Why one size can cover 30 or 40 square metres
Both figures come from the same brand, which tells you how rough room-size guides are. What moves a room up or down the band is everything around the floor: the ceiling, the glass, the insulation and the climate outside.
ActronAir says a well insulated home likely won't need as much capacity as a poorly insulated one, and that tropical and subtropical parts of Australia ask much more of a system than cooler regions. A shaded, well-insulated room at the small end of the band is what 3.5 kW is pitched at. A sunny room of the same size in a hot climate may need the next size up.
3.5 kW is what it delivers, not what it draws
The kW on the box is capacity: how much heating or cooling the unit puts into the room. Mitsubishi Electric says power consumption is measured separately, as the total input power, also in kilowatts. On your power bill, the input figure is the one that counts.
Find it in the manual, then put it into the running cost tool with your own power rate.
Comparing two 3.5 kW units
- Compare like with like. The ACT Government's guide says the most meaningful comparison is between systems of similar capacity.
- Use the zone that matches you. The zoned label groups Brisbane and Darwin as hot, Adelaide, Perth and Sydney as average, and Canberra, Hobart and Melbourne as cold.
- Read both colours. Mitsubishi Electric says blue stars are for cooling and red stars for heating, and more stars mean more efficient.
- Check the yearly energy figure. A lower number means the unit uses less to do the same job.
- In a cold zone, look at the heating capacity at 2 degrees outside, not just the headline number.
Your part, and the installer's part
You can
- Measure the room, or run it through the size calculator, and see where it falls against Rinnai's two figures.
- Note the sunniest window and the ceiling height.
- Read the labels of the units you're quoted, using the checklist above.
Leave to a licensed installer
- A heat load that confirms 3.5 kW, or tells you to go up a size.
- Placing the indoor unit where its airflow reaches the whole room.
- Pipework, charging, wiring and commissioning, all of it licensed work.
What we'd do
Our take
If the room is under 30 square metres, insulated and out of the afternoon sun, we'd be comfortable with 3.5 kW. Between 30 and 40, or if heating through a cold-zone winter is the main job, we'd ask the installer to show us their heat load before agreeing to the size. A split specialist does that sum all the time.
