The Physics of Heat Loss
This calculator uses the basic formula for **conduction heat transfer** through a surface. The total heat loss for a structure is the sum of the heat loss through all its components (walls, windows, floors, roof, and air leakage).
The Heat Loss Formula ($\text{Q}$)
The rate of heat transfer ($\text{Q}$) through a material is proportional to its area ($\text{A}$) and the temperature difference ($\Delta \text{T}$) across it, and inversely proportional to its thermal resistance ($\text{R}$).
The **$\text{U}$-Value** (Thermal Transmittance) is the inverse of the $\text{R}$-Value: $\mathbf{U} = \mathbf{1}/\mathbf{R}$.
$$\text{Q} \ (\text{BTU}/\text{hr}) = \text{Area} \ (\text{sq ft}) \times \text{U-Value} \ (\text{BTU}/(\text{hr}\cdot\text{sq ft}\cdot\text{°F})) \times \Delta \text{T} \ (\text{°F})$$
Key Terms Explained
$\mathbf{R}$-Value (Thermal Resistance)
A measure of a material's resistance to conductive heat flow. The higher the $\text{R}$-value, the better the insulating capability. Standard wall insulation is typically $\text{R}$-$\text{13}$ to $\text{R}$-$\text{21}$.
$\mathbf{U}$-Value (Thermal Transmittance)
A measure of the rate of heat transfer through a material. It is the reciprocal of the $\text{R}$-value. The lower the $\text{U}$-value, the better the material prevents heat loss.
Using the Results
- **Total Load:** To size a heater or furnace, you must sum the heat loss for **every surface** (walls, windows, doors, roof, floor) and add the heat loss due to **air infiltration** (drafts).
- **Infiltration:** Air leakage (or infiltration) often accounts for $\mathbf{25}\%-\mathbf{40}\%$ of total heat loss and is not accounted for in this simple surface calculation.
- **Design $\Delta \text{T}$:** The result is based on the coldest expected temperature. Your furnace will run at maximum capacity only when the outdoor temperature hits your Design Outdoor Temp.