Adding HVAC Zones to Existing Commercial Buildings

What You Need to Know

Most commercial buildings run one thermostat for a whole floor. The boardroom freezes while the open-plan area sweats. Zoning fixes this by giving each area its own temperature control.

You do not have to replace the whole air conditioning system to do it. Three retrofit paths exist: motorised zone dampers, VAV box conversion, or a VRF overlay. The right path depends on your existing system, your budget, and how the building is used.

Done well, zoning cuts energy use by 15 to 30 percent and lifts tenant comfort scores. Done badly, it freezes coils, trips the unit, and creates more complaints than it solves. The biggest design risks are coil airflow, ventilation per zone, and fan power.

• Each zone must control its own temperature with its own thermostat. Mixing actively heated and actively cooled air to control temperature is not allowed. (NCC 2025 Part J6)
• Outdoor air rates per AS 1668.2:2024 must still be met when any zone damper is throttled. Closing a zone cannot starve people of fresh air. Set a minimum damper position to keep ventilation flowing. (AS 1668.2:2024)
• Minimum airflow across the cooling coil must stay above the equipment manufacturer's limit. For DX coils this is typically 50 to 60 percent of design airflow. Below that, the coil can freeze. (AIRAH DA09)
• New ductwork crossing fire-rated walls or floors needs fire dampers. Smoke dampers apply at smoke compartment boundaries. (AS 1668.1:2015, NCC 2025 Part C)
• Fans above 125 W must run at variable speed, and the system must meet the NCC 2025 fan power ratio cap of 0.84 W per L/s. Restrictive zoning that drives static pressure up can blow this limit. (NCC 2025 Part J6)
• All new actuators, transformers, and controllers must be installed to the Wiring Rules. (AS/NZS 3000:2018)
• VRF retrofits using R-32 or R-454B must check A2L charge limits in occupied spaces. (AS/NZS 5149:2016)
• Pre-1990 buildings need an asbestos survey before any duct modification. (WHS Regulation 2017, SafeWork NSW Code of Practice)

The biggest design constraint on a zoning retrofit is the cooling coil. When zone dampers close down, less air flows across the coil. If airflow drops too far, the refrigerant in the coil cannot pick up enough heat. The coil freezes, the unit trips, and the building goes warm.

Every zoning design needs a way to keep the coil fed with air. The three common methods are a bypass damper from supply to return, a variable speed drive (VSD) on the supply fan with a low airflow limit, or oversized ductwork that lets even one open zone carry 70 percent of system airflow.

Ventilation is the second constraint. AS 1668.2:2024 sets the minimum outdoor air for each room. When a zone damper closes, the room behind it still needs its share of fresh air. The fix is a minimum stop on every zone damper, typically 30 percent of design airflow. The damper never fully closes. People still get fresh air even when their zone is "off."

Cost varies widely by approach. Use the table below as an indicative guide for a typical 600 m2 commercial floor in the Sydney metro area.

The cheapest option is not always the right one. A Class A office floor with high tenant expectations usually justifies VAV. A small strata-titled office may only need dampers. A building with failing ducted plant and limited ceiling access often suits a VRF overlay because VRF needs only refrigerant pipework, not ducts.

Zoning is not always the answer. An oversized system gets worse with zoning, not better, because closing zones pushes more capacity onto fewer rooms and triggers short-cycling. A single open-plan space with no thermal partitions gains nothing from zoning. A system at the end of its life should be replaced, not zoned. Always start with a mechanical condition assessment before specifying a zoning retrofit.

A 600 m2 office floor served by a single 60 kW packaged DX rooftop unit on the roof. One return-air thermostat in the open-plan area. Persistent perimeter discomfort complaints in the boardroom (north) and the meeting rooms (south) because the open-plan area drives the thermostat.

Design: convert to four zones (north, south, east, west), each roughly 150 m2.

• Four motorised zone dampers (around 600 mm diameter) with electric actuators on each branch.
• Bypass damper from supply to return, sized to maintain 60 percent of design airflow at the coil with one zone open.
• VSD added to the supply fan with low airflow limit set to manufacturer minimum.
• Four zone thermostats wired back to a common zone controller, integrated to the existing BMS via BACnet.
• Minimum damper position set to 30 percent at commissioning to maintain AS 1668.2:2024 outdoor air per zone.

Indicative cost: $25,000 to $33,000 total, supply, install, controls, and commissioning.

Outcome: 15 to 25 percent reduction in unit runtime through reduced overcooling of unoccupied zones. Comfort complaints typically fall by half within the first month. NCC 2025 Part J6 satisfied because each zone now has its own thermostat. AS 1668.2:2024 satisfied because total supply airflow envelope and minimum zone openings preserve outdoor air per occupant.