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Evgenii Konkin
Evgenii Konkin

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Why "400 Square Feet Per Ton" Gets the AC Size Wrong for Every House

#hvac #engineering #energy #building

The most common AC sizing method in residential HVAC is a rule of thumb: 400 to 600 square feet per ton. It is fast. It requires no calculation. And it gets the answer wrong almost every time.

Two houses, same square footage, different loads 

House A: 2,000 sq ft
  Single story, slab on grade
  North-facing main windows
  R-38 attic, double-pane windows
  Tight envelope (3 ACH50)
  Manual J cooling load: 2.2 tons

House B: 2,000 sq ft
  Two story, vaulted ceiling
  Large west-facing window wall
  R-19 attic, dark roof
  Leaky envelope (10 ACH50)
  Manual J cooling load: 5.1 tons
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Same city. Same year built. Same square footage. One needs 2.2 tons. The other needs 5.1.

The rule of thumb says 3.3 to 5.0 tons for both. It oversizes House A by 80% and undersizes House B by 22%.

What the rule ignores 

Factor                        Impact on Load
──────────────────────────────────────────────────
Window area + orientation      ±2.25 tons
  (300 sqft west vs 100 sqft north)

Insulation level               ±1.0 ton
  (R-19 vs R-38 attic)

Air sealing                    ±1.5 tons
  (2 ACH50 vs 12 ACH50, humid climate)

Ceiling height                 ±0.5 tons
  (8 ft vs 12 ft vaulted)

Duct location                  ±0.8 tons
  (attic ducts vs conditioned space)

Climate zone                   ±1.0 ton
  (Phoenix vs Portland at same sqft)
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Any one of these can shift the load by a full ton. Combined, they explain why two identical floor plans need completely different equipment.

What Manual J calculates

ACCA Manual J is the standard residential load calculation. Inputs:

1. Outdoor design conditions (ASHRAE data, your city)
2. Indoor setpoint (typically 75F cooling)
3. Orientation (compass direction of each wall/window)
4. Window specs (U-factor, SHGC, shading)
5. Wall/ceiling/floor R-values
6. Infiltration rate (blower door or estimate)
7. Duct location and insulation
8. Internal gains (people, appliances)
9. Ventilation requirements
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Output: design cooling load in BTU/hr (sensible + latent). Divide by 12,000 = tons.

The rule of thumb range is too wide 

2,000 sq ft house:
  At 600 sq ft/ton = 3.3 tons
  At 400 sq ft/ton = 5.0 tons

  Range: 1.7 tons (50% spread)
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For a $3,000-$7,000 decision that affects comfort for 15 years, a 50% uncertainty is not engineering — it is guessing.

Cost of getting it wrong 

Oversized (House A got 4T, needs 2.2T):
  - Short cycling, poor dehumidification
  - Higher equipment cost (+$2,000)
  - Shorter compressor life
  - House feels clammy at 72F

Undersized (House B got 4T, needs 5.1T):
  - Runs nonstop on design days
  - Cannot maintain 75F setpoint
  - Reaches 78-80F on extreme days
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The better approach 

1. Get a Manual J calculation ($150-300)
2. Or use a detailed calculator with:
   - Climate zone
   - Orientation
   - Window area and type
   - Insulation levels
   - Air sealing quality
3. Size equipment to 100-110% of calculated load
4. Never round up "just in case"
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Square footage is one input. It is not the answer.

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