Heat Loss / Gain Calculator
Estimate room heat loss in winter and heat gain in summer using areas insulation U or R values infiltration and internal loads. Get component breakdown and sizing with safety factor plus exportable results for reports.
Developed by: Nohman Habib
Example Data
| Input | Example Value |
|---|---|
| Indoor / Outdoor | 21°C / 0°C |
| Floor Area / Height | 20 m² / 2.8 m |
| Wall Area / U | 35 m² / 0.35 W/m²K |
| Roof Area / U | 20 m² / 0.25 W/m²K |
| Windows Area / U / SHGC | 6 m² / 1.8 W/m²K / 0.50 |
| Infiltration | 0.50 ACH |
| Internal Loads | 2 people, 100 W lighting, 200 W equipment |
Formula
- Conduction: Q = U × A × ΔT
- U from R: U = 1 / R
- Infiltration (ACH method): Q ≈ 0.33 × ACH × V × ΔT
- Solar (simplified): Q = Awin × SHGC × SolarFactor
- Internal gains: Q = (People × W/person) + Lighting + Equipment
- Safety factor: Qsized = Q × (1 + Safety%)
How to Use This Calculator
- Select metric or imperial units, then choose Heat Loss, Heat Gain, or both.
- Enter indoor and outdoor temperatures for your design condition.
- Enter floor area and ceiling height to estimate room volume.
- Enter areas and U-values (or R-values) for walls, roof, floor, windows, and doors.
- Set infiltration using ACH (recommended) or CFM if you know measured airflow.
- For summer, add SHGC, solar factor, and internal loads, then press Calculate.
- Use the CSV/PDF buttons to download your inputs and results.
FAQs
1) What is heat loss and heat gain?
Heat loss is the heating power needed to maintain indoor temperature during cold weather. Heat gain is the cooling load added by hot outdoor air, solar radiation, and internal sources like people and equipment.
2) Should I use U-value or R-value?
Use whichever you have. U-value measures how easily heat passes through a surface. R-value measures resistance. The calculator converts R to U automatically using U = 1/R.
3) What ACH value is reasonable?
As a quick guide, 0.2 ACH is tight construction, 0.5 ACH is average, and 1.0 ACH is leaky. If you have blower-door or ventilation data, use that for better accuracy.
4) What is the solar factor input?
Solar factor is a simplified estimate of solar intensity on your windows after orientation and shading. If you are unsure, keep the default and adjust it based on local conditions or shading devices.
5) Why include a safety factor?
Real buildings have uncertainties like thermal bridges, wind-driven infiltration, and varying insulation quality. A modest safety factor (5–15%) helps avoid undersizing heating or cooling equipment.
6) Is this calculator a full HVAC load calculation?
It is a practical estimate for rooms and small zones. For final HVAC design, engineers may use detailed standards that include humidity, solar timing, duct losses, and precise material assemblies.