How do you estimate a scoped heating loss in Btu per hour?
Enter design temperature difference and the areas and U-values of exterior components. The tool calculates steady-state conduction and optional unrecovered outdoor-air heat loss. It reports required heating output in watts, kW and Btu/h for the scope you supply.
Furnace size searches use this heating-load worksheet. Compare required delivered heat with rated output capacity, not fuel input capacity alone. Building heat-loss assumptions and equipment selection require a verified project design; floor area alone does not set the correct furnace.
Which surfaces and U-values should you include?
Use net opaque wall, exterior roof or ceiling, exterior floor, and window or door areas as separate components. Each entered area needs its matching U-value. Subtract glazing from opaque wall area to avoid counting it twice; do not include a component facing a conditioned room.
| Input | Units |
|---|---|
| Design indoor-outdoor temperature difference | as labeled |
| Net exterior opaque wall area (optional) | ft² / m² |
| Wall U-value (W/m²K, optional) | as labeled |
| Exterior roof / ceiling area (optional) | ft² / m² |
| Roof U-value (W/m²K, optional) | as labeled |
| Exterior floor area (optional) | ft² / m² |
| Floor U-value (W/m²K, optional) | as labeled |
| Window / door area (optional) | ft² / m² |
| Window / door U-value (W/m²K, optional) | as labeled |
| Unrecovered outdoor air | as labeled |
| Design air density (kg/m³) | as labeled |
| Air specific heat (J/kgK) | as labeled |
| Explicit design margin (%) (optional) | as labeled |
| Heating efficiency (%) (optional) | as labeled |
What is the heat-loss formula for each component?
Conduction is U × area × temperature difference, using W/m²K, m² and °C or K difference. Outdoor-air loss is density × specific heat × volume flow × temperature difference. The tool requires design air properties when that component is enabled.
Envelope heat loss W = Σ(U × area × temperature difference); outdoor-air loss W = air density × specific heat × volume flow × temperature difference; Btu/h = W × 3.412141633
What does a simple wall example show?
For 100 m² of wall at U = 0.3 W/m²K and a 20°C indoor-outdoor difference, loss is 0.3 × 100 × 20 = 600 W. Multiplying by 3.412141633 gives about 2,047.285 Btu/h, before windows, roof, floor or outdoor air.
How do temperature-difference and area units convert?
Imperial geometry uses square feet and a Fahrenheit temperature difference. Metric uses square meters and a Celsius difference. A temperature difference converts by 5/9 without subtracting 32. All U-values remain SI W/m²K as labeled, regardless of dimension selection.
How should ventilation or infiltration be entered?
Enter the scoped unrecovered outdoor airflow in CFM or L/s, with design air density and specific heat. Use combined airflow that avoids double counting the same source. Heat recovery and varying pressures need appropriately adjusted inputs; the tool does not select an infiltration rate.
What is the difference between heating output and input?
Output is heat delivered to cover the scoped loss. Optional efficiency divides that output by the entered efficiency fraction to estimate input. This percentage model fits combustion or resistance efficiency; a heat pump COP is a different relationship and is not entered as a percentage above 100.
Does this replace a complete heating load calculation?
No. It is a transparent component worksheet. Thermal bridges, distribution losses, different boundary temperatures and room-by-room design conditions must be addressed in the supplied scope or another calculation. No universal watts-per-floor-area assumption is applied.
How can you compare insulation or window changes?
Save the current component areas and verified U-values, then change the affected U-value and calculate again. Compare component watts and total output. An explicit margin is applied once after adding components, so the effect remains separate from the underlying heat loss.
Which tool handles cooling tonnage instead?
Use the AC Tonnage Calculator for cooling-capacity conversion and screening. The CFM Calculator helps with supplied airflow relationships; heating Btu/h and cooling tons describe different design checks.
