Formula Used
Temperature difference: ΔT = Indoor design temperature - Outdoor design temperature
Surface heat loss: Q = U × A × ΔT
Total envelope loss: Q envelope = Sum of all surface heat losses
Infiltration loss: Q air = 0.018 × ACH × Volume × ΔT
Thermal bridge loss: Q bridge = Q envelope × Bridge percentage
Duct loss: Q duct = Base heat loss × Duct loss percentage
Final design loss: Q final = Base loss + Duct loss + Safety margin
Output conversion: kW = Btu/h ÷ 3412.142, Tons = Btu/h ÷ 12000
How to Use This Calculator
Enter the indoor and outdoor design temperatures first. Add each surface area and its U value. Use total exterior wall, roof, floor, window, and door areas. Enter house volume and air changes per hour. Add duct loss, thermal bridge allowance, and safety margin. Press calculate to view the design heat loss.
Example Data Table
| Input |
Example Value |
Unit |
| Indoor temperature |
70 |
F |
| Outdoor design temperature |
5 |
F |
| Wall area and U value |
1800 and 0.065 |
ft², Btu/h-ft²-F |
| Roof area and U value |
1200 and 0.035 |
ft², Btu/h-ft²-F |
| House volume and ACH |
14400 and 0.50 |
ft³, ACH |
| Duct loss and safety margin |
8 and 10 |
% |
Whole House Heat Loss Guide
A whole house heat loss estimate shows how fast heat leaves a building. It helps compare insulation, air sealing, glazing, and heating capacity. The method is simple. Each surface loses heat through its area and U value. Air leakage also removes warm indoor air.
What The Calculator Measures
This calculator uses design indoor and outdoor temperatures. It finds the temperature difference first. Then it multiplies each assembly area by its U value and that difference. Walls, roof, floor, windows, and doors are counted separately. The tool also estimates infiltration from house volume and air changes per hour.
Why U Values Matter
A U value shows heat flow through one square foot for each degree of temperature difference. Lower U values mean better insulation. A modern insulated wall may have a low U value. Older windows often have a higher value. This is why small window areas can still create large losses.
Infiltration And Duct Loss
Air leakage can be a major load. The formula uses house volume and ACH. A tighter home has lower ACH. A leaky home needs more heat. Duct losses are added after envelope and infiltration losses. This helps when ducts pass through attics, garages, or crawl spaces.
Safety Margin And Equipment Size
The final load includes a safety margin. Use it carefully. A small margin helps cover uncertainty. A large margin can oversize the heating system. Oversized equipment may short cycle. It may waste fuel and reduce comfort. The result should guide planning, not replace a professional load calculation.
Using The Result
The peak heat loss is shown in Btu per hour. The calculator also converts it to kilowatts and heating tons. These values make equipment comparison easier. The annual estimate uses heating degree days. It gives a rough fuel need and cost. Actual use depends on weather, thermostat habits, solar gain, internal gains, and equipment control.
Good Input Practice
Measure exterior surfaces as accurately as possible. Use rated U values when available. If unsure, enter conservative values. Run several cases. Compare old windows with new windows. Compare poor sealing with better sealing. This turns the calculator into a design tool. It can show which improvement may reduce heat loss most.
FAQs
What is whole house heat loss?
It is the rate at which a house loses heat during cold conditions. It includes heat moving through surfaces and heat carried away by air leakage.
What unit does this calculator use?
The main result is shown in Btu per hour. It also shows kilowatts and heating tons for easier equipment comparison.
What is a U value?
A U value measures heat transfer through a building part. A lower U value means better insulation and lower heat loss.
Why is air leakage included?
Air leakage brings cold air inside and pushes warm air outside. This can create a large heating load in older or drafty homes.
Should I add a safety margin?
Yes, but keep it reasonable. A small margin covers uncertainty. A very large margin can oversize the heating system.
Does this replace a Manual J calculation?
No. It is a planning tool. A certified load calculation may be needed for final equipment sizing and code compliance.
How do heating degree days affect the result?
Heating degree days estimate seasonal heating demand. More heating degree days usually mean higher annual fuel use and cost.
Can I compare upgrades with this calculator?
Yes. Change U values, air changes, or duct losses. Then compare results to see which improvement reduces heat loss most.