Heat Loss Calculator

Room heat loss has two parts. Envelope loss: Q = U x A x dT, where U is the assembly U-factor (BTU/hr-ft2-F), A is the surface area in square feet, and dT is indoor minus outdoor design temperature in F. Ventilation loss: Q = 0.018 x ACH x volume x dT. Example: a 12x15x8 ft bedroom (180 sq ft, 1,440 ft3) with 2x4 R-13 walls (382 ft2 net, 2,062 BTU/hr), double-pane windows (30 ft2, 900 BTU/hr), a solid wood door (20 ft2, 480 BTU/hr), an R-30 ceiling (180 ft2, 396 BTU/hr) and an R-13 floor (180 ft2, 691 BTU/hr) at 0.5 ACH, 70F indoor and 10F outdoor design (dT = 60F): envelope loss is 4,529 BTU/hr, ventilation adds 778 BTU/hr, for a total of 5,307 BTU/hr - about 29.5 BTU/hr per sq ft, or 1.56 kW.

A widely published rule of thumb (PickHVAC furnace sizing guide) is 30 to 60 BTU/hr per square foot depending on climate zone: Zone 1 = 30, Zone 2 = 35, Zone 3 = 40, Zone 4 = 45, Zone 5 = 50, Zone 6 = 55, Zone 7 = 60 BTU/hr per sq ft. For example, a 2,000 sq ft home in Zone 5 needs roughly 2,000 x 50 = 100,000 BTU/hr of heating output. Use it only as a sanity check against a room-by-room calculation - PickHVAC itself recommends going slightly bigger rather than slightly too small, and a professional Manual J calculation for final equipment selection.

Use Q = U x A x dT. The U-factor is 1 divided by the wall's total R-value, but for framed walls use the parallel-path method because wood studs (about 25% of the wall area at 16 in. on center, per ASHRAE 90.1 Appendix A) conduct more heat than insulation. A 2x4 wall with R-13 fiberglass has a series R of 14.92 (U = 0.067), but the parallel-path effective value is U = 0.090 (R-11.11). So 100 sq ft of that wall at a 60F temperature difference loses 0.090 x 100 x 60 = 540 BTU/hr. The same wall with an empty cavity (U = 0.297) loses 1,779 BTU/hr - insulation cuts wall loss by about 70%.

Window heat loss is Q = U x A x dT, with U the whole-window U-factor. Take a 15 sq ft window at a 60F temperature difference: a single-pane window (U = 1.0) loses 1.0 x 15 x 60 = 900 BTU/hr, while a double-pane low-e argon window (U = 0.28) loses 0.28 x 15 x 60 = 252 BTU/hr - 72% less. Typical U-factors: single-pane 1.0, double-pane clear 0.50, double-pane low-e 0.40, double-pane low-e with argon 0.28, triple-pane low-e 0.18. Always check the NFRC label on the actual window, since frame type shifts these values.

Sensible ventilation heat loss is Q = 1.08 x CFM x dT (BTU/hr), where 1.08 combines air density, specific heat and the minutes-per-hour conversion at standard conditions (ASHRAE). With air changes per hour it becomes Q = 0.018 x ACH x volume(ft3) x dT. Example: the 1,440 ft3 bedroom above at 0.5 ACH and 60F dT loses 0.018 x 0.5 x 1,440 x 60 = 777.6 BTU/hr, about 778 BTU/hr. For a ventilation target, ASHRAE 62.2 requires Qtot = 0.03 x floor area + 7.5 x (bedrooms + 1) CFM - for a 1,800 sq ft, 3-bedroom home that is 0.03 x 1,800 + 7.5 x 4 = 84 CFM continuous.

Thermal bridging is heat bypassing insulation through more conductive paths - mainly wood or steel framing. Wood studs occupy about 25% of a wall's area at 16 in. on center (ASHRAE 90.1 Appendix A / CA Title 24 JA4). A 2x4 R-13 wall ignoring framing is R-14.92 (U = 0.067); with the parallel-path framing correction it is effectively R-11.11 (U = 0.090) - framing adds about 34% more heat loss than the insulation-only math suggests. Steel framing is far worse and needs the ASHRAE zone method; this calculator models wood framing only and will underestimate steel-framed assemblies.

Multiply the design heat loss by a 1.15 sizing margin, then divide by the heater's efficiency (AFUE as a decimal) to get the required input rating, and pick the next standard size up. Example: a house losing 48,000 BTU/hr needs 48,000 x 1.15 = 55,200 BTU/hr of output; with a 92% AFUE furnace the required input is 55,200 / 0.92 = 60,000 BTU/hr, so you would select a 60,000 BTU/hr furnace (residential furnaces commonly run 40,000 to 140,000 BTU/hr). Electric resistance heat is 100% efficient at the point of use, so input equals output.

Annual cost = (heat loss in BTU/hr x heating hours per year / fuel energy content / efficiency) x fuel price. Fuel energy contents (EIA): natural gas 100,000 BTU/therm, heating oil 138,500 BTU/gal, propane 91,333 BTU/gal, electricity 3,412 BTU/kWh. Example: 48,000 BTU/hr x 1,500 hours = 72,000,000 BTU per season. On natural gas at $1.20/therm and 92% efficiency that is 782.6 therms = $939.13; on electric resistance at $0.17/kWh it is 21,102 kWh = $3,587.34. Adjust the heating hours to your climate - the calculator leaves it as an input for that reason.

Slab edges lose heat to the ground, calculated as Q = F x P x dT, where F is the slab F-factor (BTU/hr-ft-F, ASHRAE 90.1 Table A6.3), P is the exposed slab perimeter in feet, and dT is the indoor-to-outdoor temperature difference. Example: 120 ft of exposed perimeter at 60F dT. Uninsulated (F = 0.73): 0.73 x 120 x 60 = 5,256 BTU/hr. With R-10 vertical edge insulation 2 ft deep (F = 0.54): 0.54 x 120 x 60 = 3,888 BTU/hr - about 26% less. Deeper or full-under-slab insulation (F = 0.48 down to 0.36) cuts it further.

No. This is a simplified steady-state estimate: it sums U x A x dT over your envelope, adds ventilation at a single design temperature, and models wood framing with the parallel-path method. A professional ACCA Manual J accounts for room-by-room design temperatures, duct losses, internal gains, solar gain, air leakage testing and more. Use this tool for planning, comparing insulation options and sanity-checking quotes - not as the final word before buying equipment. Estimates only; always confirm sizing with a qualified HVAC professional.