Quick Answer
Use square-foot rules only as a screening check, never as final sizing. Many homes loosely fall around 1 ton per 400–600 square feet, but a Manual J heat-load calculation should account for climate, windows, orientation, air leakage, insulation and ceiling height. Compare high-efficiency equipment with a simple payback calculation using realistic annual savings.
1. Manual J Is the Real Sizing Starting Point
Manual J estimates how much heat enters and leaves the home under design conditions. It considers climate, orientation, glass area, insulation, infiltration, ceiling height and internal loads.
2. Use Square Footage Only as a Sanity Check
A rough screening range is often 1 ton per 400–600 square feet. For a 2,000-square-foot home that suggests about 3.3–5 tons, which is deliberately too broad to select equipment from.
Conditioned Floor Area ÷ 400–600 sq ft per ton = Rough Tonnage Range3. Compare SEER2 Operating Cost
For a simplified same-load comparison, cooling energy is approximately inversely proportional to SEER2.
Baseline Annual Cooling Cost × (Baseline SEER2 ÷ New SEER2) ≈ New Annual Cooling CostIf a 14.3 SEER2 system costs $1,200/year for cooling, a rough 20 SEER2 comparison is $1,200 × 14.3 ÷ 20 ≈ $858/year, or about $342/year lower.
4. Calculate Simple Payback
Efficiency Upgrade Premium ÷ Annual Energy Savings = Payback YearsA $3,500 premium divided by $342/year is about 10.2 years.
5. Do Not Ignore the Duct System
| Duct Scope | Planning Range | Main Drivers |
|---|---|---|
| Repair / replacement | $35–$65 per linear foot | Access, diameter, insulation, plenums, dampers and registers. |
Leaky, crushed or undersized ducts can prevent premium equipment from delivering its rated performance.
6. Worked Comparison
| 14.3 SEER2 | 20 SEER2 | |
|---|---|---|
| Installed price | $11,000 | $14,500 |
| Annual cooling cost | $1,200 | ≈ $858 |
| Annual savings | — | ≈ $342 |
| Upgrade premium | — | $3,500 |
| Simple payback | — | ≈ 10.2 years |
Also compare heating efficiency, rebates, sound, comfort, equipment life and duct condition.
Frequently Asked Questions
Is one ton per 400 to 600 square feet an HVAC sizing rule?
It is only a rough screening range. Final equipment sizing should be based on a Manual J load calculation because climate, insulation, windows, air leakage, orientation and ceiling height can change the load substantially.
Can an oversized air conditioner increase energy use?
Yes. Oversized systems can short-cycle, reduce dehumidification, create uneven temperatures and add wear because they start and stop more often.
How do I compare 14.3 SEER2 with 18 or 22 SEER2?
For a rough comparison, cooling energy use is inversely related to efficiency. If the cooling load is the same, a higher SEER2 unit should use less electricity, but real savings depend on climate, controls, part-load performance, ducts and installation quality.
What is the simple payback formula?
Divide the additional installed cost of the higher-efficiency option by the expected annual energy savings.
How much does ductwork replacement cost?
A broad planning range around $35 to $65 per linear foot can help with early budgeting, but access, duct size, insulation, plenums, registers and attic/crawlspace conditions matter.