Installing or replacing an evaporative air conditioner in Perth, Adelaide, Melbourne, or regional Australia comes down to one core engineering principle: volume displacement. Unlike reverse cycle air conditioning that recirculates and dehumidifies indoor air in an airtight box, evaporative cooling works on a continuous tidal flow of 100% fresh, moisture-chilled air.
Get the unit capacity wrong, and you run into immediate site problems. If you undersize the rooftop unit, airflow velocity drops, the air heats up before reaching back bedrooms, and the house feels stuffy on 40°C days. If you oversize the unit without providing enough duct outlets or open window relief, backpressure stalls the fan, water droplets carry over into the ductwork, and your rooms turn into a humid, sticky swamp.
In Australia, rooftop evaporative units are engineered and sized by airflow volume in cubic metres per hour (m³/h) and litres per second (L/s) rather than thermal kilowatts. This trade guide covers the exact Air Changes per Hour (ACH) formula used by leading manufacturers like Seeley International (Breezair, Braemar, Coolair) and Climate Technologies (Bonaire), along with critical relief opening rules.
Comparing evaporative cooling to refrigerated air conditioning? If you live in a humid coastal zone or want refrigerated cooling and heating in a single sealed system, use our Residential Heat Load Calculator. For individual bedroom cooling, check our Air Conditioner Size Calculator.
Evaporative Cooler Sizing Chart
A standard 3-bedroom Australian home (140m² to 180m²) with 2.4m to 2.7m ceilings requires an evaporative cooler with an airflow rating between 10,500 m³/h and 13,500 m³/h (approx. 2,900 to 3,750 L/s). Smaller units and townhouses (100m² to 120m²) need 7,500 m³/h, while expansive 4- to 5-bedroom homes (220m² to 280m²+) require high-output units moving 14,000 to 16,500+ m³/h.
| Floor Area (m²) | Typical Layout | Volume (m³ @ 2.55m) | Required Airflow (m³/h) | Airflow (L/s) | Recommended Vents | Benchmark Aussie Models |
|---|---|---|---|---|---|---|
| 90 – 120 m² | 2-Bed Unit / Villa | 230 – 306 m³ | 7,000 – 8,500 m³/h | 1,950 – 2,350 L/s | 4 – 6 outlets | Breezair EXH170, Braemar LCQ250, Coolair CPL450 |
| 130 – 170 m² | 3-Bed Family Home | 330 – 434 m³ | 10,000 – 12,000 m³/h | 2,750 – 3,350 L/s | 6 – 8 outlets | Breezair EXH210, Braemar LCQ350, Coolair CPL600 |
| 180 – 230 m² | 4-Bed Single Storey | 460 – 587 m³ | 12,500 – 14,500 m³/h | 3,450 – 4,000 L/s | 7 – 10 outlets | Breezair ExtraRX, Braemar LCQ450, Coolair CPL850 |
| 240 – 300+ m² | Large / 5-Bed Residence | 612 – 765 m³ | 15,000 – 16,500+ m³/h | 4,150 – 4,600+ L/s | 9 – 14 outlets | Breezair Supercool TBS580, Braemar LCQ550 |
Air Changes Per Hour (ACH) Sizing Formula
To size an evaporative air conditioner, multiply the total conditioned volume of the home in cubic metres (length × width × ceiling height) by the design Air Changes per Hour (ACH), then apply an insulation correction factor. In Australia, the standard design benchmark is 30 air changes per hour.
Air Changes per Hour represents how many times per hour the entire air volume of your home is completely replaced with fresh, cooled outdoor air. Because evaporative cooling relies on continuous air motion to create an evaporative chilling breeze across your skin, low airflow rates feel stuffy.
- 25 to 28 ACH (Low Heat Load): Modern homes complying with NCC 7-star energy efficiency, equipped with R4.0+ ceiling batts, wall insulation, double glazing, and good eaves shading.
- 30 ACH (Standard Benchmark): The reliable default for brick veneer Australian suburban homes with standard R2.5–R3.5 ceiling insulation and 2.4m to 2.7m ceilings in temperate southern zones.
- 32 to 38 ACH (High Heat Load): Older uninsulated homes, timber fibro cottages, houses with cathedral raked ceilings, or properties located in harsh inland climates (such as Kalgoorlie, Mildura, or Broken Hill) with severe afternoon sun exposure.
Open Window Relief Air Requirements (The 1m² per 1,000 L/s Rule)
For every 1,000 litres per second (L/s) of evaporative cooler airflow, you must provide at least 1.0 square metre of open window or door relief area. A typical 12,000 m³/h unit moving 3,330 L/s requires approximately 3.3 to 3.5 m² of clear, open openings facing away from prevailing winds.
Running an evaporative air conditioner with all doors and windows tightly shut is the single most common mistake Australian homeowners make. The rooftop fan forces thousands of litres of fresh air into the roof space and ceiling registers each second. If that air cannot escape freely:
- Air Pressure Stalls the Fan: Backpressure builds inside the living areas, causing fan motor strain and reducing duct velocity to a trickle.
