The air conditioner with no outdoor unit: the flat-friendly fix that comes with a catch

With an amber heat health alert in force and temperatures pushing 35C, the flat you rent or own on a leasehold can quickly start to feel like an oven, and the obvious fix is off the table. Fitting a split air conditioner means bolting a compressor to an outside wall, and for most flat dwellers that needs your landlord's consent, freeholder permission or, on some facades, planning approval. If you cannot put anything on the outside of the building, there is one type of unit that keeps everything indoors, the monobloc air conditioner.

What a monobloc air conditioner actually is

Unlike a split, a monobloc holds all of its working parts in a single body that sits inside the room. It pushes hot air outside through one flexible hose that you run through a tilted window or a hole in the wall. There is also a fixed version that vents through two small holes in an external wall, again with no separate box outside. Neither version needs the bulky outdoor compressor that causes all the permission problems, and that is exactly why it suits a flat.

The audience is specific: renters who need something they can unplug and take with them at the end of a tenancy, and leaseholders in blocks where the freeholder or managing agent will not sign off an external unit. A portable monobloc lets you cool a room without asking anyone's permission first. Our guide to portable air conditioners runs through the main types.

The catch: it costs more to run

Here is the part the showroom does not lead with. For the same amount of cooling, a monobloc is less efficient than a split, with a lower EER and SEER rating. A single-hose portable has a built-in flaw that shows up on every hour of your bill.

The mechanism is simple. To send hot air outside, that one hose draws air out of the room, air you have just paid to cool, and pushes it outdoors. That air has to be replaced, so warm air is pulled back in through the door, the window frame and every gap in the room. The upshot is that a chunk of your cooling is quietly cancelled out, and the machine works harder to reach the temperature you set. On top of that it is noisier than a split and only really cools the room it stands in.

What it costs to run

The running cost is easy to picture. A 1 kW portable at the current price cap of around 26p per kWh costs roughly 26p an hour, which is over £6 a day and close to £188 a month if you leave it running around the clock, before you add the daily standing charge of about 57p. Left on all day through a heatwave, an inefficient unit turns a comfort buy into a serious line on the bill. You can soften that with the habits in our guide to saving electricity.

How to choose one without wasting money

Before the price tag, get the size right. The BTU rating has to match the room's floor area and how much sun it gets: an undersized unit runs flat out and never quite cools the space, while an oversized one switches on and off constantly and wastes energy each time. Matching the BTU to the room matters far more than the sticker price.

Two practical steps do the rest. Seal the window kit tightly around the hose, because an unsealed gap lets much of your cooling escape and drags the running cost up, and if your budget stretches to it, choose a dual-hose model, which draws its exhaust air from outside and limits the warm air pulled back in. A well sealed, correctly sized monobloc closes much of the efficiency gap with a split. For the wider picture on fixed systems, see our air conditioning guide.

With the heat where it is, a monobloc remains the most realistic option if you cannot fit a split, as long as you choose it with your eyes open. It is not the most efficient machine on the market, but sized correctly, sealed carefully and used only in the room you are in, it is a sensible answer for a flat. Knowing what each hour of cooling actually costs is what separates a smart buy from a nasty surprise at the end of the month.