FCU Plenum Boxes — Supply & Return

Smart Ventilation Systems fabricates custom plenum boxes for fan coil units to order in galvanised steel. Choose your FCU manufacturer and model — Daikin, Samsung, Mitsubishi Electric and more — or enter the connection size from the unit drawing, and build the plenum around it in our interactive 3D configurator.

Built to your fan coil unit

  • Supply and return air — discharge plenums with spigots to diffusers, or return plenums with optional filter sections
  • Stepped or tapered body — flat step plate to keep the box short, or a gradual taper where space allows
  • Acoustic lining — 6mm or 12mm internal foam lining to cut noise breakout
  • Spigots anywhere — click any face in the 3D view to place circular spigots exactly where the drawing calls for them

Live pricing from the body dimensions

Prices exclude VAT and update as you design. Baseline: a 600×200mm body, 400mm deep, with two Ø150 spigots is £64.00 unlined, £68.69 with 6mm foam or £73.39 with 12mm. A 980×180 body, 400mm deep, with four Ø150 spigots is £110.00 unlined. Width is interpolated between those anchors; height and depth add £0.05/mm above 200 and 400 (no discount below 200); each spigot after two adds £10.00; spigots over Ø150 add £0.05 per extra mm. Lining uses the gross six-face body area: approximately £5.3333/m² for 6mm and twice that for 12mm, calculated at the full rate before penny rounding. Openings are not deducted. Filter sections require confirmation from our team.

Manufacturer presets are estimated, editable starting points — not verified fits. See the FCU plenum brand guide for every preset and a pre-order site checklist.

FCU plenum box FAQs

What is an FCU plenum box?

A plenum box (or plenum chamber) connects a fan coil unit to the ductwork. On the supply side it distributes air from the FCU outlet into circular spigots feeding diffusers; on the return side it collects air back to the unit, often through a filter section.

Can you make a plenum to suit my fan coil unit?

Yes — every plenum is made to order to match your unit's connection size. Pick your manufacturer and model in the configurator and we'll build the plenum around it, or enter the connection size yourself from the unit drawing.

What is the difference between a stepped and tapered plenum?

A stepped plenum changes size with a flat step plate, which keeps the box short. A tapered plenum transitions gradually, which is gentler on airflow where space allows. Both can be made with a flat top or flat bottom to suit ceiling void constraints.

Do you line plenum boxes acoustically?

Yes — we can line the inside of the box with 6mm or 12mm acoustic foam to reduce noise breakout from the fan coil unit.

How much does an FCU plenum box cost?

The designer prices your plenum live, excluding VAT, from the body dimensions. A 600×200mm body, 400mm deep with two Ø150 spigots is £64.00 unlined, £68.69 with 6mm foam or £73.39 with 12mm foam. Lining is calculated from the gross six-face body area at approximately £5.3333 per m² for 6mm; 12mm is twice that rate. Extra width, height, depth and spigots add to the box price. Filter sections require confirmation.

See also our FCU plenums overview, the FCU plenum brand guide and FCU ductwork packages, or send us your FCU schedule to price the full set. Call 07710 984 957 — Mon–Fri, 8am–5pm.