An open external door or a wide-open window throws away the cooling or heating a split has just paid for. The unit will run and the room will still track the garden. An internal door is different: a little air will share with the next space, and it will not reliably condition that space unless the survey sized the indoor unit for both and the layout actually allows the air to move. Treat a split as a room system. Treat a shop door as a design problem, not as a habit.
Patio doors standing open through the afternoon
Every minute a patio door stands open, warm humid air arrives and cooled air leaves. Short openings to let a dog out are nothing. An afternoon with bi-folds fully open is not air conditioning. It is a fan pointed at the weather. Decide which afternoon you want. The same pattern happens in heating mode on a January evening, with a defrost cycle trying to catch up.
The compressor chasing the garden
The thermostat sees the warmth and holds the compressor up. Electricity use rises. Comfort near the doorway never settles. Energy Saving Trust's air-to-air pages describe a machine that moves heat in a room it can hold.
Drift through an internal door
With an internal door open, some cool air drifts through. You may take the edge off a small adjacent space. You should not expect a second bedroom, a closed kitchen or an upstairs landing to reach the same setpoint. Door heads, curtains and the direction of the louvres all limit the drift. If two rooms must be one comfort zone, say so at survey. The answers are a larger indoor aimed across an opening that stays open, two indoors, or a ducted unit with a plenum and two grilles. An open door is not a control strategy.
- External openings defeat the setpoint.
- Internal doors share air only weakly.
- A split cools the room it is sized for.
A high-street door and the till
Retailers open doors to look welcoming. The cooling plant then tries to condition the high street. Staff at the till feel a draught and the sales floor stays warm. An air curtain at the door, plus cooling aimed at the occupied zone, is the usual grown-up pairing. Leaving a cassette at 18C with the door wedged open is the expensive pairing. HSE still expects a reasonable workplace temperature for the people who cannot step outside. Spending the electricity on the pavement does not meet that aim. Size the system for the door position you will really trade with. A shop should run air conditioning with the door open only if the system was designed for that opening. An ordinary cassette and a propped door will not hold a setpoint. Where the plant is large enough, GOV.UK air conditioning inspections are a separate duty.
Stop asking the split to cool the garden
Stop if the plan is to cool or heat with the external openings left open all evening. Cool or heat with them shut. Ventilate in a deliberate burst when the outdoor air is actually fresher, and pause the unit while you do. Use internal doors as zone boundaries: shut them when the room has its own indoor, open them only when you have accepted that the next room is outside the design. If you want the doors open all evening, buy shade, a fan and a cold drink for those hours, and keep the split for the part of the day when you are willing to close up. The machine cannot do both jobs.
Frequently asked questions
Will the unit break if I leave the door open?
It will usually just run for long periods and fail to reach setpoint. That wastes energy and wears the equipment harder. Close the external door while cooling or heating.
Can an open internal door cool two rooms?
It can take the edge off a small linked space. It will not replace a second indoor unit. If both rooms matter, design both zones.
Core buying guides
Start with these high-intent guides, then dig into the rest of the library.
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.