- Moisture Trapping: Because the system continuously introduces water vapour, closed rooms quickly reach 90%+ relative humidity, saturating timber, carpet, and furniture.
- Zoning via Windows: You can guide the cool breeze wherever you want simply by opening windows in the rooms you occupy (e.g. bedrooms at night, living zones during the day) while keeping doors closed to unoccupied rooms.
For security-conscious households or bushfire-prone areas where leaving windows wide open overnight is unsafe, installers fit automated ceiling relief vents (such as Air-Relief or Secura-Vent dampers) into hallway ceilings. These dampers pop open automatically under positive air pressure, discharging the exhaust air directly into the roof cavity and out through the eaves or gable vents.
Climate Suitability: Dry Inland vs Coastal Humidity
Evaporative air conditioning works exceptionally well in hot, dry Australian climates like Perth, Adelaide, Melbourne, and inland country towns. It performs poorly in humid coastal regions like Brisbane, Sydney, and Darwin because high outdoor humidity prevents water from evaporating from the cooler pads.
Evaporative cooling works through adiabatic cooling: when hot, dry air passes over wetted honeycomb filter pads (such as Seeley's Chillcel pads), heat energy is consumed as water evaporates into vapour. This causes the air temperature to drop substantially—often by 10°C to 14°C below outside ambient air on a dry 38°C day.
- Prime Regions (High Performance): Perth & South-West WA, Adelaide & SA regional, Melbourne & regional Victoria (Shepparton, Bendigo, Mildura), Riverina (Wagga Wagga, Griffith), Central West NSW (Dubbo, Orange).
- Marginal Regions (Summer Humid Spells): Western Sydney (Blacktown, Penrith), coastal Wollongong, and Newcastle. Units work well on scorching dry north-westerly days, but struggle during humid maritime easterly sea breezes.
- Unsuitable Regions: Brisbane, Gold Coast, Sunshine Coast, Mackay, Townsville, Cairns, and Darwin. In these sub-tropical and tropical zones, relative summer humidity routinely exceeds 70%, meaning evaporative discharge air will feel wet, heavy, and hot.
Running Costs & Water Usage: Evaporative vs Ducted Reverse Cycle
A residential evaporative cooler costs roughly $0.15 to $0.35 per hour in electricity to operate, representing an 80% reduction in running costs compared to an equivalent whole-house ducted reverse cycle refrigerated air conditioner ($1.80 to $3.50/hr).
| System Type | Electrical Power Draw | Hourly Electricity Cost (@ $0.32/kWh) | Hourly Water Usage | Season Operating Cost (500 hrs) |
|---|---|---|---|---|
| Evaporative Cooler (12,000 m³/h) | 0.65 – 1.0 kW | $0.21 – $0.32 / hr | 12 – 22 L / hr | $120 – $180 |
| Ducted Inverter Reverse Cycle (14 kW) | 3.8 – 5.5 kW | $1.22 – $1.76 / hr | 0 L / hr (closed loop) | $610 – $880 |
| Older Non-Inverter Ducted (17 kW) | 6.0 – 8.0 kW | $1.92 – $2.56 / hr | 0 L / hr | $960 – $1,280 |
Water consumption on modern Australian evaporative coolers is managed by intelligent microprocessor drain valves (such as the Breezair WaterManager system). Rather than continuously bleeding water onto the roof like vintage units, modern systems monitor water salinity. When total dissolved solids (TDS) rise, the tank drains clean and refills, protecting the pads from calcium crusting while minimising total water wastage.
Dropper Box & Duct Outlet Sizing Guidelines
A high-capacity evaporative cooler requires correctly sized roof droppers and ductwork conforming to AS 4254. Because evaporative systems move two to three times the air volume of refrigerated systems, installing undersized ducts or closing too many ceiling diffusers will generate severe air noise and static pressure loss.
- Roof Dropper Sizing: Small units (up to 8,500 m³/h) use a 450mm × 450mm square dropper. Medium and Large units (10,500 to 14,000 m³/h) require a 550mm × 550mm dropper box. Extra Large commercial or two-storey units require 650mm × 650mm or twin droppers.
- Supply Duct Air Velocity: Main distribution trunks should maintain air velocity below 5.0 to 6.0 m/s, and branch flexible ducts to bedrooms should stay below 3.0 to 3.5 m/s to ensure whisper-quiet operation.
- Minimum Open Outlets: Never operate an evaporative unit with fewer than 4 to 5 supply outlets open. Shutting off bedroom grilles concentrates massive volume through remaining vents, whistling loudly and potentially lifting ceiling registers.
- Winterisation & Damper Flaps: Ensure your unit has an automatic internal damper flap (e.g. Seeley MagIQtouch Autoweathershield). In winter, when your central gas heater is running, this seal prevents costly warm room air from leaking up through the ductwork and out through the roof cowl.