\[ \text{size} = \text{BTU}_{\text{rec}} \times A \times F_s \times F_i \times e \]
Where:
| Climate Zone | Annual Average Temperature (°C) | BTU Recommendation (Min) | BTU Recommendation (Max) |
|---|---|---|---|
| Zone 1 | 21°C and up | 30 | 35 |
| Zone 2 | 16 to 20°C | 35 | 40 |
| Zone 3 | 11 to 15°C | 40 | 45 |
| Zone 4 | 6 to 10°C | 45 | 55 |
| Zone 5 | -4 to 5°C | 55 | 70 |
| Zone 6 | -9 to -5°C | 70 | 80 |
| Zone 7 | -19 to -10°C | 80 | 90 |
| Zone 8 | -24 to -20°C | 90 | 100 |
| Zone 9 | -25°C and below | 100 | 100+ |