🔀What size mini split does this room need?

About 350 to 450 sq ft at an eight-foot ceiling, which is a large bedroom or an ordinary living room. It is the smallest head most manufacturers make, so anything smaller than that room still gets the 9,000 — a 150 sq ft study will simply run its compressor at the bottom of its range.

Yes, though less badly than a single-speed system. An inverter modulates down to roughly a third of rated capacity, which forgives a lot; below that it cycles like any other unit. A head at double the room’s load will hold temperature but leave humidity high, because the compressor never runs long enough at low speed to condense much water.

No. Manufacturers publish a combination ratio — commonly anywhere from 100% to 130% of the outdoor unit’s capacity in connected head capacity — precisely because the bedrooms and the living room do not peak at the same hour. Adding heads and buying that number in an outdoor unit is the single most common way multi-zone systems end up oversized.

Almost all of them are heat pumps, and the heating capacity is usually a little higher than the cooling capacity at mild temperatures and considerably lower when it is very cold. If the unit is your main heat source, size it on the heating load in your climate zone rather than on cooling — use the heat pump calculator, and check the manufacturer’s capacity at your design temperature rather than the headline number.

Air does not go around corners. A single large head in a hallway will not condition the bedrooms off it, no matter how big it is. The rule is one head per space you want controlled, and open-plan areas count as one space. Doors that stay closed mean separate zones.

High on a wall that lets the air throw across the long axis of the room, not into a wall a metre away, and not directly over the seating. Keep it clear of the return path so it does not recirculate its own output, and leave the manufacturer’s clearances above and to the sides or the capacity you sized for will not be the capacity you get.