How to vent a portable air conditioner in a UK home

British windows were not designed with air-conditioner hoses in mind, and the hose is the part of portable AC ownership nobody warns you about. Vent well and the unit performs close to its rating; vent badly and you pay full running costs for half the cooling. Here is how to do it properly in the window types UK homes actually have — without drilling, if you rent.

An air conditioner does not destroy heat; it moves it. The refrigeration circuit absorbs heat from room air and rejects it — plus the heat of its own electricity use — through the exhaust hose. That hot air must leave the building. Run the unit with the hose disconnected or pointing back into the room and you have built a machine that consumes a kilowatt to make the room warmer: all the rejected heat stays inside, along with the input energy.

Venting has two jobs, and most setups only do the first: get the hose outside, and seal the opening around it. The seal matters because a single-duct unit blows room air out, which pulls replacement air into the room through every available gap — and the biggest available gap is usually the open window the hose is hanging out of. An unsealed opening lets the hot air you just expelled, plus warm outside air, flow straight back in past the hose.

Vertical sliding sashes — standard in Victorian and Edwardian terraces and conversions — are actually one of the easier cases. Raise the lower sash a little more than the hose outlet's height and fill the opening with a rigid infill panel: purpose-made sash vent kits exist, or a board of plywood or rigid foam board cut to the width of the frame with a hole for the hose connector. The sash then closes down onto the panel.

• Seal the edges of the panel with self-adhesive foam strip — the goal is no visible daylight around it.
• Fill the gap between the two sash panes (created by opening the window) with a foam strip too; it is the leak everyone forgets.
• For security on lower floors, add a sash stop or lockable restrictor so the window cannot be lifted further from outside.

Side-hinged casements and tilt-openers — the standard on most post-1980 UK housing — cannot hold a board, so the usual answer is a fabric seal kit: a zipped sheet that sticks with hook-and-loop tape around the window frame and the opening casement, with the hose passed through the zip. The window stays "open" but the opening is sealed to the hose.

• Measure the full perimeter of the opening — frame plus casement — before buying a kit; kits are sold by total seal length.
• Fit the tape to clean, dry frames and let the adhesive bond before hanging the fabric on it.
• Tilt-and-turn windows in tilt position work particularly well: the fabric bridges the tilted pane and the frame with the hose exiting at the top.
• Do not try to vent through trickle vents or letterbox-style openings — the duct needs its full diameter; throttling it chokes airflow and cooks the unit.

Everything above is reversible: boards lift out, fabric kits peel off leaving at most tape residue, and nothing needs a landlord's permission in practice — though checking your tenancy terms costs nothing. Balcony and patio doors can take a tall version of the same approach, either a fabric kit sized for doors or a hinged infill strip. What does not work is "venting" into the hallway or an unused room: the heat stays inside the flat's envelope, migrates back, and you have paid to rearrange it.

If the unit will live in one place for years and the wall is yours to drill, a through-wall vent — a cored hole sized to the duct, sleeved, with an external louvred or self-closing cap — is the tidy permanent answer: better sealing, no window compromise, faster setup each summer. It is a proper masonry job with building-regs and lintel considerations, so if in doubt have a tradesperson do the hole.

• Keep the hose as short and straight as the setup allows — extra length and tight bends add back-pressure, and the exhaust hose itself radiates heat into the room along its whole run.
• Do not extend the duct with improvised piping beyond what the manufacturer supports; the fan is specified for the supplied duct.
• An insulating duct sleeve (or a DIY wrap) over the hose is a cheap upgrade that noticeably cuts the heat the hose gives back to the room.
• Check for daylight around the seal after installation, and again after windy days — hook-and-loop fabric kits work loose.

Cooling air condenses moisture out of it, and the water has to go somewhere. Most modern portables re-evaporate the condensate onto the hot condenser coil and expel it through the exhaust as vapour, so in ordinary use the tank fills slowly or not at all. In a muggy UK heatwave — exactly when the unit works hardest — expect the tank to need emptying more often, and expect the unit to stop with a full-tank warning at 3am if you ignore it.

• Site the unit on a level floor so the internal tank and pump behave as designed.
• If the unit offers a continuous-drain spigot and there is a lower drain point nearby, a gravity drain hose removes tank duty entirely.
• Empty the tank and dry the filters before storing the unit for winter — a damp sealed unit grows exactly what you would expect.

One last planning note: seal kits, infill boards and duct insulation share the portable-AC demand curve — when a heatwave empties the shelves of units, the venting accessories often follow. If you are buying a unit in the calm season, buy the venting solution for your window type in the same trip.