Ducted Air Systems in Australian Homes

What it is

A ducted air system distributes conditioned air from a central plant through a network of supply ducts to ceiling, wall or floor diffusers and pulls cool or warm air back through return-air grilles. In Australian homes the plant is most often a reverse-cycle heat pump, sometimes a gas furnace and sometimes a hydronic fan coil. The ducts themselves and how they are arranged decide whether the system is quiet and efficient or whistles, leaks and short-cycles.

Duct sizing

Duct sizing starts with the airflow rating of the indoor unit at the static pressure the manufacturer specifies (commonly 100 to 150 Pa external static for residential ducted units). Each branch then has to deliver the share of that airflow needed for the room it serves, sized off the room cooling load.

Undersized ducts raise velocity, raise noise and raise static pressure. Oversized ducts cost more and can cause poor throw from the diffuser. The usual residential target is supply velocity in the trunk at or below 6 m/s and at the diffuser at or below 3 m/s. Static regain or equal-friction methods are both used, the equal-friction method being more common in residential.

Return-air

Return-air sizing is where most residential ducted installs fall down. The return grille and return duct have to pass the full unit airflow at low pressure drop, typically with a face velocity at or below 2 m/s. A single central return on a high-airflow unit needs a grille of around 600 by 600 mm or larger. Undersized returns starve the unit, drop capacity and pull air around door undercuts in a way that whistles.

Each conditioned room either needs its own return path or a generous door undercut and a transfer grille. Closing the bedroom door on a system with a single hallway return is a classic complaint source.

Duct fabrication

Rigid sheet metal duct follows AS 4254.1. The standard sets sheet thickness by duct dimension. It also sets joint and reinforcement spacing, sealing classes A B and C plus pressure-test methods for each sealing class. Most residential supply duct is Class B.

Flexible duct follows AS 4254.2. Each piece is capped at 6 m. It has to be pulled taut, with bends radiused at least to the duct diameter and never crushed or kinked. Joints to the rigid spigot are made with a saddle clamp on the inner core then a second clamp on the outer jacket over the insulation.

Insulation

Ducts outside the conditioned envelope (in the roof space, in subfloors, in unheated garages) need insulation. The NCC calls this up under Volume Two energy efficiency provisions, generally R1.5 to R2.0 wrap depending on the climate zone. Inside the conditioned envelope ducts can be uninsulated, but acoustic lagging is sometimes added to quieten the system.

Ventilation interface

A ducted air-conditioning system is not the same as a ventilation system. Recirculating ducted air does not bring in outside air. Where mechanical outside-air ventilation is needed (typically apartment dwellings on a single aspect, or any space that cannot meet NCC Volume Two Part 10.6 openable-area requirements) the ventilation system is designed under AS 1668.2.

A common residential pattern is to leave ventilation to natural means (windows) but add bathroom and laundry exhaust fans discharged to outside, with the kitchen rangehood handled separately. AS 1668.2 sets the design flow rates for the mechanical ventilation case.

What to check on site

Walk the roof space before plasterboard goes up. Look for: trunk duct supported off battens at the spacing AS 4254 calls up; flex pulled straight with no sags; collar clamps tight on both the inner core and the outer jacket; return-air box sized to the unit's airflow; diffusers in positions that throw air into the room not at the cornice; condensate drain to a stormwater point or tundish; commissioning data sheet showing measured airflow at each diffuser.