Ventilation heat loss is estimated from airflow and temperature difference:
- Q (W) = (ρ · cp / 3600) · V̇ · ΔT
- V̇ (m³/h) = ACH · Volume(m³) when using ACH.
- Net loss accounts for runtime fraction and optional heat recovery.
Where ρ is air density (kg/m³), cp is specific heat (J/kg·K), V̇ is airflow (m³/h), and ΔT is the indoor–outdoor temperature difference (°C).
- Choose ACH or airflow, then enter the zone volume.
- Enter indoor and outdoor temperatures in your chosen unit.
- Add runtime fraction and recovery efficiency if applicable.
- Set season hours and prices to estimate energy and cost.
- Press Calculate, then export CSV or PDF if needed.
| Scenario | Method | Volume | Flow input | Indoor/Outdoor | Recovery | Instant loss (W) | Season energy (kWh) |
|---|---|---|---|---|---|---|---|
| Small office zone | ACH | 250 m³ | 1.2 ACH | 20°C / 5°C | 0% | ~1500 W | ~3240 kWh |
| Workshop with heat recovery | Airflow | 600 m³ | 600 L/s | 18°C / 2°C | 70% | ~2420 W | ~2614 kWh |
| Corridor ventilation | Airflow | 900 m³ | 1200 m³/h | 22°C / 10°C | 0% | ~4800 W | ~5184 kWh |
Examples are illustrative and assume standard air properties and full runtime.
Ventilation heat loss in load calculations
Ventilation losses represent the sensible heat required to warm incoming outdoor air or cool it in summer. In early-stage design, this calculator quantifies the impact of outside air rates, air-change assumptions, and temperature differences on peak loads and seasonal energy.
Input data that most affects the result
The largest drivers are airflow and ΔT. When ventilation is defined by ACH, verify the conditioned volume and avoid including unserved voids. When airflow is measured, confirm whether the value is supply, exhaust, or balanced. Use runtime fraction to reflect schedules and demand control strategies.
Document the ventilation basis used for compliance: outdoor air per person, per area, or process exhaust. Convert those requirements into steady design flow, then apply diversity only when justified. For multi-zone systems, calculate losses per zone and sum, because ΔT and schedules may differ across spaces significantly.
Accounting for heat recovery and controls
Heat recovery reduces net loss by transferring heat between exhaust and supply streams. Enter effective recovery efficiency based on commissioning data or manufacturer performance at the intended flow. For variable flow systems, align runtime fraction with realistic seasonal operating hours.
From watts to seasonal energy and cost
Instantaneous loss (W) supports equipment sizing checks, while seasonal energy (kWh) supports budgeting and comparative studies. For retrofit evaluation, keep all assumptions constant except the variable under study to isolate savings from recovery upgrades or ventilation rate changes.
Example data you can replicate
Example: Volume 250 m³, 1.2 ACH, indoor 20°C, outdoor 5°C, runtime 1.0, recovery 0%, season 12 h/day for 180 days. This yields roughly 1.5 kW instantaneous loss and about 3,240 kWh seasonal energy under standard air properties.
| Parameter | Value | Notes |
|---|---|---|
| Flow basis | ACH | V̇ = ACH × Volume |
| ΔT | 15°C | Indoor–outdoor difference |
| Season hours | 2160 h | 12 × 180 |
1) Should I use ACH or airflow?
Use ACH when ventilation is specified as air changes for a known zone volume. Use airflow when you have measured or designed supply/exhaust rates, especially for mechanical systems.
2) Why does the calculator use absolute temperature difference?
Heat transfer magnitude depends on the size of the indoor–outdoor difference. The direction changes between seasons, but the sensible load magnitude uses |ΔT|.
3) What does runtime fraction represent?
Runtime fraction scales the load to reflect intermittent operation, demand-controlled ventilation, or occupancy schedules. For example, 0.6 approximates systems running about 60% of the time.
4) How should I set heat recovery efficiency?
Use an effective value that matches expected flow and temperature conditions. If unsure, start with 50–70% for HRV/ERV systems, then refine using commissioning data or manufacturer performance curves.
5) Does this include latent (moisture) loads?
No. This tool estimates sensible heat loss only. If humidity control matters, add a separate latent load calculation using outdoor humidity ratio, indoor setpoint, and ventilation mass flow.
6) Why do my CFM results look different from m³/h?
CFM and m³/h are converted using fixed factors. Differences usually come from input rounding or selecting a different unit basis. Re-check the airflow unit and the zone volume if using ACH.
7) How can I validate the output?
Check that losses increase linearly with airflow and ΔT. Verify the coefficient (ρ·cp/3600), then confirm season hours and recovery settings match your design intent.