Home Heating Load Calculator
Estimate the heating load (BTU/hr) required to keep your home comfortable during cold weather using the Manual J simplified method.
Formulas Used
1. Temperature Difference:
ΔT = Tindoor − Toutdoor
2. Conduction Heat Loss (each surface):
Q = U × A × ΔT
where U = thermal transmittance (BTU/hr·ft²·°F), A = surface area (ft²)
3. Infiltration Heat Loss:
Qinf = 0.018 × ACH × Volume × ΔT
where 0.018 BTU/ft³·°F is the volumetric heat capacity of air, ACH = air changes per hour
4. Total Design Load:
Qdesign = (Qwalls + Qwindows + Qceiling + Qfloor + Qinf) × 1.25
5. Wall Area Estimation:
Perimeter = 4 × √(Floor Area / Stories) (assumes square footprint)
Net Wall Area = Perimeter × Ceiling Height × Stories − Window Area
Unit Conversions:
1 ton = 12,000 BTU/hr | 1 BTU/hr = 0.000293071 kW
Assumptions & References
- Based on ACCA Manual J (Residential Load Calculation, 8th Edition) simplified method.
- Ceiling U-value fixed at 0.026 BTU/hr·ft²·°F (≈ R-38 attic insulation), a common code-minimum for cold climates.
- Home footprint is assumed square for perimeter estimation; irregular shapes may increase wall area and load.
- A 25% safety factor (×1.25) is applied per ACCA Manual J to account for unusually cold days, duct losses, and equipment cycling.
- Infiltration constant 0.018 BTU/ft³·°F = density of air (0.075 lb/ft³) × specific heat (0.24 BTU/lb·°F).
- Outdoor design temperature should be the 99% heating design temperature for your location (available from ASHRAE or local weather data).
- Internal heat gains (occupants, appliances, solar) are not subtracted in this simplified model, providing a conservative (safe) estimate.
- Duct losses are not explicitly modeled; for forced-air systems in unconditioned spaces, add 10–30% to the design load.
- References: ASHRAE Fundamentals Handbook; ACCA Manual J 8th Ed.; U.S. DOE Building Energy Codes Program.