Two identical houses in two cities need different equipment, and the difference is not small. It comes from two properties of the local climate.
Design temperature
The 1% summer design dry-bulb temperature is the outdoor temperature exceeded only 1% of hours in a cooling season — roughly 35 hours a year.
Sizing to that rather than to the record high is deliberate. A system sized for the hottest hour ever recorded would be oversized for the other 8,750 hours of the year, and would spend the summer short cycling.
The load is driven by the difference between that temperature and the indoor setpoint, usually 75°F.
| City | 1% design | ΔT to 75°F |
|---|---|---|
| Phoenix, AZ | 108°F | 33 |
| Houston, TX | 96°F | 21 |
| Atlanta, GA | 93°F | 18 |
| Chicago, IL | 91°F | 16 |
| Seattle, WA | 84°F | 9 |
A ΔT of 33 is not twice a ΔT of 16 in its effect, because envelope losses are only part of the load — but it is the single largest term.
Humidity, and why it splits the map
Total cooling load has two parts. Sensible load lowers air temperature; latent load removes moisture.
In dry climates, latent load is nearly zero. Nearly all the capacity goes to temperature, and evaporative cooling is a genuine alternative.
In humid climates, latent load can be 30% or more of the total. That capacity is spent condensing water and produces no reading on a thermometer, which is why a system that is adequate in Denver is not adequate at the same tonnage in Tampa.
This is also why the same nominal tonnage delivers less apparent cooling in the Southeast, and why run time matters more than capacity there.
The pattern that results
Hot and dry — high tonnage per square foot, but the capacity is almost all sensible. Systems can be selected for high sensible heat ratio, and long run times are less critical.
Hot and humid — high tonnage and a large latent fraction. Correct sizing matters most here, because oversizing kills dehumidification.
Mixed — moderate tonnage, and the heating load often governs the equipment choice on a heat pump.
Cool — low cooling load, and in a heat pump installation the winter heating requirement decides the size entirely.
Where a heat pump changes the question
A heat pump does both jobs with one machine, so it has to satisfy two loads that peak in opposite seasons.
In the South, cooling governs and the heating capacity comes along free.
In the North, heating governs, and a unit sized for the heating load is substantially oversized for cooling — the classic argument for a variable-capacity or dual-fuel system that can turn down in summer.
Microclimate within a region
Design temperature data is published by station and varies inside a metro area. Elevation, distance from water and urban heat island effects all move it.
A house on the coast in a coastal city and a house twenty miles inland can differ by several degrees of design temperature and noticeably in humidity, which is enough to change a half-ton decision.
Using this properly
These figures set expectations and catch a quote that is far outside the plausible range. They do not replace a load calculation.
What they are genuinely good for is the conversation before the calculation: if a contractor proposes 5 tons for a 1,600 sq ft house in a mixed climate, this is the arithmetic that says the number needs explaining.