Natural AHU

Can we ventilate, heat and cool a building without a traditional air conditioning unit?

In short, yes you can.

Heating and cooling a building has been done for a long time, without a chiller and air handling unit.

As a concept, ignore for now the detail, comfort criteria and the like and concentrate on what an air handling unit does in a building. This is summarised below:

  • Delivers outside air for occupants to breath and removes or dilutes odours and the like – FAN
  • Provides cooling, due to internal and external heat gains – COOLING
  • Provides heating – HEATING

Fan, Cooling and Heating – Passively

Fan: Moving air through a building can be achieved by many methods including:

  • Cross ventilation
  • Thermal chimney
  • Room Stratification

Cooling: Cooling can be done, examples include:

  • Pre Cooling via thermal labyrinth
  • Wet bulb cooling via water features
  • Thermal mass in contact with ground

Heating: Heating can be done, examples include:

  • Suns energy direct to a space, often combined with thermal mass
  • Pre warming air using the concept of SOL air temperature

If we can design our buildings to capture, release and utilise the above then we have created a ‘natural AHU’.

So why don’t we do this more?

Simple, as society develops, we expect perfect internal conditions and for these conditions to be maintained within close tolerances. We expect to be able to wear summer clothing all year round. If this pre meditation is removed the ‘Natural AHU’ would be able to provide ‘acceptable’ conditions for a large number of buildings.

Design Process

As a starting point for each building the design team should start with trying to get the ‘natural AHU’ to work. Then in a step fashion tweak the design to the final solution keeping the concept of the ‘Natural AHU’ as intact as possible.


Free CO reduction/control

Many, many carparks, can be naturally ventilated.  However there is no BCA requirements to force or encourage a design to be made naturally ventilated.  So, the majority of car parks end up with fully fanned power consuming car parks. Going the hard yards to allow for extra excavation or perimeter openings is often ‘too difficult’. From an energy prospective, Im always frustrated. But in reality we are all working for a client and its simply hard dollars.

Free Cooling

Many small to medium offices are provided with hybrid systems (both natural and mechanical means to control the internal environment). The term office natzi is often used here. This is a person who’s role it is to managed the opening and closing of windows and switching off the air conditioning button, if someone turns it on. Its quite funny that most of us in our home lives wouldn’t dream of using the air conditioning (due to cost), but at the office, no problem. So in this type of office acceptable conditions can be achieved.

Nursing homes, offer a great opportunity to provide ‘natural AHU’s’. These developments now often include internal garden areas opening onto large internal circulation spaces. Provide operable roof to these external gardens, add plants (free oxygen) and a water feature (free cooling) and you’ve just created a ‘natural AHU’. Air can be drawn into the courtyard, directed over the water feature (free cooling) and into the space. Further work within the space will allow for this air to be directed through the corridors and into each room (before being removed naturally or by the toilet exhaust system).

For the neigh Sayers

I’m often on the side of the neigh Sayers, as an Engineer, and can come up with a significant list of issues that seem to prevent the ‘natural AHU’. A hospital I hear you say, can’t be naturally conditioned, a laboratory and so on.

Well even these specialist buildings can have significant parts of the floor plate conditioned with a ‘natural AHU’.

For a city office block, with high CO levels surrounding the site, buildings on 3 sides and high ambient noise levels, then the challenges to the natural AHU start mounting up. At this point we do what we can (refer to Design Process paragraph above). ‘Do what we can’ isn’t a cop out…with some out of the box thinking we can significantly reduce the reliance on the ‘mechanical AHU’.

What Next

To get the ‘Natural AHU’ concept working needs an alternative thought process.

  • An aspirational client
  • An Architect willing to invest time in passive control and able to Architecturally make the passive control part of the Architectural solution.
  • An Engineer, who wants to challenge the norm and not just knock out a chilled water VAV system.

Author: Jorgen Knox

Date: 11/08/2014

Contact: e:, t: 02 800 33 100, w: KAE, LI

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