Formula
Heating load is conduction plus infiltration. Conduction is U times area times (indoor winter minus outdoor winter). Cooling conduction uses outdoor summer minus indoor summer, with no cooling-load temperature difference. Infiltration airflow is air changes times volume divided by 60. Heating infiltration is 0.018 times air changes times volume times the winter difference. That 0.018 is 0.075 lb/ft³ times 0.24 Btu/lb-°F. Cooling infiltration sensible is 1.08 times cfm times the summer difference. Latent is 0.68 times cfm times the grains per pound you type.
Q = U × A × ΔT
cfm = ACH × V / 60
Qheat,inf = 0.018 × ACH × V × ΔT
People are counted at 230 Btu/h sensible and 190 Btu/h latent each, the seated light-work figures used in the worked example. Window solar is area times SHGC times the solar factor you type. One Btu/h is 0.29307 W. One ton is 12,000 Btu/h. A U-factor in Btu/h·ft²·°F times 5.678 is W/m²·K. Manual J adds orientation, design-day weather, duct loss, and a fuller latent treatment. Oversizing equipment can hurt comfort and humidity. This page does not pick the equipment.
Worked examples
Heating. UA 175, ΔT 60 °F, conduction 10,500 Btu/h, infiltration 4,320, total 14,820 Btu/h, about 4.34 kW. A 10 percent margin is 16,302.
Cooling. Conduction 3,500, solar 4,000, infiltration sensible 1,440 and latent 1,360, people 920 and 760, appliances 1,200. Sensible 11,060, latent 2,120, total 13,180 Btu/h, 1.098 tons.
Reference
R-20 is U 0.05. U 0.5 is R-2. U 0.05 in IP units is 0.2839 W/m²·K.
Questions
How do I calculate HVAC load?
Add conduction, U times area times the design temperature difference, for the walls, windows, ceiling, floor, and doors. Add infiltration. On the cooling side, add the solar, people, and appliance inputs you type. This page shows that sum. It is an estimate, not Manual J.
What is the difference between this and Manual J?
ACCA Manual J is a room-by-room procedure with design-day data, orientation, duct gains, and a full latent treatment. This page is a steady-state U·A·ΔT estimate. It is not an ACCA-approved Manual J and not a free Manual J calculator.
How many BTU per square foot?
Divide the load this page computes by the floor area. The example house is about 14.8 Btu/h per square foot for heating and about 13.2 for cooling. That is this example, not a rule of thumb.
How many tons for my house?
Cooling Btu/h divided by 12,000 is tons-equivalent. The example is 1.098 tons. The tonnage calculator is a separate planning rate. Neither page names a model.
What is a good R-value?
It depends on climate and the assembly. This page does not publish a recommended R. The U.S. Department of Energy insulation guidance is the place to start, and the label on the product is the R you type.
What is infiltration?
Outdoor air leaking in. Airflow in cfm is air changes per hour times volume in cubic feet, divided by 60. You type the air changes. Above 3 per hour, the page warns.
Does this replace a contractor?
No. A licensed contractor and the local code still do the load calculation that equipment is chosen from.
Sources
- Conduction Q = U A ΔT, with U = 1/R. Infiltration heating uses 0.018 = 0.075 × 0.24. Cooling sensible uses 1.08 × cfm × ΔT. Latent uses 0.68 × cfm × grains per pound. Cooling conduction has no CLTD.
- People: 230 Btu/h sensible and 190 Btu/h latent, the seated light-work pair used for the worked example. This page did not re-read the current ASHRAE Handbook table. ACCA Manual J is the full procedure this estimate is not.
- 1 Btu/h = 0.29307 W. 1 ton = 12,000 Btu/h. U in W/m²·K = U in IP × 5.678. Design temperatures and R-values are inputs. DOE insulation guidance is where climate R guidance lives; no city table is embedded here.