Heating Load Calculator
Calculate heating load accurately for your build. Get material quantities, waste allowances, and project cost breakdowns.
Formula
Q = (U x A x Delta-T) + (1.08 x CFM x Delta-T)
Total heating load equals the sum of envelope heat loss and infiltration heat loss. Envelope loss is the overall U-value times total envelope area times temperature difference. Infiltration loss uses the sensible heat formula: 1.08 times the infiltration airflow in CFM times the temperature difference. The result in BTU/hr tells you the furnace or boiler capacity needed.
Worked Examples
Example 1: Average Insulated Home in Cold Climate
Problem: A 2,000 sq ft home with 8 ft ceilings, average insulation, indoor temp 70F, outdoor design temp 10F.
Solution: Envelope area = 4 x 44.7 x 8 + 2000 = 3,430 sq ft\nEnvelope loss = 0.30 x 3,430 x 60 = 61,740 BTU/hr\nInfiltration = 1.08 x (16000 x 0.5/60) x 60 = 8,640 BTU/hr\nTotal = 70,380 BTU/hr
Result: Total heating load is approximately 70,380 BTU/hr; recommended furnace size is 80,000 BTU/hr
Example 2: Well-Insulated Home in Moderate Climate
Problem: A 1,500 sq ft home with 9 ft ceilings, good insulation, indoor temp 68F, outdoor design temp 25F.
Solution: Envelope area = 4 x 38.7 x 9 + 1500 = 2,893 sq ft\nEnvelope loss = 0.18 x 2,893 x 43 = 22,390 BTU/hr\nInfiltration = 1.08 x (13500 x 0.35/60) x 43 = 3,660 BTU/hr\nTotal = 26,050 BTU/hr
Result: Total heating load is approximately 26,050 BTU/hr; recommended furnace size is 40,000 BTU/hr
Frequently Asked Questions
What is a heating load calculation?
A heating load calculation determines how much heat energy (in BTU per hour) a building loses during the coldest expected conditions. This tells you the capacity needed for the furnace, boiler, or heat pump to maintain a comfortable indoor temperature. The calculation considers the building envelope (walls, ceiling, floor), insulation levels, air infiltration, and the temperature difference between indoors and outdoors. Professional Manual J calculations are more detailed but this method gives a reliable estimate.
How does insulation level affect heating load?
Insulation is the single largest factor in heating load. An uninsulated home can lose two to five times more heat than a well-insulated one. Upgrading from R-11 to R-19 wall insulation can reduce wall heat loss by roughly 40%. Adding attic insulation from R-19 to R-38 cuts ceiling losses in half. Better insulation means a smaller furnace is needed, lower utility bills, and improved comfort. The upfront cost of insulation often pays for itself within a few heating seasons.
What is air infiltration and how does it affect heating?
Air infiltration is uncontrolled air leakage through cracks, gaps, and openings in the building envelope. Cold outdoor air enters and warm indoor air escapes, requiring the heating system to constantly warm replacement air. In older homes, infiltration can account for 25-40% of total heat loss. Sealing air leaks around windows, doors, electrical outlets, and plumbing penetrations is one of the most cost-effective ways to reduce heating load.
How do I calculate the load-bearing capacity of a beam?
Beam capacity depends on material, cross-section dimensions, span length, and support conditions. For a simple rectangular wood beam, bending strength = (F_b x b x d^2) / 6, where F_b is allowable stress, b is width, and d is depth. Always consult a structural engineer for critical applications.
Is my data stored or sent to a server?
No. All calculations run entirely in your browser using JavaScript. No data you enter is ever transmitted to any server or stored anywhere. Your inputs remain completely private.
How accurate are the results from Heating Load Calculator?
All calculations use established mathematical formulas and are performed with high-precision arithmetic. Results are accurate to the precision shown. For critical decisions in finance, medicine, or engineering, always verify results with a qualified professional.