Many Australians use reverse-cycle air conditioners to heat their homes during winter. These systems are usually very energy efficient. However, sometimes a room can still feel unevenly heated. The air may feel warm in some parts of the room, while people sitting on the couch may still feel cold.
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Since many people already have reverse cycle air conditioners (RCACs) but still use gas for heating, it is important to educate them on how to maximise comfort, as well as cutting heating costs and reducing carbon emissions by using their RCAC.
Governments are beginning to offer incentives to retrofit insulation and shift from gas heating. For example, the Australian Capital Territory and the Victorian government are phasing in incentives for insulation and rental efficiency standards.
Insulation is one important solution that both improves comfort and saves energy. However, our recent computer modelling shows that another less obvious factor can also make a difference: the type and placement of furniture, for example, blocking the gap under a couch.
Using computational fluid dynamics (CFD) modelling, we found that air coming from a wall-mounted reverse-cycle air conditioner can travel underneath an elevated couch. The heat is still inside the room, but the air cools as it moves across the room, so much of the air passes under the people sitting on the couch, creating a “wind chill” effect.
When we blocked the gap, the air could not continue underneath the couch. Instead, warm air was guided upwards in front of the couch and towards the people sitting there. This increased the temperature in the seating area by about 0.7°C and reduced the flow of air over their feet.
The better the insulation, the more comfortable people were, while energy savings of 10 to 35 per cent were identified.
Why can reverse-cycle heating feel uneven?
RCACs are normally installed high on a wall. This position works well for cooling because cold air naturally moves downwards.
Heating is more difficult because warm air naturally rises. The air conditioner must push the warm air down and across the room. The fan inside the unit helps to do this, but the final movement of air also depends on the room layout, insulation, furniture and position of the air conditioner. The RCAC draws air in to reheat it. In a poorly insulated room, this air may be cool, so people sitting under the RCAC may feel a cold draft.
As a result, the thermostat may show a comfortable temperature, but the areas where people are sitting may still feel colder.
We created a virtual living room
Our multidisciplinary research team used CFD to study how warm air moves inside a typical living room.
CFD is a computer-modelling method used by engineers to study airflow and temperature. We completed six simulations with different levels of insulation and different couch arrangements.
In every simulation, one RCAC was installed high on the wall. In all cases, it supplied the same amount of air at the same temperature – 80 litres per second at 30 degrees Celsius. This allowed us to directly compare the effects of insulation and the gap underneath the couch.
Of course, real homes are different in their size, layout, windows, draughts, climate and heating systems. Therefore, our model cannot predict the exact temperature or energy savings for every home. However, it helps us understand how these factors can affect heating and comfort.
Insulation made the largest difference
The largest improvement in both comfort and energy savings came from insulating the ceiling, walls and floor and double glazing the windows.
With the same heating conditions, the fully insulated room was about 3°C warmer than the uninsulated room.
Insulation reduces the amount of heat escaping from the building. It also keeps the internal surfaces of the walls, ceiling and floor warmer. This is important because our comfort depends on more than air temperature. Cold surfaces can take radiant heat from our bodies and make us feel cold, even when the air temperature seems comfortable.
Our modelling also showed different levels of energy saving. Adding ceiling insulation alone reduced the energy needed to reheat the circulating air by about 10 per cent. When the ceiling, walls and floor were all insulated, the energy saving increased to over 30 per cent.
Apart from making the room feel warmer, insulation and better furniture placement can reduce the heating capacity required for the room. This may allow a smaller-capacity reverse-cycle air conditioner to be installed, which can be cheaper to purchase and may also cost less to run. However, the system should still be correctly sized by a qualified installer.
These results are estimates from our modelling. They do not mean every household will reduce its electricity bill by exactly 10 per cent or 30 per cent. Actual savings depend on the climate, building condition, thermostat setting, air-conditioner efficiency and how often the heating is used.
The surprising role of the couch
The effect of the couch was smaller than insulation, but it was more surprising.
When there was an open gap under the couch, some of the air travelled underneath it and bypassed the occupants. While it was still relatively warm, this would create some “wind chill” effect. When the gap was blocked, the couch acted as a barrier. This guided the warm air upwards in front of the couch and into the area around people’s legs and upper bodies.
This does not mean households need to buy a new couch. A removable fabric skirt or another suitable material may be used to block the gap. However, any barrier should not cover electrical equipment, heating vents, ventilation openings, or the inlet or outlet of the air conditioner.
What can households do?
Insulation should be the first priority because it provides the largest improvement in our modelling. Households can check their ceiling insulation, consider wall and underfloor insulation, seal unwanted draughts, use suitable curtains or blinds over windows and install high performance glazing.
As noted earlier, government support may also help reduce the cost. These discounts should reduce average installation costs.
As Australian homes become more electrified, choosing an efficient heater is only one part of the solution. Insulation provides the largest benefit, but even a small detail, such as the gap under a couch, can affect whether warm air reaches the people sitting in the room.
