Ducted systems are popular. They promise whole-home comfort, but do they deliver it, and at what cost?
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According to the 2021 Residential Baseline Study ducted heating comprises 90 per cent of Victoria’s annual residential gas heating total of 74 PJ (3.9 million tonnes of carbon emissions) at an annual gas cost of over $2 billion.
Many ducted heating systems are extremely inefficient. The single local field study of ducted heating I know of suggested that overall system efficiencies of existing ducted gas heating systems were 30 to 69 per cent, with older systems tending to have lower efficiencies.
This variation reflects not only varying ducting efficiency, but also furnace efficiency and whether or not there was a pilot light.
Many ducted systems used for heating and cooling are in hot roof spaces that undermine cooling efficiency and effectiveness, especially for rooms a long way from the cooling source.
Modern gas furnaces tend to be more efficient and use electronic ignition, as reflected in varying furnace star ratings. Ducted systems also use significant amounts of electricity for fans and controls.
After ducting was replaced, overall system efficiencies improved to 50 to 76 per cent. Replacement of the ductwork led to overall reductions in gas consumption of 14 to 30 per cent.
Many ducted systems used for heating and cooling are in hot roof spaces that undermine cooling efficiency and effectiveness, especially for rooms a long way from the cooling source.
But the problems are more fundamental.
For ducted heating or cooling to work effectively, heated air released into rooms must flow freely back to a central return air inlet.
As shown in Figure 1, flow resistance limits this airflow so the heater may preferentially draw air from outdoors, through gaps, open windows, pet doors etc. This leakage increases if internal doors are closed, as is common.
Figure 1. Impact of operating ducted heating on air leakage from a house: 1990s US study
The 1980 Australian Gas Association design manual recommends that the airflow through relief openings (grilles or gaps under doors) between outlets in rooms and the return air inlet should not exceed two metres per second. My calculations suggest this would require a gap under the door of around three centimetres if a room has one duct supplying its heated air.
Many living areas have more than one duct outlet. Many homes don’t have gaps under internal doors and, even if they do, there is still significant flow resistance due to a closed door compared with the free flow of air from a ventilated laundry or a gap under an external door near the return air inlet.
Air takes its easiest path to reach the return air inlet.
It’s common practice to locate ducting outlets near windows or external walls, to improve comfort. These areas tend to be cold because windows and uninsulated walls allow very high heat losses (or gains).
A typical single-glazed window has a U-value of around six watts a square metre per degree. This sounds very good compared with the R-values of uninsulated walls (R0.5) or even insulated walls with R2 insulation. However, the units of measurement are not the same.
A window with a U-value of six has an R-value of 0.17 as R-value=1/(U value). So the window has about a third of the R value of an uninsulated wall. The high temperature difference between the heated air and outdoors further adds to heat loss.
What’s more, around two-thirds of the window’s R value is due to the layer of still air that is assumed to “stick” to the window.
If air is blowing over the inside of the window, most of this benefit is lost. This is why double glazing, which traps a layer of still air between the two sheets of glass, can make a big difference.
Studies by the Gas and Fuel Corporation in the 1980s found that fitting simple plastic air diverters, like that shown in Figure 1, to ducted heating outlets near windows or uninsulated external walls delivered 20 per cent gas savings by directing hot air into the room, reducing temperatures near them and reducing heat loss.
So it is likely that many ducted gas heating systems are very inefficient when the losses in heat production, supply, installation practices and building design are considered. But we don’t have any recent data based on the physics.
Similar problems occur when using ducted cooling – in reverse.
Zoned ducted systems can have their problems. Often there is only one thermostat, so it can only sense temperature at one location.
One example I analysed was a two-zone system with the thermostat in the return air inlet in a hall near bedrooms. If the door to the other zone in the living area was closed and just the living area zone was conditioned, it would simply get hotter and hotter or colder and colder, as the thermostat didn’t accurately sense the temperature in the conditioned zone because not much air passed through the closed door between zones.
Another example was a two-storey house with the thermostat upstairs in a sunny area. That area would warm up while the downstairs zone stayed cold. In summer, keeping the upstairs zone comfortable meant the downstairs zone was over-cooled. And an upstairs room with high solar gains would still be hot if the door was closed.
It is difficult to maintain comfort in all rooms when the heating is controlled by a centrally located thermostat.
If it’s in a warm, sunny room, the rest of the house may be cold. If it’s in a drafty corridor, other rooms may overheat. If some rooms get lots of sun they may overheat. This can lead to people opening windows that increase energy waste, or other issues such as cold drafts.
Adjusting cooling or heating supply to each space as the heating or cooling demand varies differently over the day in different rooms can be very tricky with a ducted system. Most air outlets are fixed flow capacity.
Of course, no heating solution is perfect. Outcomes are strongly influenced by building thermal performance and detail relating to installation, as shown in a recent article on reverse cycle air conditioners at Replacing gas heating with reverse-cycle aircon leaves some people feeling cold. Why? And what’s the solution?