Keep windows closed while the split is cooling. A crack feels harmless and it lets the unit's output leave while warm, humid air comes in. The remote cannot outrun that. Dropping the setpoint toward 16C only runs the coil harder and can ice it. If the room needs fresh air, open the windows with the unit off, then close them and cool. Fan speed changes how the closed room feels. It does not make an open window efficient.
Cool air leaving through the sash
The indoor unit recirculates room air across a cold coil. A window gap short-circuits that. Cool air falls out or is pushed out, and the system sees a room that never arrives at the setpoint. A tiny crack is still a vent for the cooling.
Humid air from the garden on the coil
In humid weather you also ask the coil to wring out moisture from the garden, which is how drains get busy and rooms feel clammy. On a hot day the air coming in is the problem. Close the window while the unit runs. Do not answer a crack with a lower setpoint.
Fan-only with a window open is a fan
A fan-only mode with a window open is just a fan. It is not cooling. Fan-only is cheaper than cooling because the compressor is off. It does not reduce the room temperature below the air you just let in. It is a breeze, not air conditioning. If someone in the house cracks the sash because the air feels stale, the fix is a burst of ventilation, then cooling, or a separate fresh-air plan. The split was not sized as a ventilator.
- Windows closed while cooling or heating.
- Air the room first, then shut and switch on.
- Keep the room door shut as well.
Night purging with the compressor off
When the outdoor air is genuinely cooler than the house, and the street is safe, opening windows with the unit off can dump heat. That is night purging. It is not a crack for fresh air at 3pm with the compressor running. Mix the two and you buy the worst of both. Security, pollen and noise are reasons people refuse open windows at night. In that case the closed-window split is the tool you paid for. Use a normal setpoint. Energy Saving Trust's air-to-air description assumes the unit treats the room, not the street.
Steam, the hob and the kitchen door
Steam and cooking smells need an extract fan to outside, used as that fan intends. Running a living-room split with the kitchen window ajar does not clear the hob, and it upsets the living room. Shut the kitchen door.
A room with no opening window
If a room has no opening window and no extract, say so at survey. Cooling it is still possible. Fresh air is a building question beside the F-gas install, not a millimetre of sash left up for luck. Say if anyone needs the window open for ventilation, noise or a fire-escape habit, and have the room sized as a closed space.
Frequently asked questions
We need fresh air. How do we get it?
Open up while the split is off, then close and cool. If that is not enough, ask about a proper vent or extract. Do not leave the unit fighting a gap.
Core buying guides
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Related guides
Sources and further reading
How this guide was prepared
This guide was written by the Local AC Installers Editorial Team. We compare official guidance, legislation and established consumer information, separate general information from project-specific advice, and show publication dates and sources. We do not accept installer accreditation claims at face value.