Why demand-controlled ventilation?
A ventilation unit that runs at the same speed around the clock is doing one of two things wrong: either it is moving more air than the building needs and paying for it in electricity and noise, or it is sized too low and never quite catches up when the rooms actually fill. Demand-controlled ventilation removes the choice between the two by letting the building itself set the pace.
What demand-controlled actually means
A conventional system is balanced once, at handover, to a calculated airflow. That figure is sized for the worst case — every room occupied, full load — and the system then runs that way permanently. Demand control does not change the sizing; it lets the unit move freely below it. Full capacity is still there, it is simply used only when it is needed.
In practice that means the unit has several steps, or continuously variable fan control, and that something tells it when to change step. That something is sensors.
What the sensors read
The sensors sit in the extract air, where the air leaving the rooms passes. Each responds to a different kind of load.
| Sensor | What it reads | Typical response |
|---|---|---|
| CO₂ | Number of people in the room, 400–1200 ppm | Step up within seconds |
| Relative humidity | Showers, cooking, drying laundry | Boost until RH falls back |
| Temperature | Supply/extract differential | Post-heater and bypass control |
| Timer or switch | Known daily peaks | Timed forced boost |
CO₂ is the most useful of them in a home or an office, because it correlates directly with the number of people present. Humidity matters most in bathrooms and utility rooms, where the load arrives in short, sharp bursts.
What it costs to run — and what it saves
Fan power does not follow airflow linearly. Halve the airflow and the power demand falls to roughly an eighth. That is why running at reduced steps for most of the day is so effective:
- Lower energy use. A home that is empty for eight hours a day can sit at minimum for that entire period without anyone noticing.
- Lower noise. A fan at 40% of nominal speed is effectively inaudible in a living space.
- Longer filter life. Less air through the filter means less dust captured, and therefore longer intervals between changes.
- Less winter dryness. Cold outdoor air is dry air. Moving less of it while the house is empty keeps humidity at a more comfortable level.
Where the difference is biggest
The gain is proportional to how much occupancy varies. Some typical cases:
- Housing. Empty during the day, full in the evening, humidity peaks morning and night.
- Offices. Empty sixteen hours a day and all weekend — more than two thirds of the time.
- Meeting and teaching rooms. Zero to full load in minutes, several times a day.
- Hotels and rentals. Occupancy that varies room by room and week by week.
The converse also holds: a space with constant load around the clock has little to gain from demand control, and there a simpler system is the right choice.
What to check before you specify it
Demand control is not only a choice of unit. Four things decide whether it actually works in the building:
- Sensor placement. A CO₂ sensor in a corridor measures the corridor, not the rooms.
- Zoning. Without zones the whole building follows the most loaded room, and the gain disappears.
- Documented performance curves. You need airflow, pressure, SFP and sound at every step — not only at the nominal point.
- Commissioning. The balance between supply and extract has to hold at all steps, not only at the one that was balanced.
A system that never changes speed is sized for a moment that rarely happens, and pays for that moment all year.
Ensy units are built for this kind of control from the ground up, and every unit ships with measured curves rather than nominal values — so the figures you design with are the figures the system actually delivers at each step.