A plenum box is the sheet-metal box on a ducted or concealed indoor unit that connects the unit's outlet, and sometimes its inlet, to rigid or flexible ducts and then to grilles. It lets one hidden fan coil serve several rooms or several points in one room. It needs height in the void, a way to reach the filter, and a design that does not roar. Agree the box before the ceiling closes. A pretty grille with no thought behind it is only a hole.
Spigots, returns and a ceiling that is still open
The indoor unit sits in a bulkhead or a ceiling void. Its fan cannot blow straight into a room without a transition. The plenum is that transition: a box with spigots, insulated so it does not sweat, sized so the air leaves at a sensible speed. Supply plenums feed the rooms. A return plenum or a simple return duct brings air back to the filter. On some cassettes people say plenum when they mean a decorative adaptor. A cassette blows directly from its fascia. A plenum is the box that lets a concealed unit feed ducts and remote grilles. Ask the drawing to show a ducted indoor, the box, the duct diameters and the grille locations. Names vary by manufacturer. The air path is what matters. A joist depth that looked generous on a section can be full of downlights, soil pipes and the indoor unit itself. The plenum then has nowhere to go, and the installer is offered a crushed flex duct. Airflow collapses, noise rises, and the room furthest from the unit never cools. Walk the void with the surveyor while it is open. Mark the unit, the box, the drain fall or the pump, and the access panel. Fire stopping and acoustic separation between bedrooms belong in that same conversation, with the builder, not as a snag after handover. The builder should make the box only to the installer's dimensions and insulation specification, or the air side will be wrong. Many installers supply the box. Agree who makes it before the ceiling is ordered.
Whistle, balance and a panel you can open
Small spigots and long flex runs whistle. Linings inside the box can calm a hard metal tone. Balancing dampers let the installer share air between a bedroom and a dressing room instead of dumping it all at the first grille. Ask for that balancing to be part of commissioning, and ask what the rooms should feel like with doors shut. The filter is usually at the unit, sometimes at a return grille. If the only way to reach it is to destroy the bulkhead, the SEER you paid for will be gone by the second summer. Specify a hinged panel and a filter you can buy again. Unit model, plenum dimensions, duct sizes, grille types, access panel, drain and outdoor unit belong on the drawing. You still need an outdoor unit on a split. The plenum is only the indoor air path. The condenser, the lineset, the electrical supply and the F-gas commissioning are still required. GOV.UK publishes certification for the company that does the refrigerant work. The metalwork may be the same firm or a ductwork partner. One quote should own the join between them. As a 2026 illustration, a single concealed unit with a short plenum and two grilles in an extension often costs more than a wall split in the same room by a clear labour margin, commonly into the extra hundreds or beyond including VAT, because of the builder's work. You are paying for hidden plant. A wall unit is the better buy when the void is fiction. Where the system is large enough, GOV.UK air conditioning inspections are a separate duty. Energy Saving Trust's air-to-air pages describe the split behind the box.
Frequently asked questions
Is a plenum the same as a cassette?
No. A cassette blows directly from its fascia. A plenum is the box that lets a concealed unit feed ducts and remote grilles. They solve different ceiling problems.
Can the builder make the plenum?
Only to the installer's dimensions and insulation specification, or the air side will be wrong. Many installers supply the box. Agree who makes it before the ceiling is ordered.
Do I still need an outdoor unit?
Yes on a split. The plenum is only the indoor air path. The condenser, the lineset, the electrical supply and the F-gas commissioning are still required.
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How this guide was prepared
This guide was written by the Local AC Installers Editorial Team. We compare official guidance, legislation and established consumer information, separate general information from project-specific advice, and show publication dates and sources. We do not accept installer accreditation claims at face value.