Advanced Calculator
Example Data Table
| Input | Example value | Unit |
|---|---|---|
| Indoor design temperature | 22 | °C |
| Outdoor design temperature | 35 | °C |
| External wall area | 180 | m² |
| Wall U-value | 0.45 | W/m²K |
| Air changes per hour | 0.70 | ACH |
| Safety factor | 15 | % |
Formula Used
Temperature difference: ΔT = |Outdoor temperature − Indoor temperature|
Envelope heat transfer: Q = U × A × ΔT
House volume: V = Floor area × Ceiling height
Infiltration heat load: Q = 0.33 × ACH × V × ΔT
Internal gains: Q = People heat + Lighting watts + Appliance watts
Solar gain: Q = Window area × Solar gain × Shading factor
Cooling load: Envelope + Infiltration + Internal gains + Solar gain
Heating load: Envelope + Infiltration − Internal gain credit
Design load: Raw load × (1 + Safety factor ÷ 100)
BTU per hour: Watts × 3.412142
Cooling tons: BTU per hour ÷ 12000
How to Use This Calculator
Choose auto, cooling, or heating mode. Enter the indoor and outdoor design temperatures. Add the measured area and U-value for walls, roof, windows, doors, and floor. Enter house floor area and ceiling height to estimate air volume. Add air changes per hour for leakage or ventilation. Include people, lights, appliances, solar gain, shading, and safety factor. Press the calculate button. Review the result above the form. Use CSV or PDF download for records.
House Heat Load Calculation Guide
Why Heat Load Matters
A house heat load calculation estimates how much heat enters or leaves a home during design weather. It helps select equipment capacity. It also helps compare insulation, windows, leakage, and solar effects. A rough guess can oversize equipment. Oversized units may cycle often. Undersized systems may fail during severe weather.
Main Heat Paths
Heat moves through walls, roofs, floors, doors, and windows. The calculator uses U-values to measure that flow. A lower U-value means better resistance. The temperature difference sets the driving force. Larger surfaces and larger temperature differences create larger heat transfer. Windows often add high loads because glass has weaker insulation.
Air Leakage and Ventilation
Outdoor air also changes the load. Air leakage is entered as air changes per hour. The calculator multiplies this value by house volume. It then applies a standard air heat factor. Drafty houses can lose or gain a large amount of heat through infiltration. Sealing gaps can reduce this load.
Internal and Solar Gains
People, lights, and appliances release heat indoors. These gains increase cooling demand. During heating, part of those gains can reduce the required heat. The calculator lets you credit a selected percentage. Solar gain is based on window area, solar intensity, and shading. Good shading can lower summer load.
Design Margin
A safety factor is added after the raw load is calculated. This margin covers input uncertainty and unusual conditions. It should not be excessive. A balanced margin improves comfort without creating serious oversizing. The final result is shown in watts, kilowatts, BTU per hour, and cooling tons.
Better Input Practice
Measure exposed areas carefully. Use realistic U-values from building documents when available. Separate old windows from improved windows if needed. Use local design temperatures instead of average weather. For important projects, compare this result with a detailed room by room calculation. This tool is useful for planning, checking, and early equipment discussions.
FAQs
What is house heat load?
House heat load is the heating or cooling capacity needed to hold indoor temperature during design weather. It includes envelope transfer, air leakage, internal gains, solar gain, and a safety margin.
Should I use heating or cooling mode?
Use heating mode when outdoor design temperature is lower than indoor temperature. Use cooling mode when outdoor design temperature is higher. Auto mode chooses based on those two temperatures.
What is a U-value?
A U-value measures heat flow through a building part. It uses watts per square meter per kelvin. Lower values mean better insulation and lower heat transfer.
Why is ACH important?
ACH means air changes per hour. It estimates leakage or ventilation. Higher ACH brings more outdoor air inside, so heating or cooling demand increases.
Does this calculator size HVAC equipment exactly?
It gives a planning estimate. Final equipment selection should also consider room loads, ducts, humidity, local codes, climate data, and professional design methods.
Why include solar gain?
Sunlight through windows can add major cooling load. Solar gain depends on window area, orientation, glass type, shading, season, and local sun exposure.
What safety factor should I use?
A common planning range is 10% to 20%. Large safety factors can oversize equipment. Use better measurements instead of adding excessive margin.
Can this calculator handle old houses?
Yes. Use higher ACH values, realistic U-values, and measured areas. Older homes often have more leakage and weaker insulation, so careful inputs matter.