Bi-fold doors turn a wall into glass. The cooling load is the solar gain through that glass on a hot afternoon, plus the rest of the room, with the doors closed. You cannot air condition the garden. A single small wall unit at the kitchen end often leaves the far seating warm, or chills the people nearest the indoor unit. Survey the glass, use shading where you can, and expect a higher kW or a second indoor in a deep open-plan room.
Size the room with the bi-folds shut
When the doors are open, the unit is fighting the outside. Design for the doors closed, which is how you will actually use cooling on the hottest days. Survey glass area and orientation, not just floor area. A wide opening also means a long room. Air from one indoor unit decays before it reaches the back of a 8 m kitchen diner. Two wall units, or a cassette where there is a void, often feel more even than one oversized head blasting the sofa. Deep plans may need two indoors.
A solid return wall beside the doors, or the wall opposite, usually beats the short piece of wall above the frame. The chassis needs a flat fixing and a drain. Do not bury it in a pelmet that stops the filter coming out. If the kitchen is in the same volume, add cooking heat to the survey. The doors are not the only gain. An extractor over the hob removes cooking fumes and is not a substitute for the split.
The same glass in winter, and the condenser beside the extension
A reverse-cycle unit can take the chill off a glazed room in spring and autumn. At night the glass loses heat, so read the heating capacity. Air-to-air heating, as Energy Saving Trust describes it, serves the room the indoor unit is in. Condensate in cooling mode still needs a route that does not track across the new floor. Pumped drains are common when the indoor unit is on an internal wall far from an outside gully.
Side returns get tight once the extension lands. The condenser needs airflow, not a slot between the bi-folds and the fence. Keep the outdoor unit off the elevation you have just spent money opening. Ground stands in the garden, or a side wall with real clearances, should be decided with the door design, not after the cladding is finished. F-gas installation and, in England, Planning Portal conditions apply. A brand-new rear elevation is exactly where neighbours notice a noisy box. Ask for sound data before the slab is poured.
Frequently asked questions
Will one wall unit cool a kitchen diner with bi-folds?
Sometimes, if the room is modest and shaded. Deep, south or west glazing often needs more kW or a second indoor. Survey the glass with the doors treated as closed.
Should I run the air conditioning with the bi-folds open?
You will be cooling the garden and wasting electricity. Use the system with the doors shut on hot afternoons.
Do bi-folds mean I need commercial plant?
Not by themselves. Many homes stay on one or two domestic splits. Commercial equipment appears when the volume and the gains leave domestic ranges behind.
Do internal blinds cut the bi-fold gain?
Internal blinds help less, because the heat is already inside the glass line. Low-e glass and external shading cut the gain the split must cover.
Will heating mode fix a cold wall of glass?
It can take the chill off in spring and autumn, in the room the indoor unit is in. It does not replace curtains, lining or a serious look at comfort when the doors are a cold wall. Read the heating capacity.
Can the condenser sit in the new side slot?
Only with real airflow clearances. A slot between the bi-folds and the fence cooks the unit. Decide a garden ground stand or a side wall with gaps while the door design is still open.
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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.