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Through-wall air conditioning units: when they suit a business

Updated 23 October 2026 · SEO Dons Editorial

What is a through-wall air conditioner?

A through-wall unit is a self-contained air conditioner that sits inside the room and swaps air with the outdoors through openings in the wall. Everything a split system divides between an indoor and an outdoor unit is in one cabinet. Two product types are sold, and data sheets use different names for them.

Where do through-wall units suit?

They suit single rooms where an outdoor condenser is unwelcome: a facade a landlord or council will not allow outdoor units on, a hotel bedroom, a student room, a small office or consulting room. The models checked for this guide are small. The Unico Easy S2 lists a cooling capacity of 2.0 kW, and the Unico Pro has "two maximum power models: 3.4 kW and 3.5 kW". For scale, the worked hotel example in the gov.uk inspection guide assumes 3 kW a room. They do not suit open-plan floors or any room that needs several times that capacity, and the hotel air conditioning page shows where they sit among other room systems.

What are the trade-offs against a split or cassette?

IssueThrough-wall unitSplit or cassette
Outdoor unitNone; air passes through wall openingsOutdoor condenser and refrigerant pipes
CapacityOne room; 3.5 kW is the top of the range checkedWide range, one unit to many rooms
NoiseCompressor and fans are in the roomCompressor is outside; only the indoor fan is in the room
WallOpenings and grilles in the facadeSmaller opening for pipes
CondensateNeeds a route, or heating mode is switched offCondensate pipe from the indoor unit
Efficiency ratingRated at standard conditions (EER, COP)Rated over a season (SEER, SCOP)

Condensate. The Unico Pro page says that "in the absence of condensate drainage" the unit can be configured at installation as "COOL ONLY", which deactivates heating.

Noise. The manufacturer pages checked describe noise in words, such as acoustic comfort "at any operating condition", not in decibels. The retained ecodesign rules cap a double duct unit's indoor sound power at 65 dB(A) at standard rating conditions (Annex I, Table 3). Sound power is not the sound pressure a person hears, so ask for the pressure level at a stated distance, in the room, with the test conditions.

Efficiency. The same regulation judges a double duct unit on its rated EER and COP "at the standard rating conditions", not on seasonal figures. From 1 January 2014 the minimum EER is 2.6 where the refrigerant's global warming potential is above 150 and 2.34 where it is 150 or below, whereas a split under 6 kW has a minimum seasonal SEER of 4.6 or 4.14, again depending on the refrigerant. The two tests differ, so the numbers cannot be compared directly. The guide to SEER and SCOP explains the seasonal side, and the comparison of cassette, ducted and wall-mounted units covers the rest of the family.

Do the wall openings need planning permission or building control?

Planning. Section 55 of the Town and Country Planning Act 1990 excludes from development works that "do not materially affect the external appearance of the building". Whether two grilles do is a judgement for the council, and the planning permission guide covers the permitted development position.

Listed buildings. Section 7 of the 1990 listed buildings Act says "no person shall execute or cause to be executed any works for the demolition of a listed building or for its alteration or extension in any manner which would affect its character" unless authorised. Cutting openings in a facade can be such a work. See air conditioning in listed buildings.

Building Regulations. Regulation 3 counts as building work the "material alteration" of a building, which it defines as work that would result "in a building or controlled service or fitting not complying with a relevant requirement where previously it did". The relevant requirements include "Part A (structure)" and "paragraph B4 (external fire spread)". An opening in a load-bearing or fire-resisting wall is the case to raise with building control before work starts.

What is the F-gas position for factory-sealed units?

GOV.UK says there is "no maximum period of time between leak checks" on "hermetically sealed refrigeration and air conditioning systems unless they contain F gas equivalent to 10 tonnes of carbon dioxide". Article 4 of the retained F-gas Regulation adds a condition: such equipment is not subject to leak checks "provided the equipment is labelled as hermetically sealed". The manufacturer's page for the Unico Easy S2 carries a footnote describing it as hermetically sealed equipment containing fluorinated gas. Check the nameplate on the actual unit, because the label, not the product type, is what the rule turns on.

The inspection duty works differently. The gov.uk guide counts "one or more air conditioning units within a building controlled by a single person" as a single system, so small room units add up. For example, five units of 2.5 kW give 12.5 kW, above the 12 kW threshold. GOV.UK's leak-check page was published on 21 August 2019 and shows no later update; the GB F-gas changes guide tracks anything newer.

What should a buyer ask before choosing a through-wall unit?

  1. What capacity does the unit give at the design conditions, and for how long can it hold them?
  2. What are the sound power and the sound pressure in the room, at what distance and under what test conditions?
  3. What is the energy class, and the rated EER and COP?
  4. Where does the condensate go, and does heating mode need a drain?
  5. How large are the openings, and what wall thickness and construction do they pass through?
  6. Is the unit labelled hermetically sealed, and what is its refrigerant charge?
  7. Can it join the building controls? The Unico Pro page lists an RS485 port for external BMS control in Modbus RTU, which shows what to look for, and the controls guide covers the wider picture.

Where a through-wall unit is one option among several, the commercial air conditioning overview sets out the system families. You can send an enquiry through the form to ask for quotes against your own room list.

Sources

Common questions

Can a through-wall unit heat as well as cool?

Some can. Olimpia Splendid lists heat pump versions of its Unico range, describing the heating mode as a way to replace or support traditional heating in intermediate seasons. Its Unico Pro can be set to cool only where there is no condensate drain.

How many through-wall units trigger an air conditioning inspection?

The test is combined effective rated output above 12 kW under one person's control, not a unit count. For example, five 2.5 kW units reach 12.5 kW. Inspections are by an energy assessor at intervals not exceeding five years in England and Wales.

What does double duct mean on a data sheet?

It is the ecodesign term for an air conditioner that takes condenser air from outdoors through one duct and rejects it through a second duct, placed wholly inside the room near a wall. Such units are rated at standard conditions rather than seasonally.

Can a through-wall unit go on a listed building?

Only with the right consent. Altering a listed building in a way that would affect its character needs authorisation under the listed buildings Act, and cutting openings in a historic facade is the kind of work that engages it. Ask the council's conservation officer first.

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