Two rough guides, side by side
| Rinnai's buyers guide | CHOICE's table, via Solar Victoria | |
|---|---|---|
| Up to 20 square metres | 2.5 kW is most common | 2 to 2.5 kW |
| 20 to 40 square metres | 3.5 kW | 2.5 to 5 kW |
| 40 to 60 square metres | 5.0 to 6.0 kW | 4 to 6 kW |
| 60 to 80 square metres | 7.0 to 9.0 kW (its band for 60 and up) | 5 to 7 kW |
| Over 80 square metres | Same 60 and up band | 6 to 9 kW |
Solar Victoria, which reproduces the CHOICE table, says the supplier's assessment of your home should be followed over it. Rinnai's separate size guide pitches things a little differently again: 3.5 kW for up to about 30 square metres, 5 kW up to 40 and 7 kW up to 60.
That spread between guides is the point. They're rough, and your room decides.
What makes a room need more, or less
Mitsubishi Electric calls a professional heat load calculation essential. The things it lists are the climate, which way the building faces, insulation, windows and doors, what the house is built from, and the ceiling height.
In plain terms, a room needs more when it has a high ceiling, a big window catching the afternoon sun, little or no insulation, or an open doorway to a stairwell. It can get by with less when it's insulated, shaded and closed off by a door. Rinnai says much the same: higher ceilings demand more, good insulation holds the temperature, and a sun-facing room may need more cooling in summer.
Climate counts too. ActronAir says tropical and subtropical parts of Australia have much greater demands than cooler regions. It also says a unit loses cooling capacity once it's above 35 degrees outside, with the example of a 14 kW system dropping to 10 kW or 12 kW.
Work out a starting size yourself
Measure the floor area
Length times width, in metres. A bedroom 4 metres by 5 metres is 20 square metres. For an open-plan space, measure everywhere the air has to reach, not just the lounge end.
Note the ceiling height
ActronAir's rough formula changes with it: 150 watts per square metre for a 2.4 metre ceiling, 160 for 2.7 metres and 175 for 3 metres.
Run the numbers
Multiply the area by that figure. A 20 square metre bedroom with a 2.4 metre ceiling comes out at 3,000 watts, or 3 kW. ActronAir itself warns the method leaves out many important factors, and Rinnai's guide puts that room at 2.5 kW, so you now have a range rather than an answer.
List what's unusual
A wall of glass, a skylight, a kitchen in the space, an uninsulated ceiling, a room over the garage. These are exactly what an installer needs to hear about.
Try the size calculator
Our size calculator uses ActronAir's ceiling-height figures and Rinnai's room bands, so you can test a few rooms in a minute.
Ask how the quoted size was worked out
A number calculated from your room is worth more than one picked from a brochure. If the installer can't say how they got it, ask for a heat load.
Why bigger isn't safer
It's tempting to go up a size just in case. Rinnai says an oversized system can switch on and off too often (short cycling), take less moisture out of the air and wear faster. An undersized one struggles to heat or cool, uses more energy and may not last as long.
If you're torn between two sizes, tell the installer how you use the room. A bedroom you only cool at night with the door shut is a different job from a sunroom you sit in all afternoon.
Sizing for heating in a cold climate
If your winters are cold, check the heating side as closely as the cooling side. The example label in the ACT Government's guide shows cooling capacity at an outdoor temperature of 35 degrees, and heating capacity at both 7 and 2 degrees. Its climate map puts Canberra, Hobart and Melbourne in the cold zone.
If that's you, the heating capacity at 2 degrees is the figure to compare between models. The same guide suggests asking whether a split has an H2 rating, which it says means the unit will work efficiently in a Canberra winter. Our page on energy ratings explains the rest of the label.
Sizing a whole house
A ducted system is sized for the house, not one room. ActronAir's own example of its formula is 100 square metres times 150 watts, which is 15,000 watts, or 15 kW. That's a starting point only.
FairAir, from AIRAH, says its online calculators assume a single split system in a single room, and that experienced air conditioning contractors or designers give the most accurate estimates. For a whole home that's even more true. A ducted specialist should work through each room, the zones you want and the duct runs before settling on a size.
If several splits are on the cards instead, each room gets sized on its own. Our guide to split systems or ducted helps with that choice.
What you can work out, and what the installer should
You can
- Measure each room and note the ceiling height.
- Note which way the main windows face and whether anything shades them.
- Run the rooms through the size calculator so you know roughly what to expect.
- If winters are cold where you live, find the heating capacity at 2 degrees on each label.
Leave to a licensed installer
- A heat load calculation for the room or the whole house.
- Checking the switchboard can take the unit, and running a dedicated circuit if it needs one.
- Deciding between one large unit and two smaller ones for an awkward space.
- For ducted, sizing each zone and the ducts that feed it.
Questions to ask about the size you're quoted
- How did you work out this size, and can I see the numbers?
- What did you allow for the ceiling height, the windows and the insulation?
- What's the heating capacity at 2 degrees, not just the headline figure?
- Would a smaller unit cope if we keep the door shut?
- For a big open area, would two units do it better than one?
What we'd do
What we'd do
We'd measure the room, run it through the size calculator, and go into the quote with a range in mind. Then we'd ask the installer to show how they got their number. If it's well above or below the rough guides, a good installer will have a reason, like a west-facing wall of glass or a 3 metre ceiling, and will happily walk you through it. One who can't explain the size is the one to be wary of.
