Ducted air conditioning

Where ducted is the right answer

• Period and listed interiors, where a wall unit is visually unacceptable and frequently would not get consent – see retrofitting cooling into period buildings.
• Rooms where wall space is precious or where there is no suitable high wall.
• Serving several rooms from one indoor unit, where a loft or ceiling void allows it.
• Where appearance genuinely matters – a formal reception room, a bedroom somebody objects to a box in.
• Open-plan spaces, where multiple grilles distribute air more evenly than a single wall unit can.
• Where noise at the grille matters, because the fan is remote from the room.

Where it is not the right answer: a single room in a house with no void, on a tight budget, where a wall or floor unit would do the job for a fraction of the cost and disruption.

What it needs

Designing the ductwork

Duct design is where ducted installations succeed or disappoint, and it is invisible once the ceiling is closed.

1. Rigid duct wherever possible. Flexible duct is quick and it accommodates a sloppy route, and it costs airflow at every sag and bend.
2. Short runs. Every metre and every bend reduces the air actually delivered.
3. Swept bends rather than tight ones. Two gentle turns beat one sharp one.
4. Consistent diameter. Reductions increase resistance disproportionately.
5. Balanced branches, so the furthest grille gets its share. This is the same principle as balancing a heating system and it is skipped for the same reason.
6. Insulated through cold space, with a vapour barrier.
7. Supported properly, so it does not sag into a low point.
8. Return path planned, whether that is a return grille, a plenum, or transfer through door undercuts.

A well-designed duct system is quiet, even and unobtrusive. A poor one is noisy at the grilles, uneven between rooms, and delivers considerably less than the unit is capable of.

Installation

1. SurveyVoid depths measured, routes established, grille positions agreed, and the load calculated per room – see sizing air conditioning.
2. Agree the access provisionWhere the service hatch goes, before anything is built. This is the conversation people skip and regret.
3. Indoor unit mountedIn the void, on anti-vibration mounts, level, with the condensate fall established from that position.
4. Ductwork runInsulated, supported, with balancing provision.
5. Refrigerant pipework and condensateTo the outdoor unit and to drainage.
6. Grilles fittedPositioned for distribution, and adjustable where useful.
7. Pressure test, evacuate, chargeThe invisible half that determines how long the system lasts – see what an installation involves.
8. Commission and balanceAirflow measured at each grille and adjusted, not left to chance.

Living with one

• Filters still need cleaning, and on a ducted unit that means using the access hatch. Monthly in heavy use.
• The condensate path still blocks, and in a ceiling void the consequence is worse than water down a wall. It is checked at every service.
• Grilles need occasional cleaning, and they show dust.
• Noise comes from the grilles, not the unit, and a well-designed system is very quiet. Whistling at a grille usually means airflow velocity is too high, which is a balancing or sizing issue.
• Servicing is more involved than a wall unit, which is the trade-off for the appearance.
• Zoning between rooms on one unit is possible with motorised dampers, and it adds complexity – worth it in some buildings and not in others.

See air conditioning servicing for what a proper visit covers.