Office Air Conditioning Design: System Selection Guide

What You Need to Know

Office air conditioning is the most common commercial HVAC application in Australia. The right system depends on the building size, number of floors, and whether you are fitting out an existing shell or designing a new building from scratch. A small office under 200 sqm can use split systems. A medium office from 200 to 2,000 sqm suits VRF or packaged systems. Large offices over 2,000 sqm and multi-storey buildings typically need chilled water with central air handling units.

Ventilation is non-negotiable. AS 1668.2:2024 requires 10 L/s of outdoor air per person in offices, with a minimum of 0.35 L/s per sqm of floor area. Standard occupancy density is 1 person per 10 sqm. Open plan hot-desking layouts at 1 person per 5 sqm double the fresh air requirement.

Typical cooling loads for offices range from 80 to 120 W/sqm for internal zones and 120 to 180 W/sqm for perimeter zones with glazing. Section J of the NCC sets energy efficiency requirements for all office HVAC systems, including fan power limits, ductwork insulation, and mandatory economy cycles above certain capacities.

Mechanical engineering design fees for a standard single-level office are $5,000 to $12,000. Multi-storey office buildings with central plant design run $15,000 to $30,000 or more.

• Offices need 10 L/s of outdoor air per person. The minimum outdoor air rate must not drop below 0.35 L/s per square metre, even with demand control ventilation. Meeting rooms at 1 person per 2 sqm need proportionally more fresh air per square metre than open plan areas. (AS 1668.2:2024)
• Economy cycles are mandatory above certain system capacities. Systems with a total cooling capacity above 40 kW serving a single air handling zone must have an outdoor air economy cycle. This uses cool outdoor air for free cooling when conditions allow, reducing compressor energy. (NCC 2025 Section J)
• Time switches must be fitted to all systems above 2 kW cooling or 1 kW heating. Office HVAC must be able to shut down automatically outside occupied hours. Seven-day programmable timers are the minimum requirement. (NCC 2025 Section J)
• Fan power limits apply. The total fan power (supply plus return plus exhaust) must not exceed the limits set in Section J. For a typical office system with ductwork, the limit is around 2.5 W per L/s of supply air. Oversized ductwork and low-pressure-drop diffusers help meet this. (NCC 2025 Section J)
• Ductwork must be insulated and sealed. Supply air ductwork in unconditioned spaces needs thermal insulation to R1.0 minimum. All ductwork must be sealed to Seal Class B as a minimum to prevent conditioned air leaking into ceiling voids. (NCC 2025 Section J, AS 4254)
• Server rooms and comms rooms need 24/7 cooling. These spaces must be on a separate system from the office hours air conditioning. A dedicated split system or fan coil unit running year-round is standard practice. (AS 1668.2:2024, best practice)
• Toilet exhaust must not recirculate into occupied spaces. Toilets and kitchenettes need dedicated exhaust, discharged to outside. The exhaust rate depends on the number of fixtures. (AS 1668.2:2024)

Consider a 1,500 sqm office across two floors of a Sydney CBD building. Standard occupancy at 1 person per 10 sqm gives 150 people. At 10 L/s per person, the building needs 1,500 L/s of outdoor air. Add six meeting rooms at 20 sqm each (10 people per room at 1 per 2 sqm), and each meeting room needs 100 L/s when fully occupied. The total outdoor air demand peaks at around 2,100 L/s when all meeting rooms are in use.

For cooling, the internal zones (away from the facade) sit at 80 to 100 W/sqm. Perimeter zones with north or west-facing glass can hit 150 to 180 W/sqm due to solar gains. The total cooling load for this office would be approximately 150 to 200 kW. A VRF system with 3 to 4 outdoor units can handle this comfortably. A chilled water system would also work but is harder to justify at this scale unless the base building already provides chilled water.

Zone control is the key to comfort and efficiency. Perimeter zones need separate control from internal zones because the solar load varies throughout the day. A north-facing zone heats up in the morning while a west-facing zone peaks in the afternoon. Internal zones have relatively constant loads from people, lighting, and equipment. Without separate zones, the system overcools one area to keep the other comfortable.

Open plan offices are simpler for HVAC distribution because large areas can be served by a few air handling units or fan coil units with linear diffusers. Cellular offices with individual rooms need more zones, more ductwork branches, and more thermostats. A floor with 20 private offices may need 20 separate VAV boxes or fan coil units, compared to 4 to 6 zones for the same floor in open plan layout.

Server rooms and comms rooms are a separate problem. A single server rack generates 2 to 10 kW of heat. This runs 24 hours a day, 365 days a year. The office HVAC shuts down at 6pm, but the server room cannot. A dedicated 5 to 15 kW split system on the server room costs $3,000 to $6,000 installed and runs independently of the main office system. Skipping this is the most common cause of IT equipment failures in offices.