Air conditioning without an outdoor unit: the wall monobloc, honestly
Contents (8)
Across urban Europe, the obstacle to cooling a flat is rarely the machine. It is the façade: the wall belongs to the building, the building has an opinion, and in a conservation area the municipality has one too. That is why a product category thrives here that barely registers in the United States or India — the fixed wall monobloc, an air conditioner with no outdoor unit, its compressor and condenser inside the indoor casing, breathing through two holes in an external wall.
What a wall monobloc actually is
Strip the marketing and you have a complete refrigeration circuit in one box roughly the size of a tall radiator, fixed against an outside wall. Two ducts pass through that wall, core-drilled at 160 to 200 mm — 162 and 202 mm have become the de facto European bore sizes, which is why replacing an existing unit can often reuse the old holes. One duct draws outdoor air across the condenser; the other pushes the heated air back out. The refrigerant circuit never leaves the casing: welded, factory-charged, hermetically sealed.
In EU regulatory vocabulary this is a double-duct air conditioner, a category defined alongside single-duct portables in Commission Regulation 206/2012 on ecodesign for air conditioners up to 12 kW. That distinction matters more than any specification on the box. A single-duct portable rejects heat through one hose and takes its condenser air from the room, so it depressurises the flat and drags hot outdoor air back in through every gap — the documented reason portables underdeliver, covered in our comparison of portable versus split air conditioners in Europe. A double-duct monobloc takes and returns its condenser air outside. No infiltration penalty, no negative pressure, no machine quietly re-swallowing the heat it just expelled.
The price you pay, in three numbers
| Wall monobloc (double duct) | Single-duct portable | Wall split | |
|---|---|---|---|
| Efficiency, typical | EER about 2.6–3.3 | EER about 2.4–3.1 declared, less delivered | SEER about 6–8.5 |
| Sound in the room | ~40–50 dB(A): the compressor is indoors | ~50–65 dB(A) | ~19–26 dB(A) at low speed |
| Capacity per unit | roughly 2–3.5 kW, one room | roughly 2–3.5 kW nominal | 2–7 kW, multi-room versions available |
| What it needs from the building | two wall bores and grilles | a window and a socket | outdoor unit position, consent, certified installer |
Read the efficiency row carefully, because the energy label makes the comparison harder than it looks: double-duct and single-duct units are rated on EER, a full-load figure, while splits are rated on seasonal SEER. The letters are therefore not interchangeable — an A+ monobloc and an A+ split are measured on different scales, a trap set out in our guide to reading the EU energy label. The honest summary is that a monobloc uses roughly twice the electricity of a good split for the same cooling, with the true gap slightly narrower than the raw ratio suggests because part-load seasonal testing flatters the split.
Put a season on it. A bedroom needing 2.5 kW of cooling for 400 hours delivers 1,000 kWh of cooling. At EER 3.0 the monobloc draws about 330 kWh; a SEER 7 split draws about 145 kWh. At an indicative European household rate of €0.25/kWh, that is roughly €83 against €36 — a difference near €50 for the summer, perhaps €500 across ten of them. Real, but rarely decisive against a split installation you are not allowed to carry out.
Noise is the deal-breaker, not efficiency
The compressor lives in the room. Manufacturers have worked hard on this — inverter models running at low speed are far quieter than the first generation — but a machine that reaches 40 to 50 dB(A) in a bedroom is a different proposition from a split indoor unit whispering at 20. Check the sound power level printed on the energy label rather than the brochure adjective, and if the unit is for a bedroom, plan its position away from the head of the bed and expect to sleep with it in its lowest speed.
What the installation job involves
- An accessible external wall. Standard duct kits handle walls up to about a metre thick; the two bores are diamond core-drilled with a slight fall towards the outside so that driven rain runs away from the room.
- Grilles that respect the spacing. Intake and exhaust must not short-circuit into one another, which is exactly what happens when someone improvises the hole positions to suit the furniture.
