Preliminary HVAC Heating & Cooling Load Estimator
Auditable preliminary block load
Enter envelope, design weather, air exchange and internal gains
Result
Calculation summary
Enter values to see the result
Heating and cooling balance
Sensible and latent block load
How to use this calculator
- 1Choose SI or imperial thermal-property inputs and enter conditioned floor area, ceiling height and explicit envelope component areas.
- 2Enter project-specific U-values, window SHGC, shading and design solar irradiance instead of selecting a generic building age preset.
- 3Enter local indoor and outdoor heating/cooling design temperatures, relative humidity, air changes, occupants and internal gains.
- 4Use duct and sizing allowances only when justified, then treat the result as a preliminary block-load comparison rather than equipment selection.
Formula
Transmission = Σ(U × area × ΔT); infiltration sensible = mass airflow × cp × ΔT; latent = mass airflow × hfg × humidity-ratio difference
Cooling also includes entered solar and internal sensible/latent gains. User-entered duct and sizing allowances are applied multiplicatively.
Calculation steps
- Convert all areas, heights, temperatures, U-values and solar inputs to SI units.
- Sum U × area for walls, ceiling, floor, windows and doors, then apply heating and cooling temperature differences.
- Convert air changes per hour and building volume to airflow and calculate sensible infiltration.
- Estimate cooling latent infiltration from indoor and outdoor humidity ratios at standard atmospheric pressure.
- Add window solar gain, occupants, lighting and appliance loads.
- Apply explicit allowances and report heating, sensible cooling, latent cooling, total cooling, SHR, kW, Btu/h and cooling tons.
Worked example
The default 150 m² example uses explicit envelope U-values, 0.5 ACH, four occupants and entered design conditions. The result separates transmission, outside-air, solar and internal loads so the assumptions can be audited instead of hiding one BTU-per-area rule.
Assumptions
- This is not ACCA-approved software and does not perform ANSI/ACCA Manual J. It is an educational preliminary block-load estimate only.
- Envelope areas are entered as net component areas and are not inferred from floor area. U-values and SHGC must match the actual assemblies.
- All surfaces use one outdoor design temperature; ground coupling, adjacent zones, thermal mass, exposure diversity and hourly solar geometry are simplified.
- Air changes represent combined infiltration and entered outdoor-air exchange. Wind, stack effect, mechanical ventilation schedules and heat recovery are not modeled separately.
- Humidity ratio uses entered dry-bulb temperature and relative humidity at standard atmospheric pressure.
- Duct location/leakage, room-by-room distribution, blower heat, moisture migration, dehumidification targets, equipment performance, safety factors and code documentation require professional procedures.
Sources
Frequently asked questions
Is this a Manual J calculator?
No. It is not ACCA-approved and cannot produce a compliant Manual J report.
Can I select equipment from the displayed tons?
No. Equipment selection requires an approved load procedure, manufacturer performance data and the applicable equipment-selection standard.
Why enter component areas and U-values?
Actual envelope geometry and thermal properties are more transparent than a single BTU-per-square-foot shortcut.
What is sensible heat ratio?
It is sensible cooling divided by total sensible plus latent cooling under the entered design assumptions.
Are internal gains credited against heating?
No. This simplified estimator does not reduce the heating load for occupants, lights or appliances.
Why can humidity change cooling load?
Humid outdoor air adds latent moisture-removal load when its humidity ratio exceeds the indoor design condition.