When to use: Use to estimate the peak heat loss for a building during the coldest outdoor design temperature. This determines the required furnace, boiler, or heat pump heating capacity. Accounts for wall, ceiling, floor, and window conduction losses plus infiltration. Enter the 99% design outdoor temperature for your location (ASHRAE Fundamentals, Chapter 14). Tighter envelopes (lower ACH) dramatically reduce heating load.
This free heating load calculator estimates a building's peak heat loss at winter design conditions so you can size a furnace, boiler, or heat pump correctly. Following ACCA Manual J principles, it adds up conduction (transmission) losses through walls, ceilings, floors, windows, and doors plus infiltration and ventilation losses to produce a total heating requirement in BTU/hr (and MBH). It's built for HVAC contractors, mechanical engineers, energy auditors, and homeowners who want a realistic capacity instead of a rule-of-thumb guess.
Total heating load is the sum of two parts: transmission (conduction) heat loss through the building envelope and infiltration/ventilation heat loss from air exchange.
Transmission loss through each surface uses Q = U × A × ΔT, where U is the assembly U-factor (1 ÷ R-value, in BTU/hr·ft²·°F), A is the surface area in ft², and ΔT is the indoor-to-outdoor design temperature difference. Sum it across all walls, roof/ceiling, floor, windows, and doors.
Infiltration and ventilation loss uses Q = 1.08 × CFM × ΔT, where 1.08 is the sensible-heat constant for standard air and CFM is the outdoor air leaking or being introduced (often derived from air changes per hour × room volume ÷ 60).
ΔT = indoor design temperature − outdoor 99% winter design temperature. Adding transmission and infiltration losses gives the total heating load in BTU/hr.
Worked example: a wall with R-13 (U ≈ 0.077), 1,500 ft² of area, indoor 70°F and outdoor 5°F (ΔT = 65°F): Q = 0.077 × 1,500 × 65 ≈ 7,500 BTU/hr just for that wall. Repeat for every assembly, add infiltration (e.g. 100 CFM × 1.08 × 65 ≈ 7,000 BTU/hr), and total the result.
1. Enter the indoor design temperature (typically 68–70°F) and the outdoor 99% winter design temperature for your location (ASHRAE Fundamentals, Chapter 14). 2. Enter the areas and R-values (or U-factors) for each envelope component: walls, ceiling/roof, floor, windows, and doors. 3. Enter the air-leakage rate (ACH or CFM of infiltration/ventilation). 4. Read the total heat loss in BTU/hr and MBH, then review the loss breakdown to see which components dominate. 5. Use the total to select equipment, and tighten the envelope (more R-value, lower ACH) to cut the load before upsizing the furnace.
Use the 99% winter design temperature, not the record-low temperature. The 99% value is the outdoor temperature exceeded 99% of the hours in a year, so equipment is sized for typical extremes rather than the single coldest hour — sizing to the record low wastes capacity and money.
Once you have the load, avoid grossly oversizing. An oversized furnace short-cycles, runs less efficiently, and gives poorer comfort and humidity control. ACCA Manual S recommends selecting equipment close to the calculated load (commonly up to about 140% of design heating load for furnaces, less for heat pumps). Reduce the load with air sealing and insulation before stepping up to a larger unit.
Add the transmission heat loss through every envelope surface (Q = U × A × ΔT) to the infiltration/ventilation loss (Q = 1.08 × CFM × ΔT). ΔT is the indoor design temperature minus the outdoor 99% winter design temperature. The sum is the total heating load in BTU/hr, which is the ACCA Manual J approach.
For conduction through a surface it is Q = U × A × ΔT, where U is the U-factor (1 ÷ R-value), A is area in ft², and ΔT is the temperature difference. Air-exchange (infiltration/ventilation) loss uses Q = 1.08 × CFM × ΔT for sensible heat.
It depends on a Manual J calculation for your specific house — climate, insulation, windows, air-tightness, and size all matter. As a rough planning range it is about 30–60 BTU/hr per square foot, lower in mild climates and well-insulated homes and higher in cold climates or leaky homes. Always confirm with an actual load calculation.
Use the 99% winter design temperature for your location from ASHRAE Fundamentals (Chapter 14), not the record low. The 99% value is exceeded 99% of the hours per year, so it sizes equipment for typical extremes without oversizing for a rare event.
Select equipment whose rated heating output at design conditions meets the calculated load with a modest margin — ACCA Manual S typically keeps a furnace within about 100–140% of the design heating load. Avoid grossly oversizing, which causes short-cycling and poor comfort. For heat pumps, also account for balance point and any supplemental heat.
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