HVAC Load Calculator

Most load calculators multiply floor area by a factor for the climate, which is a guess dressed up as a number and sizes equipment badly. This one adds up the real paths heat takes, each surface area divided by its R-value and multiplied by the design temperature difference, plus air leakage from the volume and an air change rate, plus solar gain through sunlit glass and the heat from people and appliances on the cooling side. Every assumption is a field you can see and change, including the design temperatures, the air change rate, the solar heat gain coefficient and the duct allowance. It is still a whole-house envelope calculation and not a Manual J, which works room by room with orientation, thermal mass, measured duct leakage and local design weather.

Calculator Type
Measurements In
This moves the whole form and the answer with it. Areas, the volume, both design temperatures, the insulation values and the results are all read and reported in the system you pick here.

Conduction through every surface plus air leakage, at your winter design temperature.

Left blank this is 70 F.
The winter design temperature for your area, which is the temperature only about one percent of winter hours fall below. It is not the record low.
Square feet, or square metres if you switched above. Exterior wall area with the windows and doors taken out, since those are counted separately. Perimeter times wall height, minus the openings.
Whole assembly, not just the batt. An uninsulated frame wall is about R-4 or RSI 0.7, a 2x4 wall with batts about R-11 to R-13 or RSI 1.9 to 2.3, a 2x6 wall about R-19 or RSI 3.3.
Total glazed area, counting the frame, in square feet or square metres.
Windows are rated in U, the reciprocal of R. In BTU per hour per sq ft per F: single glazing about 1.0, ordinary double glazing 0.5, a good low-e triple unit 0.2 to 0.3. In W per m2 per K the same four are 5.7, 2.8 and 1.1 to 1.7.
Square feet or square metres. Leave at zero to fold the doors into the wall area.
A solid wood door is about R-2 or RSI 0.35, an insulated steel or fibreglass door R-5 to R-6 or RSI 0.9 to 1.1. Left blank this is R-5, or RSI 0.88 in metric.
Square feet or square metres. The area of the ceiling under the attic, or of the roof if it is a cathedral ceiling.
An uninsulated attic floor is near R-3 or RSI 0.5, older insulation R-19 to R-30 or RSI 3.3 to 5.3, a current attic R-38 to R-60 or RSI 6.7 to 10.6.
Square feet or square metres. A floor over a crawl space, an unheated basement or a garage. Put zero for a slab on grade or a heated basement below, neither of which loses heat this way.
Left blank this is R-19, or RSI 3.35 in metric.
Cubic feet, or cubic metres if you switched above. Conditioned floor area times the ceiling height: 1,500 sq ft with 8 ft ceilings is 12,000 cubic feet, and 140 m2 with 2.4 m ceilings is 336 cubic metres.
Natural infiltration, and the same number in either system because it is a ratio. A tight new build is 0.2 to 0.35, an average house 0.5, a loose older house 0.7 to 1.0 or worse. This is the biggest guess on the form.
Ducts running through an unconditioned attic or crawl space commonly lose 10 to 25 percent. Ducts inside the conditioned envelope lose nothing worth counting.
Square feet or square metres. Used only to print the load per unit of floor area, so the answer can be sanity checked against the usual rules of thumb.
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Measurements In
This moves the whole form and the answer with it. Areas, the volume, both design temperatures, the insulation values and the results are all read and reported in the system you pick here.

The same conduction, plus solar gain through the glass, the heat people and appliances give off, and the moisture that leaks in.

Left blank this is 75 F.
The summer design dry bulb temperature for your area, exceeded in about one percent of summer hours.
Square feet, or square metres if you switched above. Exterior wall area with the windows and doors taken out, since those are counted separately. Perimeter times wall height, minus the openings.
Whole assembly, not just the batt. An uninsulated frame wall is about R-4 or RSI 0.7, a 2x4 wall with batts about R-11 to R-13 or RSI 1.9 to 2.3, a 2x6 wall about R-19 or RSI 3.3.
Total glazed area, counting the frame, in square feet or square metres.
Windows are rated in U, the reciprocal of R. In BTU per hour per sq ft per F: single glazing about 1.0, ordinary double glazing 0.5, a good low-e triple unit 0.2 to 0.3. In W per m2 per K the same four are 5.7, 2.8 and 1.1 to 1.7.
Square feet or square metres. The glass that takes direct sun at the worst hour of the day, which is usually the west and south facing windows, not all of them at once.
The share of solar energy the window lets through, and the same number in either system because it is a fraction. Clear double glazing is about 0.6 to 0.7, a low-e unit 0.25 to 0.4. Internal blinds cut it by roughly a third.
BTU per hour per square foot, or watts per square metre in metric. The peak solar energy striking the glass before the coefficient is applied: around 150 to 250, or 470 to 790 W/m2, for west and south glass in summer at mid latitudes, far less for north glass or a deep overhang. Left blank this is 150, or 473 W/m2.
Square feet or square metres. Leave at zero to fold the doors into the wall area.
A solid wood door is about R-2 or RSI 0.35, an insulated steel or fibreglass door R-5 to R-6 or RSI 0.9 to 1.1. Left blank this is R-5, or RSI 0.88 in metric.
Square feet or square metres. The area of the ceiling under the attic, or of the roof if it is a cathedral ceiling.
An uninsulated attic floor is near R-3 or RSI 0.5, older insulation R-19 to R-30 or RSI 3.3 to 5.3, a current attic R-38 to R-60 or RSI 6.7 to 10.6.
Square feet or square metres. A floor over a crawl space, an unheated basement or a garage. Put zero for a slab on grade or a heated basement below, neither of which loses heat this way.
Left blank this is R-19, or RSI 3.35 in metric.
Cubic feet, or cubic metres if you switched above. Conditioned floor area times the ceiling height: 1,500 sq ft with 8 ft ceilings is 12,000 cubic feet, and 140 m2 with 2.4 m ceilings is 336 cubic metres.
Natural infiltration, and the same number in either system because it is a ratio. A tight new build is 0.2 to 0.35, an average house 0.5, a loose older house 0.7 to 1.0 or worse. This is the biggest guess on the form.
Counted at 250 BTU per hour of sensible heat and 200 of latent heat each, which is 73 W and 59 W, and is a person sitting still.
Average, not peak, and in watts either way because a watt is a watt. Everything electrical inside the envelope ends up as heat, at 3.412 BTU per hour per watt.
Grains of water per pound of dry air, or grams per kilogram in metric, and there are exactly 7,000 grains in a pound so 30 grains/lb is 4.29 g/kg. How much wetter the outside air is than the inside air: around 30, or 4.3 g/kg, in a humid summer climate and near zero in a dry one, which is why dry climates need far less latent capacity. Left blank this is 30, or 4.29 g/kg.
Applied to the sensible load only, since duct gain in a hot attic is almost all sensible heat.
Square feet or square metres. Used only to print the load per unit of floor area, so the answer can be sanity checked against the usual rules of thumb.
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