Air Conditioning Buying Guide: Split, Floor-Standing, Multi-Split or Portable?

Splits — wall, floor and multi — are fixed systems: the hot, noisy part is outside, and the refrigerant pipes go through a hole in the wall. A portable is plug-and-play, but its compressor is in the room with you and the window has to stay open for the hose.

An inverter unit varies the speed of its compressor. It runs hard to cool the room down, then slows right down to hold the temperature, instead of switching on and off. That makes it quieter, keeps the temperature steadier and uses less electricity than an on/off unit.

Every AURA unit we sell is a DC inverter heat pump using R32 refrigerant, which has a lower global-warming potential than the older R410A. Being heat pumps, they heat as well as cool.

Allow about 100 W of cooling per square metre for a typical room, and 125–150 W for a sunny, top-floor or busy one. The air conditioner size guide has a calculator and a room-size chart. Here's what our range covers:

With a split, the compressor and its fan are outside, so indoors you mostly hear moving air. AURA quote an indoor sound pressure of 45 dB for the 12,000 BTU unit, 48 dB for the 18,000 and 51 dB for the 24,000 — and like most makers' figures that's the unit on a high fan speed; it's quieter turned down.

For a bedroom, size the unit so it isn't working flat out at night, and run it on a low fan speed. Think about the outdoor unit too: site it away from your own and your neighbours' bedroom windows, on anti-vibration mounts.

Each AURA unit comes with 5 m of copper refrigerant pipe and 5 m of interconnecting cable. The usual extras:

Split systems contain refrigerant, and in Great Britain connecting, pressure-testing and commissioning the refrigerant circuit must be done by an F-Gas certified engineer. Makers' warranties usually depend on it too. You can save time by deciding where both units go and having the electrics and any trunking route sorted, but the refrigerant work is the engineer's.

• Electrics: the unit needs a suitable supply, and larger units often their own circuit — work for a qualified electrician.
• Condensate: the indoor unit's drain needs a fall to the outside or a waste pipe, or a condensate pump.
• Planning: outdoor units can need planning permission, so check with your council — especially for listed buildings, conservation areas and flats. Leaseholders also need the freeholder's consent.

The electricity a unit uses is its cooling output divided by its efficiency. A typical inverter split turns each kilowatt of electricity into about three or more kilowatts of cooling, so a 3.4 kW unit draws roughly 1.1 kW at full output. At 25p a kWh that's under 30p an hour flat out — and much less once the room is cool and the inverter slows down. Check your own tariff and the unit's efficiency rating for an exact figure.

Portables are less efficient for the same cooling, partly because the hot exhaust hose and the open window let heat back in. On heating mode, a heat pump split gives several kilowatts of heat for each kilowatt of electricity in mild weather, against one for one from an electric fan heater.