Air Conditioning Without an Outdoor Unit

Some homes make an outdoor unit difficult: a flat with no balcony, a listed building, a terrace with no space at the back, or a lease that does not allow one. There are ways to have air conditioning without a visible outdoor unit, but each one involves trade-offs. This guide explains the options honestly.

Why Most Systems Have an Outdoor Unit

Air conditioning works by moving heat out of the room. Most systems release that heat into the outside air through an outdoor unit, which holds the noisiest parts: the compressor and the main fan. That is what makes split systems quiet and efficient indoors. Without an outdoor unit, the heat has to go somewhere else: out through holes in the wall, out of a window through a hose, or into water.

Option 1: Through-Wall Units

These units, sometimes called monoblock or all-in-one units, put everything into one indoor unit mounted on an outside wall. They draw in and expel outside air through two holes in the wall, typically around 16 to 20 cm across, finished outside with grilles.

• Pros: no outdoor unit, and only two grilles visible outside.

• Cons: louder indoors than a split system, because the compressor is in the room. The quietest through-wall models are rated at around 27 dB(A), compared with 19 dB(A) for the quietest split wall units. Output is lower, with many models around 2 to 3 kW, and you still need two large holes through the wall and, for heating models, a condensate drain.

Option 2: Water-Cooled Units

Water-cooled units get rid of the heat into water instead of the outside air. There is no outdoor unit and no need for large holes in the wall, which is why they are used in flats, basements and listed buildings where nothing can be fitted outside.

There are two main types:

• All-in-one units mount on an inside wall and connect to a cold water supply and a drain. The compressor is in the room, so they are similar in noise to a through-wall unit.

• Systems with an indoor condensing unit, where the water-cooled condenser sits in a cupboard or utility space and serves an indoor unit in the room. Some larger buildings have a shared water circuit that units can connect to instead of the mains.

The drawbacks are cost and water:

• Expensive to buy. Water-cooled systems start from £5,000. The equipment costs considerably more than a standard split system, and larger systems are often designed specially for the building.

• Expensive to run. Mains-water units send water to the drain whenever they run. At full flow, some use several hundred litres of water an hour, and units that also heat use water in heating mode as well. On a water meter, that adds up quickly.

• Plumbing. The unit needs a cold water supply and a drain nearby, and the plumbing has to meet the water fittings regulations.

Water-cooled air conditioning is usually the option of last resort. It is worth it when nothing can go on the outside of the building, but it is not the first choice when an outdoor unit can go somewhere.

Option 3: Portable Units

A portable unit needs no installation, but it needs a hose out of a window, it is noisy and it is less efficient. It is a short-term answer rather than a permanent one. See do portable air conditioners work?

Option 4: A Split System With a Hidden Outdoor Unit

Often the better answer is a normal split system with the outdoor unit somewhere you will not see it. Refrigerant pipework can run further than most people expect. Many wall units allow up to 20 metres of pipework between the indoor and outdoor units, and larger models allow more. Possible positions include:

• a flat roof, at least 1 metre from the edge

• a rear wall, side passage or back yard, out of view from the street

• ground level behind a fence, hedge or screen, with space left for airflow

• a balcony or terrace, where the lease allows it

Outdoor units can also be fitted with screens or covers designed to let air through. A cover must not block the airflow, or the system will lose efficiency.

Flats and Apartments

Fitting anything to the outside of a block of flats normally needs agreement from the freeholder or management company, and your lease may set out what is allowed. Planning rules are tighter too. Under permitted development in England, an outdoor unit on a block of flats must be no larger than 0.6 cubic metres, compared with 1.5 cubic metres for a house. We can provide the details your freeholder or council will ask for. If nothing at all can go on the outside of the building, a water-cooled unit may be the only way to have fixed air conditioning.

Listed Buildings and Conservation Areas

Permitted development does not apply to listed buildings, so an outdoor unit, or new holes through the walls, will need listed building consent from the council. Conservation areas have extra restrictions, such as on walls facing a road. Because through-wall units need two large holes in an outside wall, they are not automatically the easier option on a listed building. Water-cooled units avoid changes to the outside, which is why they are often chosen for listed buildings. Listed building consent can also cover work inside, so check with the council before any pipework goes in. See our planning permission guide.

Which Option Is Right for You?

If an outdoor unit can go anywhere out of sight, a split system gives you the quietest and most efficient result, with the lowest running costs. If there is nowhere for one but the wall can take two holes, a through-wall unit is the next option. If nothing can change on the outside of the building, a water-cooled system can do the job, at a higher cost to buy and to run. We will look at the options with you at the survey and give you an honest recommendation.

Frequently Asked Questions

Is there an air conditioner with no outdoor unit and no holes in the wall?

Yes. Water-cooled units get rid of the heat into water instead of the outside air, so they need no outdoor unit and no large holes, just a cold water supply and a drain. They are expensive to buy and to run, and mains-water models use water the whole time they are running.

Do water-cooled air conditioners use a lot of water?

Mains-water models can. Water passes through the unit and into the drain whenever it runs, in heating as well as cooling. At full flow, some use several hundred litres an hour, so it is worth checking the figures, and your water tariff, before you choose one.

How far can the outdoor unit be from the indoor unit?

It depends on the model. Many wall units allow around 15 to 20 metres of pipework, and larger units allow more. Every system has to be designed within the manufacturer's limits.

Can the outdoor unit go in the loft?

Not an ordinary air-cooled one, because it needs a large, free flow of outside air to work. A water-cooled condensing unit can sit indoors, for example in a cupboard, because it uses water instead of air. Loft rooms themselves are a different matter, and our guide to loft conversion air conditioning explains the options.

Call 01772 288651 or use the form below to book a free survey. We will measure the room, recommend the right system and give you a clear written quote.

Last updated: November 2026.