Doors, windows and blinds

An air conditioner has to deal with all the heat that gets into the room it serves. An open window, a door to a warm hallway or sun pouring through glass all add to that. A few habits with doors, windows and blinds let the same system keep the room comfortable with less work.

What to do in each season

Windows while the system runs

With a window open, warm, humid air keeps coming in, and the unit has to cool and dry that as well as the air already in the room. It may never reach your setting, and more water runs from the drain. In winter, heat escapes through an open window in the same way.

A wall-mounted unit recirculates the air in the room rather than bringing in fresh air. Keep trickle vents open and use extractor fans in kitchens and bathrooms as normal. Trickle vents give background ventilation and extractor fans take moisture out where it's made, and both move far less air than an open window. If you want to air a room, do it with the system off, then close the window and switch back on.

In a hot spell, nights are often cooler than days. If it's cooler outside than in, switching the system off and opening windows, where it's safe to do so, can let cooler air through the house. Close the windows again in the morning, before the air outside warms up. Keeping cool without air conditioning has more on this.

A portable air conditioner is different. Its exhaust hose usually runs out through a window, so gaps around the hose let warm air straight back in, and a window kit that seals the gap helps. See portable air conditioners.

Blinds, curtains and shading

Sunshine through glass heats a room directly, and in a room with large or sun-facing windows it can be one of the biggest sources of heat. Shading works best outside the glass, where external blinds, shutters and awnings stop the sun before it reaches the window. Blinds and curtains inside still help, but some of the heat is already in the room.

• Light colours and reflective linings send more sunlight back out through the glass than dark fabrics do.
• Close them before the sun arrives, not once the room is already hot.
• Keep them clear of the indoor unit, so they don't block its airflow.

Which windows need attention depends on the time of day:

Conservatories and rooms with roof windows can gain heat very quickly on a sunny day. Why some rooms overheat looks at the reasons.

Doors between rooms

A wall-mounted unit is usually sized for the room it's in. With the door open, cooled air spills out low down while warmer air comes in higher up, so the unit ends up cooling the hallway or landing too. In winter it works the other way round, with warm air escaping at the top of the doorway.

Open staircases and open-plan spaces behave the same way on a bigger scale. Warm air drifts upstairs and cool air flows down, so a unit heating a downstairs room with an open staircase loses warmth to the floor above.

Leaving a door open can share some cooling or warmth with the next room, but don't expect much. The unit measures the temperature in its own room, so the other room will usually stay warmer in summer and cooler in winter. A fan in the doorway can help move air through. If you often need more than one room kept comfortable, a unit in each room, perhaps on a multi-split system, is the usual answer.

Keeping warmth in during winter

• Close curtains at dusk. Lined or thermal curtains keep more warmth in than thin ones.
• Open them on sunny days, especially on south-facing windows, and let the sun help warm the room.
• Deal with draughts. Draught-proofing gaps around doors, windows and letterboxes keeps warm air in, but don't block trickle vents, air bricks or extractor fans.
• Keep doors closed between heated and unheated rooms.