It just hit 96°F in our area, so let’s take a quick look at what that means from an HVAC perspective

It just hit 96°F in our area , so let’s take a quick look at what that means from an HVAC perspective.

You may often hear the term “design conditions” used in HVAC discussions - let's take a quick look at cooling. The 0.4% cooling design dry bulb temperature for Bradley International Airport (Windsor Locks, CT) is 91.6°F. What does that mean? It means that, based on historical weather data, temperatures are expected to exceed 91.6°F for only 0.4% of the year. That’s roughly: 0.4% × 8,760 hours/year = 35 hours/year

Is it safe to use 91.6F to size your equipment? For most non-critical buildings, yes. Why? When mechanical engineers size cooling equipment, they often assume saturated internal space heat gains + solar + load from outside air and then find a simultaneous overall peak. In real building operations, this scenario is unlikely to happen for most non critical buildings.

Are design conditions good for evaluating building/equipment performance? Not really. You probably often hear "my AC has a COP of 3.8 and my heat pump has COP of 3.5. " What they didn't mention is at what temperature and what capacity.
Those numbers reflect a point of time performance and do not reflect a period of time performance and can be very misleading.

A simple but effective method is to look at weather bin data. Weather bin data shows how many hours per year fall into temperature “bins” (e.g., 80 - 85°F, 85 - 90°F, etc.). This gives a clearer picture of the outdoor conditions your equipment operates against and helps engineers and facility managers assess annual performance and energy use. An even better way is energy simulation looking at dynamic interactions of building components against weather conditions and operation logics.

What should building owners do when the weather hits so called design conditions? This is a good time to save your equipment trend data so down the road when you are about to replace them, both you and the engineer have good reference and benchmarks.