Air conditioning FAQ

BTU stands for British Thermal Unit — a measure of heat energy. In air conditioning, the BTU rating tells you how much heat a unit can remove from a room per hour. A higher BTU rating means a more powerful unit suited to larger or warmer spaces.

The right BTU depends on room size, ceiling height, insulation, sun exposure, and how many people typically occupy the space. As a rough guide, a well-insulated living room of around 15 m² may need 5,000–6,000 BTU, but our BTU calculator gives a more accurate figure tailored to your specific room.

Most UK bedrooms (around 10–15 m²) need 5,000–8,000 BTU. South-facing rooms or rooms with large windows may need more. Use the BTU calculator for a personalised figure.

A portable air conditioner is a freestanding unit you can move between rooms; it vents hot air via a hose through a window or wall opening and requires no permanent installation. A fixed split system has an indoor unit mounted on the wall connected to an outdoor compressor unit — it is more efficient, quieter, and better suited to regular use, but requires professional installation.

Running costs depend on the unit's wattage, how long it runs, and your electricity tariff. A 1,000 W (1 kW) portable unit drawing rated input for two hours costs about 52p at 26.11p/kWh. Thermostat cycling and tariffs vary, so use our running cost calculator for your own estimate.

A typical 9,000 BTU portable unit running for 8 hours a day in summer costs roughly £1–£1.50 per day at current UK electricity rates. Try the running cost calculator for your own figures.

Yes. All single-cycle portable air conditioners must vent hot air outside via an exhaust hose, which needs to exit through a window, door, or a small hole in an external wall. Without a vent, the unit simply recirculates warm air and cannot cool the room effectively.

A single-hose unit uses one hose to expel hot air, which can create negative pressure and draw warm outside air in through gaps. A dual-hose unit uses a second hose to bring in fresh outside air for the condenser, making it more efficient and better at maintaining cool temperatures — though dual-hose models are generally larger and more expensive.

Drainage depends on the model, humidity and operating mode. Some units evaporate much of the condensate through the exhaust in cooling mode, while others use a tank or support a continuous drain. Follow the exact manual and never assume a self-evaporating unit is drain-free in every condition.

Portable units normally avoid permanent external work. A fixed system changes the exterior of the property and may need planning, listed-building, freeholder or leasehold consent depending on the location and siting. Rules differ across the UK, so check with the local planning authority and any property owner or managing agent before installation.

EER (Energy Efficiency Ratio) measures cooling output (in watts) divided by power input (in watts) at a fixed test condition — a higher number means greater efficiency. SEER (Seasonal EER) extends this across a range of temperatures to better reflect real-world seasonal performance. Both ratings help you compare units: a higher SEER/EER means lower running costs for the same cooling output.

Many modern air conditioners — particularly split systems — include a heat-pump mode that can reverse the refrigeration cycle to warm a room in cooler weather. Check the product specification for a "heating mode" or heat-pump function; some portable units also offer this. Heat-pump heating is typically two to three times more energy-efficient than electric resistance heaters.