Sizing the system to the room
Allow about 100 watts of cooling per square metre as a rough guide, so a 20 square metre bedroom needs around 2 kW (7000 BTU), a 35 square metre living room around 3.5 kW (12000 BTU), and a large open plan space 5 kW and above. Big windows, a sunny aspect, people and equipment all push that figure up. A server room needs far more than a bedroom of the same size, because its equipment gives out heat around the clock.
Running cost and efficiency
Efficiency is rated as SEER (seasonal energy efficiency ratio) for cooling and SCOP (seasonal coefficient of performance) for heating: how much cooling or heat a system gives for each unit of electricity across a season. A SCOP of 4 means heating the room takes about a quarter of the electricity a plain electric heater would use. Most modern systems also have an inverter, which slows the compressor rather than switching it off and on, holding a steady temperature on less power.
What installation involves
By law, the refrigerant pipework must be connected by an F-Gas registered installer, who will also plan the pipe run, a solid spot for the condenser and a drain for the water the indoor unit collects. The condenser usually needs its own circuit from the fuse board, fitted by a qualified electrician, and in a flat or listed building it may need permission first. After that, wash the filters every few weeks and have it serviced once a year.
