A fan coil unit, or FCU, is a simple box that heats or cools the air in a room. It has two main parts, which give it its name: a fan and a coil.

A ducted fan coil unit is one that is hidden away, usually above a suspended ceiling, and connected to the room by ducts. All you see in the room are neat grilles in the ceiling or wall.

What is inside

A coil: a set of thin pipes with metal fins, a bit like a car radiator. Hot or cold fluid flows through it.
A fan: this draws air from the room, pushes it across the coil and sends it back out.
A filter: this catches dust before the air reaches the coil.
Controls: these turn the fan up or down and open or close the flow through the coil to reach the temperature you have set.
A drain: when cooling, moisture from the air condenses on the cold coil, just like water on a cold glass. A tray and drain carry it away.

Where the heat and cold come from

The fan coil unit does not make heat or cold itself. It is fed by a heat pump or another central plant.

In some systems the coil is fed with heated or chilled water. In others it is fed directly with refrigerant from a heat pump. This second type is called direct expansion, or DX, because the refrigerant expands and absorbs heat inside the coil itself.

Some water systems have two pipes, so the whole system either heats or cools at any one time. Others have four pipes, with separate hot and cold supplies, so one room can be heated while another is cooled.

How the ducts work

Ducts carry air from the unit to supply grilles in the room. Return air comes back to the unit through another grille or through the ceiling void. One unit can serve several grilles, so air can be spread evenly across a larger space.

Some ducted units can also be connected to a fresh air supply, bringing in outside air as well as recirculating room air.

Why choose a ducted unit

Out of sight. The equipment is hidden, which suits offices, showrooms and anywhere appearance matters.
Quiet. The fan sits away from the room, so less noise reaches people working there.
Even comfort. Air can be delivered through several grilles rather than from a single point.
Heating and cooling in one system.

Things to plan for

Ceiling space. There needs to be enough depth above the ceiling for the unit and its ducts.
Access. Filters need cleaning and units need servicing, so access panels must be planned in.
Drainage. The condensate drain needs a proper route away, sometimes with a small pump.
Duct design. Ducts that are too small or badly routed make units noisy and less effective.

Ducted fan coil units work best when they are designed together with the heat pump that feeds them and with the building around them. Done properly, they give quiet, efficient heating and cooling that you barely notice is there.