We hear a lot about the failure of ventilation in quarantine hotels leading to cross contamination. But how does the design of hotels lead to these disasters? The answer lies in the somewhat arcane world of mechanical engineers designing HVAC, Heating Ventilation and Air Conditioning.

Each of those are separate issues, and we need to pick them apart to understand where things went wrong, not just in hotels, but moreover failures in both thermal comfort and energy use. Let’s start in the middle of the muddle with ventilation.


The primary purpose of ventilation in buildings is to provide fresh air, with oxygen, for occupants and to remove odours and excessive carbon monoxide / dioxide. (Ventilation can also be used for cooling, by evaporation or by night flushing, which is discussed here.)

Traditionally ‘fresh air’ ventilation was obtained by opening windows and doors, often called ‘natural’ ventilation. The Building Code of Australia requires such openings, such as 5% of the floor area of habitable rooms, a measure that has remained unchanged for more than 70 years, without empirical evidence.

The alternative to ‘natural’ ventilation is ‘mechanical’ ventilation, using fans to draw outside air through louvres and ducts to the interior. This provides better control of air volume and can allow filtration of pollutants. Often this form of ventilation is linked to heating and cooling although these are distinctly different things.


Whilst ventilation addresses the human need for fresh air, heating and cooling looks after human thermal comfort, providing a level of warmth or coolth for the occupants. Given that the climates where Western architecture was mostly developed are cool to temperate, heating was the focus. In the past this was achieved by burning fuel - in coal or wood fires, gas or oil furnaces, and heating water to be piped to radiators.


Cooling is different. Whereas heating is created directly from energy, cooling is a ‘reduction in energy’, which can only be achieved by applying energy. This is the secret of ‘air conditioning’ or AC, a misleading and incorrect term. It is better called a chiller, a ‘coolth machine’, making chilled fluid to cool the air.

Using compression, then release, of gas pressure is the basis of a chiller, and the technology dates from the early 20th century, with its invention and patent by Willis Carrier. Unlike radiators dispersing heat, the ‘chiller coil’ absorbs heat, thus cooling the air. It can also control humidity by cooling air down to a point which removes excess humidity, and then reheats it to the comfort temperature.

Carrier applied his invention to various industries (and particularly to de-humidify tobacco factories in the south of the USA), before it’s take up in buildings such as Graumans Chinese Theatre in Hollywood in 1927 (the first cinema with AC). So, cooling by AC is less than 100 years old, overturning centuries of using the building itself to create comfort, which we can call ‘building conditioning’, in favour of air conditioning.

Thermal comfort

Surprisingly our understanding of how thermal comfort works is very recent, the 1970’s in fact, in the work of P.O. Fanger discussed here. Key to why air conditioning is poor as a deliverer of thermal comfort is to know that radiant temperature is more important than convective air temperature. That is that the floors, walls and ceilings of a space are more determinant of thermal comfort than the air temperature. Thus heating, or cooling, the air does not deliver the best comfort.

This is why heating radiators are so much more effective than fans blowing warm air. In a similar way we have recently seen the development of ‘chilled beams’, another poor descriptor for what is in fact a heat absorber. Made as flat panels, not ‘beams’, and mounted on the ceiling, they can make a space cooler without the need for chilled air. But they are an expensive technology, rarely used, such that almost all buildings now use air conditioning.

Air conditioning

The basis of AC is a ‘heat pump’, which does just as it says, it pumps heat from one side to the other. The popular reversible AC can pump heat from outside to in (to provide warmth) or it can reverse to pump heat from inside to out (to provide coolth). The other main part is a ‘fan-coil’ which directs the air over pipes that carry the hot or cool fluid, before sending it into the building space.

There are many kinds of AC that everyone will have seen.

The simplest one is an integrated AC machine, a ‘window rattler’: a single box with a heat pump and fan coil in one unit. Often stuck in a window or wall the heat is ejected outside whilst a fan blows cool air inside (or the reverse). There is no fresh air ventilation involved.

A split system is slightly more sophisticated, separating the compressor outside from the fan coil in a wall unit inside. Jokingly referred to as ‘Mark Taylors’, for the well-known cricketer featured in advertisements for these types, seemingly appropriate because they are like him, highly effective but not attractive. Again, no fresh air ventilation since it is recirculating air within the room.

More sophisticated again is ducted air conditioning: the compressor is outside on the ground or roof making warmth or coolth (as before). But here the fan coil unit is commonly in the ceiling and it has outside air ducted to it and then the cool or warm air is blown through ducts into the space. The air from the space is forced to the outside or, for efficiency, is returned back to the fan coil.

This ‘return air’ is ducted back into the fan coil unit or goes through a heat exchanger so that the stale air is exhausted but the warmth or coolth is not lost. The fresh air is supplied in ducts and usually the return air goes back in ceiling space, called a plenum. Since return air is less critical some leaks are able to occur.


Most hotels are fitted with ‘ducted air conditioning’, with central compressor(s) for the whole building on the roof or a services floor, and a fan coil unit on each floor. The fresh air is taken in through louvres, conditioned, and blown into the rooms. Critically the polluted air (with the virus) can go in many directions: to the outside through the bathroom exhaust; through windows and gaps in construction; but most commonly it's returned through a plenum in the corridor back to the fan coil unit. Sometimes return air is used to maintain a comfortable temperature in the corridor (a disaster in waiting).

The ideal in a quarantine hotel is to have 100 per cent fresh air, filtered before going in. The air should not be ‘recycled’ but rather exhausted through vents to the outside or ducted to the roof, away from the air intake. If the air is to be recycled for energy efficiency, it must be in ducts to prevent contamination and air should go through a heat exchanger before being vented out.

But this is rare. More commonly there are leaks around the door; or the bathroom is not exhausted effectively; the return air in the plenum leaks; or worst of all, the return air is used to condition the corridor. All of this allows for contamination by the aerosol COVID particles back into spaces.

Whilst it is possible to filter the contaminated air, as it is in airplanes and hospitals, this is very expensive and cannot be readily retrofitted to hotels. On the contrary, hotels use the most common, and cheapest, form of ducted air conditioning, where leaks are common, contamination is possible, and the air pressure is insufficient to prevent contamination.

The solution is to separate out heating (with radiators) and cooling (with chilled absorbers) from the fresh air ventilation, which has to one pass only. Ducted air conditioning, delivering heating, cooling and ventilation together, with return air plenums, is a recent, but deeply regrettable, phenomenon. Another COVID lesson.

Tone Wheeler is principal architect at Environa Studio, Adjunct Professor at UNSW and is President of the Australian Architecture Association. The views expressed here are solely those of the author and are not held or endorsed by A+D, the AAA or UNSW. Tone does not read Instagram, Facebook, Twitter or Linked In. Sanity is preserved by reading and replying only to comments addressed to [email protected]