Air Conditioning Without an Outdoor Unit: Where Does the Heat Go?
Flats, listed buildings and difficult façades can still be cooled. Compare fixed monoblock and water-cooled systems without pretending they have the same cost, noise or building impact.
“No outdoor unit” describes two different systems
Air conditioning moves heat; it does not destroy it. A normal split carries heat in refrigerant to a condenser outside. When that box is removed, the design needs another path.
| System | How heat leaves | Visible outside |
|---|---|---|
| Fixed air-cooled monoblock | Outside air enters and leaves through two wall ducts | Two external grilles, but no condenser box |
| Open-loop water-cooled | Mains water absorbs heat and discharges to drain | Potentially nothing on the façade |
| Building water-loop system | Shared loop carries heat to central plant | Depends on the building plant |
| Portable single-hose unit | Hot air exhausts through a window hose | Temporary window opening and hose |
A portable unit is technically “without an outdoor box”, but it is not the fixed architectural solution most searchers mean. It places a noisy compressor in the room and exhausts through a window; the open or imperfectly sealed window can draw hot air back inside.
Option one: fixed through-wall monoblock
The compressor, evaporator and condenser live in one indoor cabinet. Two core-drilled ducts bring outside air across the hot side and exhaust it again. Reversible models can heat as well as cool. Installation avoids refrigerant pipework between separate indoor and outdoor units, but it still needs substantial wall penetrations, condensate management and electrical work.
This is often the most practical answer for a single flat room where an external box is refused but discreet grilles may be accepted. The indoor cabinet is larger than a typical high-wall split and the compressor is in the occupied space, so check physical dimensions and night-mode noise in person where possible.
Option two: fully internal water-cooled cooling
Water-cooled systems reject heat into water instead of outside air. In some London flats and sensitive buildings, that allows all visible equipment to remain inside. Designs range from a compact packaged unit to more concealed arrangements with internal fan coils.
The crucial distinction is between a building’s closed heat-rejection loop and an open-loop system consuming mains water and sending warmed water to drain. Ask which one is proposed. For an open loop, demand the designed flow rate, wastewater route, annual water-cost model, leak protection, shut-off controls and any landlord or water-company requirements. “No condenser” is not enough specification.
Use the dedicated water-cooled air conditioning guide to compare costs, water use, plant arrangements and the survey specification. Flat owners should also check the full freeholder, planning and installation checklist.
Installed costs in 2026
| Solution | UK planning range | Cost drivers |
|---|---|---|
| Fixed monoblock, one room | £2,000–£4,000 | Unit capacity, wall type, access, grilles, drain and electrics |
| Water-cooled, one room | £3,000–£6,000+ | Water connection, drain, concealment and specialist plant |
| Two-room specialist system | £6,000–£12,000+ | Multiple fan coils, pipe routes, controls and acoustic work |
| Standard split comparison | £1,400–£3,200 | Where an outdoor condenser is permitted |
These are editorial planning ranges, not supplier tariffs. Specialist systems can be higher, particularly in central London or where heritage approvals, difficult core drilling, ceiling work and premium acoustic treatment are involved. Compare total ownership: supply, installation, electricity, water, servicing, filters, pumps and future parts.
If planning is the only assumed blocker, read the full UK planning permission guide before paying the specialist-system premium. Some houses can use permitted development; flats and listed or sensitive buildings need more care.
Noise: compare like with like
A premium wall-mounted split can run its indoor fan very quietly because the compressor is outside. A monoblock keeps the compressor indoors. Manufacturer figures may quote sound pressure at different distances, fan speeds and room assumptions, so a lower-looking number is not automatically quieter.
For a bedroom, request low, normal and maximum indoor sound data; ask whether compressor cycling, water valves or condensate pumps are included in the test; and identify the wall construction. Lightweight walls can transmit vibration. An acoustic mounting detail and a unit that modulates steadily matter more than a marketing word such as “silent”.
Efficiency and capacity
Air-cooled monoblocks can deliver genuine fixed cooling and heating, but the compact cabinet and wall ducts constrain airflow and model choice. A split usually has a broader range of capacities and keeps the hot, noisy section outside. Water-cooled performance may remain stable in hot weather, but electricity efficiency must be considered alongside water consumption.
