Why air conditioner size matters more in Townsville and Cairns
Townsville and Cairns are both in building climate zone 1, the tropical zone, with hot, humid summers and warm winters. The national energy label puts the hot climate zone's cooling season at October to May, so the cooling performance of a unit matters far more here than its heating.
Daikin, one of the major manufacturers, says someone in North Queensland will have very different requirements from someone in Tasmania.
The rule of thumb for sizing an air conditioner
Air conditioner capacity is measured in kilowatts, written kW. The common industry starting point is about 150 watts of cooling per square metre of an insulated room. Some guides use a range of 125 to 150 watts.
Measure the room, multiply length by width, then multiply by 150. A 4 by 4 metre bedroom is 16 square metres, which comes to about 2.4kW, so a 2.5kW unit. A 6 by 5 metre lounge is 30 square metres, about 4.5kW. A 9 by 6 metre open-plan living area is 54 square metres, about 8kW.
Treat that number as a starting point. The Australian Government and the major manufacturers all advise sizing the unit from the room, the building and the climate, with professional advice, rather than from floor area alone.
What pushes the size up or down
The direction the room faces is a big one. East and west-facing rooms take more sun and run warmer, while south-facing rooms stay cooler. Large windows let in more heat, and good window coverings cut it.
Higher ceilings mean more air to cool, which is common in older Queenslanders. Insulation, or the lack of it, changes the load a great deal. People, computers, lights and cooking appliances all add heat, so an open-plan living area joined to the kitchen needs more than a bedroom of the same floor area.
Too small and too big both cost you
A unit that is too small runs flat out for long stretches. It costs more to run, wears faster and may never reach the temperature you set.
A unit that is too big costs more to buy and switches on and off too often, which gives uneven temperatures and more wear. In a humid climate there is a second problem. It cools the room quickly and stops before it has removed the moisture, so the room can feel cold and damp at the same time. Daikin calls this especially important in the tropics. That is why we do not simply go one size up to be safe.
Getting the most out of the unit in the humidity
The Australian Government's guide for northern Australia recommends a unit with a dry or dehumidify mode, set between 25 and 27 degrees. It uses less energy than full cooling and stays comfortable, especially with a ceiling fan running, which feels about 3 degrees cooler. Each extra degree of cooling adds about 5 to 10 percent to running costs.
Inverter split systems are the most efficient choice for most homes. When comparing models of the same size, check the cooling stars for the hot zone on the energy label, because that is the zone that applies up here. Keep the filters clean, because dirty filters cut airflow and efficiency.
How we size and install a split system
Before we install, we inspect the room and the house. That covers where the indoor and outdoor units will go, how the unit will drain, and whether the switchboard has the capacity for it. Most installs get their own dedicated circuit from the board.
Our electricians hold an ARC licence for air conditioning and refrigerant work, number AU59000. We supply and install split systems, install units you have bought, and clean existing ones.