Replies

1. GBA Editor
   Martin Holladay | May 06, 2022 04:05pm | #1

   Andrew,
   Why "consistently"? The design temperature is an annual minimum. It has nothing to do with day-to-day variations, so where does "consistently" enter the picture?

   As Allison Bailes explained in one of his blogs, "Winter design temperature [is] defined as the temperature that a location stays above a certain percentage of the hours in a year. The 99% design temperature is the one usually used. That means a place stays above the 99% design temperature 99% of the hours in a year. The total number of hours in a year is 365 x 24 = 8760. Ninety-nine percent of that is 8672 hours, which means that if your 99% deign temperature is 0° F as it is in Fort Collins, Colorado, your temperature will be below 0° F for only 88 hours per year, on average."

2. 
   Andrew3GP | May 06, 2022 05:00pm | #2

   Martin, thanks for your response. "Consistently" in the sense that if it is -10F at the weather station, it is -5F here; if it is 1F at the weather station, it is 6F here. Not exactly, of course, and not every hour of every day, but let's say "almost always."

   So, if I am understanding correctly, at the weather station the winter temp is below the 99% design temperature for only 88 hours per year; but at our house the winter temp is below that 99% design temperature for far fewer hours.

   My practical concern is whether using the Man. J winter outdoor design temp without adjusting for the average (~5F) winter temperature difference between the weather station and our house will skew the outcome toward an oversized system?