How to use it
Measure the floor
Length and width in metres, including any kitchen or hallway the air will flow into. For an L-shaped space, run each part separately and add the two results.
Pick the ceiling height
Choose the closest of 2.4 m, 2.7 m or 3 m. If you're not sure, a tape measure from floor to ceiling settles it.
Read the result against the sizes
Find the nearest size in our split system sizes section and read what it suits.
Where the number comes from
ActronAir's rough formula multiplies the floor area by 150 watts per square metre for a 2.4 m ceiling, 160 for 2.7 m and 175 for 3 m. Its own example: 100 square metres at 150 watts is 15,000 watts, or 15 kW.
ActronAir is upfront that the method doesn't consider many of the important factors. Rinnai's room bands won't always line up with it either, since Rinnai puts rooms up to 20 square metres at 2.5 kW. Rough guides landing a little apart is normal, so treat the result as a size to discuss.
What the answer doesn't tell you
It can't see the room. Mitsubishi Electric says a professional heat load calculation is essential because climate, orientation, insulation, windows and doors, building materials and ceiling height all change the answer. ActronAir adds that above 35 degrees an air conditioner loses some of its cooling capacity.
For ducted, the gap is wider. FairAir, from AIRAH, says its own calculators assume a single split system in a single room, and that experienced contractors or designers give the most accurate estimates. A ducted specialist will want to see the ceiling or underfloor space and every room the ducts reach.
What we'd do
What we'd do
Use the result to choose which size page to read, not which unit to buy. If it lands between two sizes and the room gets afternoon sun or has a high ceiling, don't round down. Then ask the installer for their heat load in writing and compare it with your number.