Formula Used
Envelope conduction: Load = U × Area × Temperature Difference.
Cooling sensible outside air: Load = 1.08 × CFM × Cooling Temperature Difference.
Heating outside air: Load = 1.08 × CFM × Heating Temperature Difference.
Cooling latent outside air: Load = 0.68 × CFM × Grains Difference.
Window solar gain: Load = Window Area × SHGC × Solar Rate × Shade Factor.
Lighting heat: Load = Watts × 3.412.
Cooling tons: Tons = Total Cooling BTU/hr ÷ 12,000.
Final loads: Final Load = Base Load + Duct Allowance + Safety Allowance.
How to Use This Calculator
Enter the building area, ceiling height, envelope areas, and U values. Add design temperatures for summer and winter. Enter air leakage, ventilation, window solar data, internal gains, duct allowance, and safety factor. Press calculate. The result appears above the form and below the header section.
Use the output table to compare heating load, sensible cooling load, latent cooling load, total cooling capacity, and cooling tons. Adjust one input at a time to test insulation, window, shading, duct, or air sealing improvements.
Residential Load Planning Basics
A residential load calculation estimates how much heat a home gains or loses at design conditions. It supports heating and cooling equipment selection. A small error can affect comfort, humidity, noise, and energy use. This calculator follows a practical Manual J style approach. It separates conduction, solar gain, air leakage, ventilation, internal gains, and duct allowance.
Why Inputs Matter
Every building part changes the answer. Walls, ceilings, floors, doors, and windows transfer heat by area and U value. Lower U values usually mean better insulation. Windows also add solar heat through SHGC. Orientation, shade, and glass type can make two similar rooms perform very differently. Air leakage adds sensible load when outdoor air has a different temperature. It adds latent load when outside moisture enters the home.
Cooling Load View
Cooling load has two main parts. Sensible load changes air temperature. Latent load removes moisture. People, lights, appliances, windows, ducts, and infiltration all affect the total. The tool converts total cooling load into tons by dividing by 12,000. This is a sizing reference, not an automatic equipment choice. Real selection also needs airflow, static pressure, blower data, humidity targets, and manufacturer performance tables.
Heating Load View
Heating load focuses on heat loss. The winter indoor and outdoor design temperatures create a temperature difference. Higher differences increase wall, roof, floor, window, door, infiltration, and ventilation losses. Duct leakage or duct location can increase required capacity. Oversizing may cause short cycles. Undersizing may reduce comfort during design weather.
Useful Field Checks
Measure dimensions carefully. Use actual window areas, not rough guesses. Confirm insulation levels from drawings, labels, or inspection notes. Pick design temperatures from local guidance. Separate additions, basements, sunrooms, and bonus rooms when their exposure differs. Review each room before combining loads. Document each assumption because small entries can shift capacity, duct sizing, and comfort expectations across the house.
How To Use Results
Use the breakdown to find the largest load sources. Improve insulation, reduce leakage, add shading, or upgrade windows, then recalculate. Compare several scenarios before design decisions are fixed. For permits, rebates, or final equipment selection, use certified software and a qualified professional. This planning tool helps organize assumptions and spot weak areas early.
FAQs
Is this a complete certified Manual J report?
No. It is a planning calculator using common load concepts. A certified report needs approved software, local design data, room level inputs, duct details, and professional review.
What is a U value?
A U value shows heat transfer through a surface. Lower values reduce heating and cooling loads. Use product labels, energy reports, drawings, or reliable inspection data.
Why are sensible and latent loads separate?
Sensible load changes air temperature. Latent load removes moisture. Air conditioners must handle both. High latent load can cause poor humidity control even when temperature seems acceptable.
What does ACH mean?
ACH means air changes per hour. It estimates leakage through cracks and gaps. A tighter home has lower ACH and usually lower heating and cooling loads.
How do ducts affect the answer?
Ducts can lose heat or cooling through leakage and conduction. Losses are higher in attics, crawl spaces, and unconditioned areas. Use a realistic allowance.
Why does window shading matter?
Solar heat through glass can be large. Overhangs, screens, trees, blinds, and orientation change the cooling load. Adjust the shade factor for each scenario.
Can I size equipment from tons alone?
No. Tons are only one reference. Equipment selection also needs airflow, static pressure, humidity performance, capacity tables, climate data, and installer judgment.
Should I add a large safety factor?
Usually no. Large safety factors can oversize equipment. Oversizing may cause short cycling, noise, poor humidity control, and lower comfort. Use careful inputs instead.