📖 Overview
This lightweight HVAC sizing model helps users avoid under and oversizing mistakes.
It approximates BTU load from room envelope and exposure factors to suggest a practical tonnage band.
🧪 Example Scenarios
Use these default and higher-pressure example inputs to explore how sensitive this calculator is before using your real numbers.
| Input | Base Case | Higher Pressure Case |
|---|---|---|
| Conditioned Floor Area (sq ft) | 1,850 | 2,128 |
| Ceiling Height (ft) | 9 | 10.35 |
| Insulation R Value | 19 | 21.85 |
| Window Orientation Code (1 North 2 Mixed 3 South) | 3 | 3.45 |
| Climate Severity Code (1 Cold 2 Mixed 3 Hot) | 3 | 3.45 |
⚙️ How It Works
Provides a simplified cooling load estimate that adjusts base BTU demand by volume insulation orientation and climate severity.
The Formula
BTU Load ≈ Area × BaseFactor × VolumeFactor × Envelope Factors
| Area | Conditioned floor area |
| Ceiling Height | Used to scale interior volume load |
| R-Value | Insulation performance adjustment |
| Orientation Code | Solar gain proxy from window orientation |
| Climate Code | Regional cooling severity proxy |
💡This calculator is scenario-based. Better input quality leads to better decision quality.
⚠️This is not a full ACCA Manual J replacement. Use it as a screening tool before formal HVAC design.
Quick Reference
| Estimated Load | Approx Tons | Typical Guidance |
|---|---|---|
| 18,000 BTU/h | 1.5 ton | Small efficient zone |
| 30,000 BTU/h | 2.5 ton | Mid-size residential zone |
| 42,000 BTU/h | 3.5 ton | Larger or high-gain zone |
When To Use This
- Use this tool when you need a fast decision during active planning or execution.
- Use this before committing money, time, or tradeoffs that are hard to reverse.
- Use this to compare options using the same assumptions across scenarios.
Edge Cases To Watch
- Results can be misleading if key inputs are missing, stale, or unrealistic.
- Very small or very large values may amplify rounding effects and interpretation risk.
- If assumptions change mid-decision, recalculate before acting.
Practical Tips
💡 Oversizing can reduce humidity control and comfort.
💡 Undersizing leads to long run cycles and poor peak performance.
💡 Verify duct and airflow capacity before final tonnage decisions.
Frequently Asked Questions
❓ Why round to half-ton sizes?
Residential systems are commonly selected in half-ton equipment steps.
❓ Can orientation really change load?
Yes solar gain through south and west exposure can materially increase cooling demand.