Two air handling units can look identical and yet behave completely differently, because of how they’re set up to deliver air. The two approaches — constant volume and variable air volume (VAV) — have a big effect on comfort and, especially, on energy. Here’s the difference, and why it matters.
Constant volume
A constant volume system does what it says: it delivers a fixed amount of air, all the time, regardless of whether the building actually needs it. It’s simple and reliable, and for spaces with steady, predictable demand it works perfectly well.
The drawback is energy. If the building is half-empty, or the weather’s mild, a constant volume unit is still moving and conditioning full airflow — paying to treat air nobody needs.
Variable air volume (VAV)
A VAV system varies the volume of air it delivers to match actual demand, zone by zone. When a space needs less heating, cooling or fresh air, the system reduces the airflow to it; when it needs more, it ramps up. Because the fans and conditioning scale with demand, VAV systems can save a significant amount of energy in buildings with variable occupancy or load.
The catch is that VAV relies on good controls. The air handler, the VAV terminals and the building management system all have to work together intelligently — which is why control strategy is as important as the hardware. (Our piece on AHU controls and building management goes deeper on this.)
Which suits your building?
Steady, uniform demand can be served well and simply by constant volume. Variable demand — offices with changing occupancy, mixed-use buildings, anywhere load swings through the day — is where VAV earns its keep through energy savings.
Few buildings have demand as variable as an airport, and on our air handling work at Heathrow, matching air delivery to constantly shifting load was central to keeping both comfort and energy under control. The same principle scales down to any building whose use changes through the day.
If your building’s occupancy varies and you suspect you’re paying to condition air you don’t need, it may be worth looking at whether a VAV approach or a controls upgrade would pay for itself. Get in touch and we’ll assess it.
