Ceiling cassette or wall mounted air conditioning for an office?

The two options in plain terms

Both are the indoor half of a split system. Refrigerant pipes connect each one to an outdoor unit, which in a City building is usually on a roof plant deck or in a designated plant area.

• A ceiling cassette sits in the ceiling void with only a square grille showing. Most office cassettes blow air out four ways and draw it back in through the centre.
• A wall mounted unit hangs high on a wall and blows air out of one side across the room.

	Ceiling cassette	Wall mounted
Needs a ceiling void	yes	no
Air spread	four directions, even across open plan	one direction, best for small rooms
Visibility	flush grille	unit on the wall
Typical use	open plan floors, larger meeting rooms	small offices, studios, treatment rooms
Installation	more work, ceiling grid and void	simpler, quicker
Condensate	usually built in pump to lift water	gravity drain or pump
Cost to fit	higher	lower

Open plan City floors

Much City office space is let with a suspended grid ceiling as part of a Cat A fit out, the landlord’s basic finish. Cassettes are designed for that. They drop into a ceiling tile position, so the layout can follow the desk plan, and the four way throw avoids the strong draught you get standing in front of a wall unit. On a deep floor plate, wall units cannot reach the middle of the room, which is the main reason cassettes win in open plan.

Condensate is easy to forget. Most cassettes include a small pump that lifts water up and into a drain run in the ceiling void. That run needs a route to a soil or waste pipe, which in a multi let building means agreeing a connection with the building manager.

Meeting rooms and small offices

In a 1970s or older building around Bank, Cheapside or Moorgate with plastered ceilings and no void, or in a small office over a shop in Farringdon or Clerkenwell, wall units are often the only practical option. They are also a sensible choice for single rooms on a budget. Meeting rooms are a common problem: lots of people, screens and glass walls in a small space. A dedicated unit for each meeting room, controlled separately from the open plan, stops one busy room overheating while the rest of the floor is comfortable.

Cat B fit outs

If you are taking a new floor and doing a Cat B fit out, which is the tenant’s own layout of rooms, kitchens and finishes, that is the right time to design cooling. You can:

• place cassettes to suit the new layout
• add ducted units above meeting rooms with only slot grilles visible
• plan comms room cooling with its own unit, see our guide to server room air conditioning
• coordinate pipe routes with lighting, sprinklers and data in the ceiling

Landlords in the Square Mile usually want to see the design, method statements and the installer’s F-gas certification before signing off a licence for alterations. Our guide to office air conditioning and landlord consent explains that process.

Rules that apply either way

Whatever indoor unit you choose, a few legal points apply to office systems:

• F-gas: anyone working on the refrigerant needs an F-gas qualification, and the company must be certified by an approved body, listed on gov.uk
• Leak checks: equipment holding refrigerant equal to 5 tonnes of CO2 or more must be leak checked at set intervals, with records kept (gov.uk)
• TM44: air conditioning with an effective rated output over 12kW must be inspected by an accredited energy assessor at least every five years (gov.uk guide)
• Planning: new outdoor plant on commercial property normally needs planning permission, and the City of London has 28 conservation areas

Working in a City building

City buildings run to tight rules: lift bookings, loading bay slots, permits for hot works, and restrictions on noisy drilling during office hours. Cassette installs involve more ceiling work, so they are often done in the evening or at weekends, floor by floor. Wall units are quicker and less disruptive. Our City of London page covers the local picture, and the office air conditioning page explains how a commercial job runs from survey to handover.

Which should you choose?

If you have a grid ceiling, an open plan floor and a landlord who will approve the work, cassettes give the most even comfort. If you have a few small rooms, no void or a tight budget, wall units are the practical answer. Many offices end up with both: cassettes on the open floor and wall or ducted units in meeting rooms.