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VAV systems explained: variable air volume for offices

Updated 23 October 2026 · SEO Dons Editorial

How does a VAV system work?

A variable air volume system sends air from a central air handling unit to terminal boxes in each zone, and each box changes how much air enters its room. In the simple system CIBSE Journal describes, every terminal receives primary air from the central unit at the same temperature, so cooling is adjusted room by room through airflow.

  • Supply fan. Its flow is modulated to hold the supply air static pressure high enough to serve every terminal, while keeping duct static pressure as low as possible. The exhaust fan is varied to suit building pressurisation and the supply flow.
  • Terminal box. It contains a damper that modulates so the cool air entering the room offsets the cooling load.
  • Reheat. Terminal units often integrate reheaters to meet local heating loads, for example to offset perimeter winter heat losses.
  • Control. A pressure-independent box uses two cascading loops: the room sensor sets the airflow required, and a second loop holds that flow whatever the primary air pressure.
  • Supply temperature. Traditionally fixed, often about 13 degrees C, because the original job of VAV was cooling core areas. Some systems now reset it seasonally.

Why is VAV common in larger offices?

Because the zones in an office do not peak together. CIBSE Journal explains that a constant air volume multizone system must supply the cumulative total of every zone's peak airflow continuously, whereas VAV supplies only the concurrent zone loads. Fan energy is the second reason: it says the air power needed, and so the fan power, reduces by the cube of any reduction in volume flow.

That cube relationship holds when duct pressure drop follows the square of the flow, which CIBSE Journal gives as the rule for a duct system with fixed geometry. The table is simple arithmetic on that ideal case, not a measured result.

Airflow, share of designIdeal fan power, share of design
100%100%
80%51%
60%22%
50%13%

Real systems fall short of the ideal. Minimum box flows, the static pressure the fan holds, fan efficiency at low speed and filter loading all eat into it. An engineer quoted in a CIBSE Journal article on a housing scheme says that, on VAV systems, commissioning engineers often ramp up the static pressure to about 400 pascals to make the controls easier to set. A fan holding that pressure at part load uses more energy than one allowed to ease off.

How does VAV compare with constant volume, fan coils and VRF?

Each moves heat to the room differently, and the differences are in energy, zoning and space. The table keeps to what CIBSE Journal's modules say. Fan coil detail is on the fan coil units page, and refrigerant systems are covered in split, multi-split and VRF.

SystemPublished advantagePublished trade-off
VAVLower fan power and reheat than constant volumeNo zonal humidity control, outdoor air to each zone, air movement at maximum turn-down
Fan coils with ducted fresh airSmaller and fewer air handling units, smaller ducts, chilled water at a higher temperatureLess airflow, so less scope to cool with outdoor air. All-air may suit large ventilation loads such as conference rooms
VRFFor 5 kW, a pair of pipes under 15 mm against a duct of about 350 mm. Heat recovery models move heat between zonesVentilation air is often supplied by a separate dedicated outdoor air system

The pipes-against-ducts point has an energy side. In a deliberately simplified CIBSE Journal example, a 10 kW cooling load served through a 25 metre duct or a 50 metre pipe run, the cost to distribute waterborne energy was less than 7% of using air. That excludes fittings, controls and terminals, so it is a pointer, not a verdict. CIBSE Journal adds that, for buildings with several controlled rooms, hybrid systems can bring reduced distribution space, improved controllability and reduced operating costs.

What goes wrong with VAV systems?

  • Poor turndown. CIBSE Journal says boxes have typically been selected to turn down to 30% of maximum flow, and that cold air dumping and poor mixing of reheated air at low flow are associated with a poorly conceived or operated system. A separate module notes the minimum airflow is often set by the lowest flow the control device can achieve, and the turn-down ratio can leave it high with larger dampers.
  • Zones that miss out. The VAV module also lists the lack of zonal humidity control, and ensuring enough outdoor air for individual zones, as challenges.
  • Complex commissioning. The module says the complexity of multizone VAV control and monitoring makes commissioning and troubleshooting increasingly complicated, and describes fault detection modules that work with the building management system to identify faults in terminal boxes.
  • Noisy boxes and drifting controls. None of the sources used here gives figures, so ask the supplier for sound data at the design and minimum flows, and trend each box's measured flow against its setpoint.

What do the rules say about the unit behind a VAV system?

The ecodesign regulation for ventilation units sets the floor for the air handling unit. Legislation.gov.uk publishes the UK version, and Article 1 lists the units it excludes. For units within scope, since 1 January 2018, every ventilation unit except a dual use unit must have a multi-speed or variable speed drive, which is what lets the supply fan follow demand. A bidirectional unit, meaning one with both supply and exhaust fans, must have heat recovery with a thermal by-pass. Minimum thermal efficiency is 73% for most heat recovery systems and 68% for run-around systems. The regulation also caps internal specific fan power.

For buildings, GOV.UK says its Approved Document L page holds the current guidance on the energy efficiency requirements of Part L. The 2026 edition applies to buildings subject to the 2026 standards, and earlier versions continue to apply to buildings subject to previous standards. That guidance sits in PDFs this page does not reproduce, so read the volume that applies to the project for its fan power limits.

How can you check an existing VAV system?

  1. Trend the duct static pressure and see whether the setpoint ever moves.
  2. Trend fan speed. A fan sitting near full speed all day has lost the benefit of VAV.
  3. Compare each box's minimum flow with what its zone needs for ventilation.
  4. Look for reheat running in zones above setpoint.
  5. Check filter pressure drop, since a loaded filter raises fan energy. The filter grades guide covers this, and the AHU maintenance checklist covers the rest of the unit.

Controls detail belongs on the building management systems page. Readers wanting a survey of an existing VAV system can send an enquiry through the site's form, and the office air conditioning page covers the wider choice.

Sources

Common questions

What is a VAV box?

It is the terminal unit in each zone of a VAV system. CIBSE Journal describes it as containing a damper that modulates so the cool air entering the space offsets the cooling load, with an integrated pressure sensor measuring airflow and often a reheater.

Why does a cooling system need reheat?

A box is typically selected to turn down to 30% of maximum flow, not to zero, to maintain air distribution. CIBSE Journal says terminal units often integrate reheaters to meet local heating loads such as perimeter winter heat loss, and that VAV still reduces reheat compared with constant volume.

Is VAV only for big offices?

No. CIBSE Journal reports growing use of the VAV principle in small single-zone applications such as cafes, restaurants, teaching spaces, offices and shops, where the zone sensor varies both capacity and fan speed.

Does VAV save energy over constant volume?

It can, mainly in fan energy, because fan power falls by the cube of the airflow in the ideal case. How much is saved in practice depends on the static pressure strategy, the minimum box flows and how well the system was commissioned.

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