Use air changes only to convert a defined building volume into a first airflow; final supply and extract duties must follow local requirements and commissioning measurements.
1. Define the connected air volume
Measure the floor area and average clear height of spaces connected by the planned air paths. Exclude closed garages, unconditioned voids and other zones that belong to a different system.
For sloping ceilings or mixed-height rooms, calculate each volume and add them, then divide total volume by total floor area to obtain average height. This weights height by floor area, not room volume. For example, 20 m² × 2 m + 80 m² × 3 m = 280 m³; over 100 m² the average is 2.8 m, not the unweighted 2.5 m. For a sloping ceiling, use the actual geometry rather than applying peak height to the entire floor.
2. Establish the requirement before selecting equipment
Applicable standards may combine whole-building outdoor air with room-specific supply or extract duties. Occupancy, floor area, bedrooms, cooking and bathing can all affect the governing value.
An air-change rate is therefore an input to review, not a universal recommendation. Record its source and compare the resulting whole-building flow with every room requirement.
3. Translate total flow into real air paths
Plan where clean outdoor air enters, where transfer occurs and where moisture or pollutants are extracted. The sum alone does not prove that bedrooms receive air or bathrooms exhaust enough.
Duct pressure, filters, terminals and balancing change actual flow. Select equipment from its duty point and provide measuring points for commissioning.
4. Check heat and fan energy together
Fan electricity is based on input power and operating time. Ventilation heating load depends on outdoor airflow and temperature difference; heat recovery can reduce sensible heat loss but adds fans, controls and maintenance.
Compare both calculators at the same operating point. Recheck after filter changes and seasonal mode changes because airflow and power can move in different directions.
5. Commission and keep a record
Measure supply and extract at terminals, confirm pressure balance and record fan setting, filter condition and measured power. A control percentage or catalogue maximum is not a commissioning result.
Review combustion safety, noise, condensate drainage, frost protection and maintenance access before normal operation.
Ventilation quantities to keep separate
| Quantity | Purpose | How to verify |
|---|---|---|
| Building volume | Converts air changes to flow | Measured geometry |
| Required airflow | Design target | Applicable rule and room schedule |
| Delivered airflow | Actual outdoor air | Terminal measurements |
| Fan input power | Electricity use | Rated duty point or meter |
Worked example
A 120 m² home
Average height is 2.6 m and the reviewed whole-building rate is 0.5 air changes per hour.
Volume = 120 × 2.6 = 312 m³ Airflow = 312 × 0.5 = 156 m³/h Flow = 156 / 3.6 = 43.33 L/s
Compare 156 m³/h with the room schedule, then select and commission the system at the required duct resistance.
What to have ready
- Measured conditioned volume
- Source for the chosen airflow requirement
- Room-by-room supply and extract schedule
- Fan duty point and input power
- Commissioning and maintenance record
Common mistakes
- Treating one air-change value as a universal rule.
- Ignoring rooms that need specific extract flow.
- Using maximum catalogue airflow as delivered airflow.
- Counting heat recovery while ignoring fan electricity.
Tools for these measurements
Prepare the measurements with these instrument guides.
Measure air speed across a grille and turn a careful area average into an approximate ventilation flow.
How to use it
Compare two pressure points to screen filters, ducts and room pressure without confusing pressure with airflow.
How to use it
Use carbon dioxide trends to understand occupied-room ventilation without confusing CO₂, carbon monoxide and total air quality.
How to use itUse and limitations. Sources include US public agencies. Principles are general; regulations and design values must be checked where you live.Worked examples use stated hypothetical inputs.