How Furnace Size (BTU) is Calculated
The **Furnace Size Calculator** estimates the necessary furnace size in **BTU/hr** (British Thermal Units per hour) required to heat a space. Sizing is crucial: an undersized furnace won't heat the home adequately, and an oversized furnace cycles too frequently, leading to inefficiency and premature wear.
Key Formulas Used
This estimation uses a simplified method based on square footage and a standard BTU factor. For a highly accurate result, a detailed **Manual J load calculation** is required.
$$\text{Required Output BTU} = \text{Square Footage} \times \text{BTU per Sq. Ft. Factor}$$ $$\text{Required Input BTU} = \frac{\text{Required Output BTU}}{\text{Furnace Efficiency (\text{Decimal})}}$$
Understanding the BTU Factor
Typical BTU/Sq. Ft. Factors
- **$25-30$:** Very Mild Climate (e.g., South Florida), Excellent Insulation/New Construction.
- **$30-40$:** Moderate Climate (Most US regions), Average Insulation.
- **$40-60$:** Cold Climate (e.g., Northern US, Canada), Older Homes/Poor Insulation.
Efficiency (AFUE)
- **AFUE** (Annual Fuel Utilization Efficiency) is the percentage of fuel converted to usable heat.
- **Standard:** $\mathbf{80}\%$ AFUE.
- **High-Efficiency:** $\mathbf{90}\%+$ AFUE.
Sizing and Ordering Tips
- **Input BTU is Key:** Furnace models are typically marketed and sold based on their **Input BTU/hr** rating (the amount of fuel consumed), which is higher than the actual heat output.
- **Avoid Oversizing:** Oversizing by more than $\mathbf{10}\%$ is not recommended. An oversized unit is less comfortable (it heats too quickly) and less efficient.
- **Ductwork and Sealing:** Ensure your home's insulation and ductwork are sealed and maintained. This can significantly reduce the required BTU factor, allowing for a smaller, more efficient furnace.