Air conditioning without an outdoor unit: does it work in a London flat?

Why people search for this in London

In a lot of London flats, the outdoor unit is the problem. The freeholder will not allow one, the only free wall faces the street in a conservation area, or there is simply nowhere it can go with enough air around it. Camden is a good example. The borough has 40 conservation areas covering about half its area, including Bloomsbury, Hampstead, Primrose Hill and Kentish Town, and much of its housing is Georgian and Victorian terraces split into flats.

The Greater London Authority’s heat risk study also named Camden among the central boroughs with higher heat risk for homes, partly because of its high share of flats. So demand is real, and so are the limits.

How an all in one unit works

An ordinary split system has two parts: the indoor unit and an outdoor unit with the compressor, joined by refrigerant pipes. An all in one unit puts everything in a single box inside the room. It draws outside air in through one duct, uses it to carry heat away, and pushes it back out through a second duct. The Energy Saving Trust describes the two holes as about 15 to 20cm wide, which makes them much less visible than an outdoor unit. Many models are reversible, so they can heat as well as cool.

Because the refrigerant circuit is sealed in the factory, there is no pipework to join on site. The engineer’s work is mostly cutting the holes accurately, fixing the unit, fitting the outside grilles and connecting power and a condensate drain.

How it compares with a split system

	All in one (monobloc)	Wall mounted split
Outdoor box	none, just two grilles	yes, on a wall, roof or frame
Holes in the wall	two, about 15 to 20cm	one small pipe hole
Noise in the room	higher, compressor is inside	lower, compressor is outside
Cooling power	lower	higher, wider range of sizes
Typical position	low or high on an outside wall	high on a wall
Best for	small rooms, no outdoor space allowed	most rooms where an outdoor unit is possible

The Energy Saving Trust also notes that indoor units can be noisier than other heat pumps, which is even more true when the compressor is in the same box.

Will it be quiet enough for a bedroom?

This is the main question to ask. In a bedroom, the indoor sound level at night matters more than anything else. Check the manufacturer’s published sound level at the lowest fan speed and compare it with split system indoor units of a similar output. Some people find an all in one fine in a living room or study but too noisy to sleep next to. If the bedroom is the priority, it is worth pushing harder for a split system position, even if it means a longer pipe run to the back of the building. Our post on bedroom air conditioning covers night noise in more depth.

Consent still applies

No outdoor box does not mean no permission. The two holes and the grilles change the outside of the building.

• Leaseholders need the freeholder’s written consent, because the outside wall is almost always part of the freeholder’s property.
• Renters need the landlord’s permission.
• Planning: grilles on a street facing wall in a conservation area can need permission. Whether an all in one counts as an air source heat pump under Class G depends on the unit and how it is used, and the “not solely for cooling” condition still applies. Ask the council if you are unsure.
• Listed buildings need listed building consent before cutting through historic walls.

In a Bloomsbury terrace, for example, a rear wall facing a garden or mews is far more likely to be accepted than the front.

Where the holes can go

Two holes of that size in a solid brick wall need care. We check:

• that the wall is solid and the holes avoid joists, lintels and window heads
• the outside surface, so grilles sit neatly on brick or render
• that nothing outside, such as a neighbour’s window, sits right in front of the exhaust
• a route for the condensate drain
• an electrical supply, usually a dedicated circuit under Part P

In flats with thick walls, the installer may need longer ducts or a different model.

When an all in one is the right answer

It makes sense when:

• the freeholder has refused an outdoor unit but will accept discreet grilles
• the only workable wall faces the street in a conservation area and grilles are acceptable where a box is not
• the room is small to medium, such as a study or a small bedroom
• you want something fixed and efficient, rather than a portable unit with a hose in the window

It is the wrong answer when you need strong cooling in a large, sunny room, or when you are a light sleeper and the unit is for a bedroom.

Other options before you decide

• A split system with a longer pipe run to a permitted roof or rear wall, if the freeholder agrees.
• A multi split, where one existing or shared outdoor unit can serve your flat, if the building already has one.
• Passive steps first: reflective blinds, shading on west windows and night ventilation.

The flat air conditioning page sets out how we approach each option, and the Camden area page covers local housing and planning points.