- Condensate handling. Most current models evaporate condensate into the exhaust airflow, so no drain is normally visible. In muggy coastal weather the machine can still produce more water than it can evaporate, which is when a small drain earns its keep — the mechanism is the same one described in our guide to water dripping from an indoor unit.
- No refrigerant handling. Because the circuit stays sealed, installation does not open it, so no F-gas certified technician is legally required for the fitting itself. This is the category’s second genuine advantage, and the reason landlords and competent owners like it.
- But still a building matter. Two 200 mm holes through a shared or load-bearing wall, and two grilles on a street elevation, are decisions the building and sometimes the planning authority get to make — the three consents are laid out in our guide to permission to install air conditioning in Europe.
Refrigerant: why this category went to propane first
Many current monoblocs use R-290 (propane, GWP 3) rather than R-32. That is not marketing: a small, welded, factory-charged circuit is precisely the architecture in which a flammable A3 refrigerant can be engineered safely, so the category moved ahead of splits under the phase-down set by Regulation 2024/573 on fluorinated gases. For a buyer it mostly means the machine will not be orphaned by the next quota step.
When it is the right machine
- The co-ownership refused an outdoor unit, or the building is listed — the everyday reality in the historic centres of Italy, Spain and France.
- You rent, a split is out of the question, and a portable is genuinely unbearable in the room concerned.
- One room needs permanent cooling — a top-floor bedroom that never drops below 27 °C in August.
- An outdoor unit would be exposed, stolen or vandalised, or there is simply nowhere to put one.
When it is not
- More than one room: there is no multi-split equivalent, so each room means another box, another pair of holes and another compressor indoors.
- Loads beyond about 3.5 kW: large or glazed south-facing living rooms exceed the category’s ceiling, and a split system is the correct answer if you can get consent for it.
- Heating-led use: reversible models heat competently in mild weather, but they are a supplement in a continental winter, not a heating system.
- Light sleepers, in a bedroom, where the indoor compressor noise is the thing that will make you regret the purchase.
Frequently asked questions
Is an air conditioner without an outdoor unit as efficient as a split?
No. Expect roughly half the efficiency: EER around 3 against a SEER of 6 to 8.5 for a decent split. The comparison is only meaningful when a split is actually installable — where consent is refused, the honest alternative is not a split but a portable, and against a single-duct portable the monobloc wins clearly on both electricity and comfort.
Can I install one myself?
Legally, in most of Europe, yes: no refrigerant circuit is opened, so no F-gas certification is involved. Practically, the two core bores are the hard part — dust, reinforcement bars, alignment, fall to the outside, and a wall that may be structural or shared. Many owners buy the unit and pay a builder for the drilling alone.
Do I still need the building’s permission?
Usually. Two discreet grilles are far less contentious than a bulky machine bolted to a balcony, and some co-ownerships that reject splits accept monoblocs for exactly that reason. But drilling collective walls and altering a façade remain building decisions, and conservation rules apply to grilles as much as to compressors. Ask before you drill, not afterwards.
Does it need a drain?
Normally no — condensate is evaporated into the exhaust air. In persistently humid weather, or if the unit is undersized and running flat out, water can accumulate faster than it evaporates and the safety cut-out stops the machine. A short drain to the outside solves it permanently.
Are they worth the money compared with a good portable?
If the installation is permanent and the room is used every summer, yes: no hose in the window, no depressurised flat, markedly lower noise than a portable and roughly a third less electricity for the same cooling. If you need cooling for two weeks a year in a rented studio, the portable is the rational purchase.
Sources
Read next
- Heating with your air conditioner through a European winter
- AC noise rules in Europe: keeping the neighbours (and the law) calm
- Multi-split systems: one outdoor unit, several rooms
- Portable vs split in Europe: renting, rules and the real math
- Why your air conditioner smells, and how to clear it for good