Air Conditioning and Hot Water: Sizing and Choosing for Queensland
Two of the largest running costs in a Queensland home, why oversizing air conditioning makes comfort worse, and which hot water system suits a new build.
The two systems that dominate your power bill
In a South East Queensland home, cooling and water heating account for the largest share of electricity use. Both are specified during the build, and both are expensive to change afterwards.
They are also where the most money is wasted, because both are commonly oversized on the assumption that bigger is safer.
Why oversized air conditioning performs worse
An oversized unit cools the air quickly, reaches its setpoint, and shuts off. In a humid climate that is a problem, because dehumidification requires sustained run time. Short cycles drop the temperature without removing moisture, and the room feels cold and clammy rather than comfortable.
A correctly sized unit runs longer at lower output, removes humidity properly, uses less energy and lasts longer because it cycles less.
Sizing should follow a load calculation based on the actual room — its volume, glazing area and orientation, insulation and shading — not a rule of thumb applied to floor area.
Reduce the load before sizing the system
Every improvement to the building envelope reduces the capacity you need to install and run. Shading western glazing, insulating the ceiling properly, ventilating the roof space and allowing cross-ventilation all shrink the cooling load.
Ceiling fans then extend the comfortable range considerably at negligible running cost. For much of the Queensland year, fans and open windows are the entire requirement, and the air conditioner is for the worst weeks rather than the default.
A house designed this way needs a smaller system, which costs less to buy and less to run for its whole life.
Ducted or split
Ducted systems condition the whole house from one unit and look tidier, but conditioning rooms nobody is in is wasteful unless the system is properly zoned. Zoning is worth the extra cost if you go ducted.
Split systems in the rooms that actually need them are cheaper to install and often cheaper to run, at the cost of visible indoor units.
Either way, decide before frame stage, because ducting routes, condensate drainage and electrical supply all have to be planned into the structure.
Hot water
Water heating is a large and constant load, and a heat pump system is generally the best fit for a new Queensland home. It uses considerably less electricity than resistive electric storage, and paired with a timer running in the middle of the day it effectively stores your rooftop solar generation as hot water.
Solar hot water with electric or gas boosting also works well in this climate, at the cost of roof space that might otherwise carry panels.
Whatever you choose, decide before plumbing rough-in, because the type and location affect the plumbing and electrical layout. Our solar guide covers how these decisions interact.
Running cost and where the savings actually are
The cheapest energy is the energy you do not use. A house that stays comfortable through orientation, shading, insulation and ventilation runs its systems for fewer hours, which saves more than any efficiency rating upgrade.
Where you do run equipment, shifting it to the middle of the day makes a large difference now that feed-in tariffs are low. A timer on a heat pump hot water system turns rooftop generation into stored hot water at no extra cost.
Zoning a ducted system, and simply not conditioning rooms nobody is in, is the other large saving and it costs nothing beyond the initial setup.