature by the number of days that the temperature was fixed for (usually small fractions of days), and then you sum all the values together to get the total heating degree days for the period in question.
> 
> The problem with that approach is that, in the real world, outside air temperature doesn't remain constant - in fact it changes pretty much all the time.  Mathematically speaking you'd need an infinite number of temperature readings to calculate degree days properly.
> 
> Fortunately, "mathematically speaking" doesn't really matter too much in this instance, and half-hourly or hourly temperature readings are plenty good enough to calculate degree days accurately using the method described above.
> 
> However, reliable half-hourly and hourly temperature readings are rarely readily available, so there are a number of other approximation methods that are used to calculate degree days from more commonly available measurements of outside air temperature.  These methods typically use either the daily maximum and minimum temperatures, or the daily average temperatures.
> 
> Personally I'm of the opinion that the the details of the approximation method used are not important, so long as it uses the data it's given to generate degree-day figures that are very close to those that would be generated by the correct method (or, more realistically, by a method that used half-hourly temperature readings or similar)." _______________________________________________
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