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Evgenii Konkin
Evgenii Konkin

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Why Climate Zone Changes AC Tonnage More Than Square Footage

#hvac #engineering #cooling #climatedesign

AC tonnage is not set by square footage. It is set by how much heat the system has to remove per hour. And that depends on where the building is.

The tonnage formula 

Cooling Load = Envelope + Occupants + Equipment + Lighting
Tonnage = Cooling Load (BTU/hr) / 12,000
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The envelope load dominates in most buildings:

Envelope = Area × Climate Factor × (Ceiling Height / 8)
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Climate factors by region 

Climate          BTU/ft²    Example cities
──────────────────────────────────────────────
Cool             20–25      Seattle, Portland, Minneapolis
Moderate         25–30      Denver, Chicago, DC
Hot-humid        30–35      Houston, Dallas, Atlanta, Miami
Hot-dry          35–40      Phoenix, Las Vegas, Tucson
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These factors account for the design outdoor temperature, solar gain patterns, and humidity levels typical for each region.

Same house, four cities

A 2,000 sq ft house with 9 ft ceilings, 4 occupants, 500W equipment, 200W lighting:

City        Climate    BTU/ft²   Envelope    Internal   Total      Tons
──────────────────────────────────────────────────────────────────────────
Seattle     Cool       22        49,500      2,400      51,900     4.3
Denver      Moderate   27        60,750      2,400      63,150     5.3
Dallas      Hot        32        72,000      2,400      74,400     6.2
Phoenix     Hot-dry    38        85,500      2,400      87,900     7.3
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Same house. Tonnage nearly doubles from Seattle to Phoenix.

Why "400 sq ft per ton" fails

The most common rule of thumb:

Tonnage = Area / 400 to 600
2,000 sq ft / 400 = 5.0 tons
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This gives 5 tons everywhere.

Seattle: needs 4.3T, gets 5.0T → oversized by 16%
  → short cycling, poor humidity control, wasted energy

Phoenix: needs 7.3T, gets 5.0T → undersized by 32%
  → runs continuously, never reaches setpoint on peak days
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The rule was calibrated for a moderate climate. It fails everywhere else.

Ceiling height multiplier

The formula uses volume, not area:

8 ft ceilings:  2,000 × 8 = 16,000 ft³
10 ft ceilings: 2,000 × 10 = 20,000 ft³
12 ft ceilings: 2,000 × 12 = 24,000 ft³
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The ceiling height adjustment:

Factor = actual height / 8

8 ft:  1.0×  (baseline)
9 ft:  1.125×
10 ft: 1.25×
12 ft: 1.5×
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A house with 12 ft ceilings in Phoenix needs 50% more tonnage than the same footprint with 8 ft ceilings.

When internal gains change the picture

In Phoenix, envelope is 85–90% of total load. Internal gains barely matter.

In Seattle, envelope is 60–70%. Internal gains are 30–40%.

Seattle office with heavy equipment:
  Envelope:  49,500 BTU/hr (70%)
  Internals: 21,000 BTU/hr (30%)
  Total:     70,500 BTU/hr = 5.9 tons

Seattle bedroom (same area):
  Envelope:  49,500 BTU/hr (93%)
  Internals:  3,600 BTU/hr (7%)
  Total:     53,100 BTU/hr = 4.4 tons
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Same area. Different internal loads. 1.5 tons difference. In hot climates, this gap shrinks because the envelope load overwhelms everything else.

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