Accuraccy of 99% design temperatures
kat_h
| Posted in General Questions on
We’ve been playing around with the NEEP heat pump sizing tool, but it doesn’t have temperature data for Canada. So we downloaded the hourly degree data for our city from 1991 to the present, in order to calculate the percentages of annual heating load served and annual load with low load cycling.
We found an interesting thing: the 99% design temperature for our city, as calculated over those years is about 5 degrees warmer than the official design temperature (about 0 degrees F as opposed to -5 degrees F).
How often are the official temperatures updated? Which is more trustworthy?
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Replies
I think the DD conditions for many locations were updated in the latest ASHRAE Handbook of Fundamentals (2022) based on weather data from 1994 to 2019, whereas the previous version used 1990 to 2014. so... every 5 years? Personally I would use the latest values from the Handbook over something I calculated myself with the caveat that if the next size down of the equipment being sized was just under the the required capacity at official design conditions and i had good evidence that they were more extreme than reality (say by looking at the TMYx.2007-2021 data), id probably go with the smaller equipment size if it was cheaper
https://www.greenbuildingadvisor.com/question/acca-outdoor-design-temperatures-trust