Ask for rated cooling output, seasonal efficiency where published, electrical input at rated conditions and performance at the design outdoor and indoor temperatures. Then get a room heat-load calculation. A fashionable 2.3kW monoblock will not rescue a large south-facing glazed room if the load is twice that.
Building permissions and physical checks
Wall construction: the survey needs wall thickness, structure, insulation, services and a safe external drilling position. Two grilles cannot be placed where exhaust air immediately recirculates into the intake.
Façade consent: no box does not mean no external alteration. Freeholders, managing agents, conservation officers and listed-building controls may care about grilles, especially on principal elevations.
Water and drainage: confirm pressure, isolation, metering, leak detection, drain capacity, noise and access. Do not accept permanent hoses or undocumented discharge.
Service access: fully internal equipment still needs filters, coils, pumps, strainers and controls maintained. Concealed units need access panels sized for component removal.
When the specialist option is worth it
Choose no-outdoor-unit air conditioning when a documented external-unit restriction, façade constraint or building-services opportunity makes a standard split genuinely impractical. It can turn an otherwise unusable top-floor bedroom, listed-property office or premium apartment into a comfortable space without a visible condenser.
Choose a standard wall-mounted split where it is allowed and a short, considerate outdoor route exists. It generally offers more brand choice, lower indoor noise, familiar servicing and lower installed cost. For several rooms, compare a multi-split system before assuming each room needs a specialist internal unit.
The quote questions that expose the design
Where exactly does the heat go? What is visible outside? What are the wall-hole sizes? What are rated and night-mode sound levels? What is the cooling output at design conditions? How much electricity and, if applicable, water is used per hour? Who obtains freeholder or planning consent? How is condensate handled? Which filters and pumps need servicing? What parts and labour warranty applies?
For refrigerant-containing systems, installation, service and leak work still require the correct F-Gas qualifications. “All indoors” does not remove that legal requirement.
Air conditioning without an outdoor unit FAQs
Can air conditioning work without an outdoor unit?
Yes, but the heat must still leave the room. An air-cooled monoblock contains the compressor indoors and rejects heat through two wall ducts. A water-cooled system transfers heat to mains or a building water circuit and then to a drain or central heat-rejection system. Neither creates cooling without rejecting heat somewhere.
How much does air conditioning without an outdoor unit cost in the UK?
A fixed through-wall monoblock typically budgets around £2,000–£4,000 supplied and installed, depending on capacity, electrics and wall work. Fully internal water-cooled systems often start around £3,000–£6,000+ per room and can cost more for multiple zones or concealed units. A survey must confirm the heat-rejection method, water arrangement, drainage and permissions.
Does a monoblock air conditioner need holes in the wall?
An air-cooled fixed monoblock usually needs two large circular ducts through an external wall, plus a smaller condensate drain or another approved disposal arrangement. Outside there is no condenser box, but there are visible grilles. Exact diameters and clearances vary by model.
Is air conditioning without an outdoor unit quiet?
Usually not as quiet as a good split system because the compressor and both heat exchangers are inside the building. Premium fixed monoblocks can be suitable for bedrooms, but compare low-fan sound-power or sound-pressure data using the same test basis. Water-cooled plant may be concealed elsewhere, yet pumps and internal equipment still need acoustic design.
Is water-cooled air conditioning expensive to run?
It uses electricity and, in open-loop designs, mains water whenever cooling rejects heat. The water cost and environmental impact depend on flow rate, operating hours and tariff, so request the manufacturer’s litres-per-hour figure at stated conditions and calculate it against your water and wastewater charges before buying.
Do I need planning permission if there is no outdoor condenser?
Removing the external box may solve the largest visual objection, but new façade grilles can still need planning, freeholder or listed-building consent. Flats, conservation areas and listed buildings deserve written confirmation before drilling. A fully internal water-cooled design may avoid grilles but still needs landlord and building-services approval.
Is a no-outdoor-unit system better than a split system?
It is better where an external condenser is genuinely prohibited or physically impossible. Where a split is permitted, the split is normally quieter indoors, has a broader choice of capacities and brands, and can be more economical. Treat “no outdoor unit” as a constraint-led solution, not an automatic upgrade.
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