Can you have air conditioning without an outdoor unit?

How does a unit with no outdoor box get rid of the heat?

Through the wall. A monobloc packs the compressor, both heat exchangers and the fans into one indoor casing about 165 mm deep. Behind it, two holes are cored through the external wall, typically 162 mm across, or 202 mm on the largest models, and fitted with self-closing grilles that sit flush with the outside wall. Outside air is drawn in through one, passed over the hot coil and blown out through the other. In winter the cycle reverses and the same two holes bring heat in. Source: Quality Heating.

Who is it for?

• Leaseholders whose freeholder refuses an outdoor unit. Two grilles are a smaller ask than a box on a bracket, though they still need consent.
• Flats in converted office blocks. Croydon has around a thousand of them in the town centre alone, usually with curtain-wall glazing and no balcony to stand a unit on.
• Blocks where the one permitted outdoor unit has already gone to someone else.
• Listed buildings, where consent for two small grilles on a rear wall may be easier to argue than consent for plant.

Source for the conversions: Inside Croydon.

What do you give up?

The efficiency gap is the one to notice. A monobloc moves less heat per unit of electricity, so a room that a split would cool for 9p an hour costs more from a monobloc, and a large or sunny room may be beyond it. Sources: Olimpia Splendid; Quality Heating; Energy Saving Trust.

Does it still need anyone's permission?

Often, yes. The permitted development conditions for heat pumps are written around an outdoor unit, so they do not arise, but two grilles are still an alteration to the outside of the building. In a flat, the wall belongs to the freeholder, and the lease will say whether consent can be withheld. In a listed building, cutting two holes through a historic wall affects its character and needs listed building consent. In a conservation area, ask the council before coring. On an ordinary house, grilles at the rear are rarely a planning matter, but we would still rather you asked than guessed. Sources: LEASE; Listed Buildings Act, section 7; Planning Portal.

Does it need an F-gas engineer?

Not to install, if the unit is sealed. A factory-charged monobloc has no refrigerant joints made on site, so the rules on who may connect a circuit do not bite, and the work is coring, fixing and the electrics. The electrics are still notifiable if a new circuit is needed. Any repair that opens the circuit, and any leak check on a larger charge, goes back to a qualified F-gas engineer. Models on R290, which is propane, also carry clearance rules of their own. Sources: Quality Heating; GOV.UK; IET.

Where does the water go?

In cooling mode the unit pulls moisture out of the air, up to 14.4 litres a day on the Unico Air, and it has to leave somewhere. A condensate connection can run to a drain, or a small pump can lift it to one. Some models evaporate part of it through the grilles. If there is no drain anywhere near the wall, say so at the survey, because that decides the model. Source: Olimpia Splendid.

Is a portable unit the same thing?

No. A portable sits on the floor and pushes its hot air out through a hose in a window, which has to stay partly open. A monobloc is fixed to the wall, breathes through sealed grilles and leaves the windows shut, which matters on a main road or a ground floor. The portable needs no holes and can move with you; the monobloc performs better and lasts longer. Our comparison covers portables in detail.