What is Heat Loss?
Heat loss, in the context of a building, is the total amount of heat that escapes from the inside to the outside during cold weather. This process occurs through conduction (heat moving through walls), convection (air currents), and radiation. The rate of heat loss is typically measured in BTUs per hour.
Calculating this value is the single most important step in choosing a new furnace or heater. A system that is too small will fail to keep the house warm on the coldest days. A system that is too large will "short cycle" (turn on and off rapidly), leading to inefficiency, increased wear, and poor temperature regulation.
Key Factors Affecting Heat Loss
| Factor | Impact on Heat Loss |
|---|---|
| Insulation Quality | The most critical factor. Good insulation (walls, attic, floors) dramatically reduces heat loss. |
| Air Leakage | Drafts from windows, doors, and other gaps are a major source of heat loss through convection. |
| Temperature Difference (ΔT) | The greater the difference between your desired inside temperature and the outside temperature, the faster heat will escape. |
| Windows & Doors | Windows, especially older single-pane ones, are significant points of heat loss compared to well-insulated walls. |
How to Use the Calculator
- Enter the total square footage of the area you want to heat.
- Enter the average ceiling height of that area.
- Select the option that best describes your home's insulation and construction quality.
- Enter the Temperature Difference (Delta T): your desired inside temperature minus the coldest expected outside temperature (e.g., 70°F inside - 10°F outside = 60°F difference).
- Click "Calculate Heat Loss" to get an estimate in BTUs per hour and Watts.
Frequently Asked Questions
What is a BTU?
A BTU (British Thermal Unit) is the amount of heat needed to raise the temperature of one pound of water by one degree Fahrenheit. It is the standard unit for measuring heating and cooling capacity.
Is this calculator a substitute for a professional "Manual J" calculation?
No. This calculator provides a simplified estimate suitable for initial planning. A professional "Manual J" heat load calculation performed by an HVAC technician is much more detailed, accounting for window sizes, specific R-values, local climate data, and more. It is highly recommended for a final decision.