What exactly is an ASHRAE design temperature?
I understand that Manual J calculations are performed based on summer and winter ASHRAE design temperatures. HVAC contractors have told me that in my area (the Raleigh NC area), summer design temp is 92 F (apparently it used to be 90 F).
What exactly does a design temp of 92 F mean? Is that supposed to be the top 1% for daily high temperature? Or is it something else like a time average of temperature above a certain point?
I ask because, in the one-month period from June 24, 2019 through July 23, 2019, the average high temperature in my area was 97.0 F. In the entire 30 day period, there were only TWO days where the high was below 92. For 25 of the days, the high was 95 or higher. It was frequently already 90 degrees by 10 am, and did not drop back below 90 until 9 pm or so. These temperature measurements were taken with a weather station located in the shade on my property, but all of the weather stations that I can view on Weather Underground had comparable readings through this 30-day period.
I have not lived in this area that long, but my understanding is that this was a rather unusual heat wave in that it lasted so long with no respite, but that it is not at all uncommon to have many different several day periods of 95 + high temperatures over the course of an average summer (just not 30 days straight…).
Anyways, if an HVAC system were sized such that it could maintain an indoor temperature of 75 degrees on days that the outside high is 92, you would have been quite toasty–in your own house–for an entire month.
So I guess what I’m asking is how can the ASHRAE design temp for my area be only 92 degrees when we have brutally hot 30-day periods like the one that ended July 23? Is this just the result of global warming? Or am I misunderstanding what an ASHRAE design temperature is supposed to represent?
Thanks for any input.