Ducted air conditioning (or heating) is inefficient.

Let me explain, and see what you think. I am not a fan (pun intended) of ducted. It seems to be very inefficient. But only one in five air con sales reps seem to agree with my reasoning.

At our previous home, in Saratoga NSW, we had a large (10kW) ducted air con system upstairs, which included all the bedrooms.

When we just wanted to cool or heat one room, we tried to close the door of that room. But the door would slam shut and whistle, as the air would try to escape. The ducted air con had one or two outlets in each room’s roof. It had one shared return vent in the hall. Turning on the air con in a single room, the air needed to return to the hall to circulate. Closing the door blocked that path and made the pump work extra hard.

The thermostat was also in the hall, as part of the control unit. We could set the target temperature of a room to say 22°C, but the room might cool down to 18° before the hall thermostat would register 22°, with a temperature gradient between the two.

The system had four separate zones. One zone was shared between the master bedroom and living room. On a hot night, if we set the air con to cool our bedroom, it would also cool the open plan living room, with no one in it. I think those two rooms shared a zone to address the “don’t close the door” problem, but it increased the wasted energy problem.

We moved to Emerald, Victoria two years ago. Our home here came with ducted gas heating. Each room has an outlet (or two) in the floor, with one shared return vent and thermostat in the hall. We can close the floor outlets in each room, similar to the zoning we had for the air con. Same problems here as we had the ducted air conditioning. If we close doors, we block air flow. The thermostat can only set the target temperature for the hall, not the actual rooms.

In contrast to ducted, a split air con system has the outlet and return vent in the same room. That means that you can close the door to the room, and cool/heat just that room. The thermostat measures the temperature of that room. All is well with the world!

In addition to ducted, we also had a split air conditioner in each house. It used a fraction of the energy that a ducted system would, even when ducted was zoned for just one room.

When I recently asked several companies for a quote to install air conditioning, most suggested a ducted system. My internal dialog was “Are you crazy? How can you still suggest ducted?” I But I politely filtered. They would tell me about zoning and the possibility of two return vents and so on. But when I asked “can I close the door?”, the answer was still no. Or I had to explain why you can’t close the door. One guy (for whom I actually have a lot of respect) said “yeah, ducted uses more power, but you have excess solar power on hot days”. I think he forgot about hot evenings and cold overcast days.

One advantage of a ducted system is having a shared central compressor. For a five room system, that means one compressor fan alongside the house. Individual split systems would have five compressors, which can be ugly.

Fortunately, one alternative is to install a multi head split air conditioning system. This shares one compressor across all the five heads (in each room). Each room still has its own combined outlet and return, and its own thermostat. When one head is already running, turning on a second head starts very quickly, because the compressor is already running.