If you have seen a long white unit high up on an office or shop wall, blowing warm or cool air, you have probably seen a wall-mounted fan coil unit. Many people simply call it an air conditioning unit.

A fan coil unit, or FCU, heats or cools a room by blowing air across a coil filled with hot or cold fluid. A wall-mounted unit does this from a single box fixed to the wall, with no ducts.

How it works

Inside the casing are a coil, a fan, a filter and some controls. The fan pulls room air in through the top, passes it through the filter and across the coil, and blows it back out of the bottom. Louvres, which are adjustable slats, direct the air around the room.

The coil is usually fed with refrigerant from an outdoor heat pump unit. The two are connected by a pair of small pipes, plus a power and control cable, through a hole in the wall. This arrangement is often called a split system, because the equipment is split between inside and outside.

In heating mode, hot refrigerant warms the coil and the room. In cooling mode, the system runs in reverse, the coil turns cold and heat is carried outside. When cooling, moisture from the air collects on the cold coil and drains away through a small pipe.

Why choose a wall-mounted unit

Simple to install. There is no need for a ceiling void or ductwork.
Room-by-room control. Each unit has its own controls, so each space can be set to suit the people in it.
Heating and cooling from one unit.
Easy to reach for cleaning filters and servicing.
Well suited to adding heating and cooling to an existing building.

Things to think about

Appearance. The unit is visible, so where it goes matters.
Airflow. Air comes from one point, so the unit needs a clear position where it can reach the whole room without blowing straight at people.
Drainage. The condensate pipe needs a route to a drain, or a small pump if there is no fall.
Pipe routes. The refrigerant pipes between inside and outside should be planned so they are short, neat and protected.

How it compares with other indoor units

Wall-mounted units are one of several types. A ceiling cassette sits flush in a suspended ceiling and blows air out in several directions. A ducted unit is hidden above the ceiling and feeds air through grilles. Floor-standing units sit low on a wall, much like a radiator.

They all work in the same way. The right choice depends on the room, the ceiling, how the space is used and how much the equipment should be seen.

Part of a wider system

A wall-mounted unit is only as efficient as the heat pump behind it. Paired with a well-sized air-source heat pump, and ideally powered by solar and batteries, it can heat and cool a room using far less energy than electric heaters or a gas boiler. As with any system, careful sizing, placement and commissioning make the difference between a unit you never think about and one that is noisy or uncomfortable.