Why we ventilate and how a ventilation system is designed
People breathe out carbon dioxide (CO₂), heat and water vapour. Cooking and bathing add smoke, smell and steam. If this air stays in the room, the air feels heavy and people feel sleepy. Ventilation takes the old air out and brings new air in.
Natural and mechanical ventilation
- Natural: wind and warm air rising push air through windows, vents and shafts. It costs nothing but it is not steady.
- Mechanical: fans move the air through ducts. It works in any weather and you can control it.
Three fan layouts
- Extract only: a fan pulls air out; fresh air leaks in. Good for toilets and kitchens.
- Supply only: a fan pushes fresh air in; the room is slightly pressurised.
- Balanced: one supply fan and one extract fan. Needed with heat recovery.
How much air? Air changes per hour
Designers pick a number of air changes per hour (ACH) for each room type. Then flow Q = ACH × room volume. For example, a room of 72 m³ at 3 ACH needs 216 m³/h. Then they size the ducts so the air speed is not too high (about 3 to 5 m/s in main ducts) because fast air is noisy.
Energy-saving ventilation
Fresh air from outside is cold in winter and hot in summer. Heating or cooling it costs energy. Good designs waste less.
- Heat-recovery ventilator (HRV): outgoing and incoming air pass through a heat exchanger (they never mix). The incoming air picks up 60 to 90 percent of the heat of the outgoing air.
- Demand control: a CO₂ sensor speeds the fan up only when many people are in the room.
- Efficient fans: fan power grows very fast with speed (about the cube), so a slightly slower fan saves a lot.
- Right size, no leaks: tight ducts and good filters keep the system working with less power.
Heat-recovery formula: supply air temperature = outdoor + efficiency × (indoor − outdoor).
Smoke-exhaust systems
In a fire, most people are hurt by smoke, not flames. Hot smoke is lighter than air, so it rises to the ceiling and then spreads down as a thick layer. A smoke-exhaust system removes this smoke so that the air near the floor stays clear and people can walk out and firefighters can enter.
Parts
- Smoke vent or damper in the ceiling or high on the wall. It opens by itself when the fire alarm rings.
- Natural smoke exhaust: the opening lets hot smoke rise out. Simple, but it needs an opening to the outside (often about 1/50 of the floor area; local codes decide).
- Mechanical smoke exhaust: a strong fan, rated for high temperature, pulls the smoke through ducts.
- Make-up air: fresh air must come in low down, or the smoke cannot go out.
- Smoke barriers split a large hall into smoke zones.
Plan: keep the clear layer at least about 2 m above the floor for the time people need to leave. Rules differ by country, so always follow the local fire code.
Key formulas and definitions
- Q = ACH × V (flow in m³/h = air changes per hour × room volume in m³)
- ACH = Q ÷ V
- Q = A × v (duct flow = area × speed)
- T supply = T out + η × (T in − T out) (heat recovery)
- Fan power ∝ speed³ (about)
Worked examples
1. A bathroom is 2 m × 2 m × 2.5 m. It needs 8 air changes per hour. Find the fan flow.
Volume V = 2 × 2 × 2.5 = 10 m³. Q = ACH × V = 8 × 10 = 80 m³/h. That is about 22 L/s.
2. A classroom of 30 students needs 8 L/s per person. Room volume is 200 m³. Find Q in m³/h and the ACH.
Q = 30 × 8 = 240 L/s. In m³/h: 240 × 3.6 = 864 m³/h. ACH = 864 ÷ 200 = 4.3 per hour.
3. Outdoor air is 5 °C and indoor air is 21 °C. A heat-recovery unit has 70 percent efficiency. What is the temperature of the fresh air after the unit?
T = 5 + 0.7 × (21 − 5) = 5 + 11.2 = 16.2 °C. The room needs far less heating.
4. A duct is 0.4 m × 0.4 m and carries 0.48 m³/s. Find the air speed. Is it too high for a quiet room?
Area = 0.16 m². v = Q ÷ A = 0.48 ÷ 0.16 = 3 m/s. This is fine for a main duct, and a little high for a very quiet room.
Common mistakes
- Putting the supply and the extract close together. Fresh air then goes straight out and the rest of the room stays stale.
- Putting a smoke vent low on a wall. Smoke rises, so the vent must be high.
- Forgetting make-up air. With no way in, a fan cannot pull air out well.
- Mixing units: 1 m³/s = 3600 m³/h = 1000 L/s.