Fresh air forever

Energy efficiency and electrification IAQ and occupant health

December 10, 2025

Clive Broadbent, L.AIRAH, reflects on the difficulties of striking a balance between acceptable CO₂, humidity and energy use in HVAC systems.

The spring 2025 issue of Ecolibrium contains a very entertaining article by Sonia Holzheimer, M.AIRAH, on AS 1668.2, our national ventilation standard. Sonia’s comments on carbon dioxide cleverly express some of the major concerns system designers (and building occupants) have long held about ventilation.

I’ve written this response to argue that CO₂ – while arguably an imprecise surrogate for indoor air quality that was never intended to function as such – is nonetheless a very useful design measure.

CO₂ in ambient air

For thousands of years, the typical quantity of CO₂ in ambient air was 280ppm. When my engineering career began in 1956, it was 300ppm; this was the concentration used in pollution dilution calculations at the time. Organisations like NASA now advise that CO₂ is running at over 400ppm. The scientific literature states that it will be 500ppm in a few decades, a level never experienced in the history of the planet. What could the consequences be?

CO₂ in indoor air

Of course, increased levels of carbon dioxide in ambient air are likely to have consequences for the HVAC industry, including an effect on acceptable ventilation rates.

Some years ago, I carried out studies in classrooms at a TAFE institution in (low humidity) Canberra using a CO₂ data recorder and my eyes to observe what I called the “drowsiness factor” among students subjected to normal classroom ventilation during lectures. The findings – which I admit are anecdotal – showed that 600ppm of CO₂ was satisfactory. However, increasing CO₂ concentration through outdoor airflow control showed that drowsiness occurred at 1,000ppm and above, with comfort temperature range maintained. Of course, there are many reasons why teenage students may become drowsy in class, but air quality could be a significant factor.

Acceptable ventilation rates for comfort have varied greatly over time as habits and other factors have changed. Think about how societal attitudes towards smoking, showering and sensitivity to odour have changed over the decades, and you’ll start to see why the levels of CO₂ we consider acceptable have also changed. CO₂ levels are generally used as a proxy measure for acceptable indoor air quality; they provide useful but incomplete results.

A level of 1,000ppm has traditionally been accepted overseas as the rate for design purposes. However, my belief is that Australians greatly value and enjoy the outdoors and that we are accustomed to better quality air than people in the northern hemisphere; 600ppm may actually be a better representation of what the wider community would expect.

Let’s split the difference and accept 800ppm as a reasonable concentration indoors. As a report by Dr Preston McNall – a highly regarded ASHRAE member and research scientist at the US Center for Building Technology’s National Bureau of Standards – states: “In building surveys, people report that CO₂ levels below 800ppm are indicative of adequate IAQ, while others report that complaints exist even at lower levels.”¹

Achieving comfort in practice

With an outdoor level of 300ppm and assuming 6 litres per minute breathing rate with 0.31ppm CO₂ for sedentary occupations (students seated – see ASHRAE Standard 62.1 for the physiological background to this CO₂ generation level), the required ventilation rate is given by:
V = N/(C1 – C2).60
where V is L/s outdoor air required for dilution, N is the breathing rate, C1 is acceptable indoor CO₂ level and C2 is the level outdoors.

The calculation shows that maintaining the 800ppm level requires:
0.31/60.(0.000800 – 0.000300) = 10L/s of outdoor air per person. This matches the AS 1668.2 level required for such applications.

For equivalent air quality and 400ppm of CO₂ outdoors, outdoor air rate is: 0.31/60.(0.000800 – 0.000400) = 13L/s of outdoor air per person. This is arguably still acceptable.

But with 500ppm outdoors, the outdoor air rate becomes: 0.31/60. (0.000800 – 0.000500) = 17L/s of outdoor air per person. There is now an unreasonable impact on energy and system needs.

When using the student satisfaction level of 600ppm for CO₂ concentration – and considering that community expectations of comfort are becoming more stringent – the calculation becomes: 0.31/60. (0.000600 – 0.000500) = 51L/s of outdoor air per person.

This would have a huge impact on system design and would fly in the face of our attempts to reduce energy consumption in buildings.

Thinking beyond CO₂

As Sonia points out, there are many factors other than CO₂ that affect our feeling of comfort within a space. One of these is humidity.

While I was reading Sonia’s article, I thought of Darwin and a special air handling system that was designed, well, around the time Sonia’s husband was a gleam in his mother’s eye! The design/construction outfit was the Commonwealth Department of Works and the project was the Royal Darwin Hospital.

Operating theatres consume copious quantities of outdoor air through ventilation. In the tropics, this brings with it high humidity levels, which presents challenges not just because of mould and microbial growth, but also because of occupant comfort.

The operating theatre system at Royal Darwin was what’s known as a run-around system. It incorporates a cooling tower accepting recirculating water from a set of coils each mounted in the outdoor air intake duct (serving an operating theatre and surrounding rooms). The system cools the recirculating water and returns it to the coils so as to accept heat from the incoming outdoor air.

The cooling tower cools the water despite the dominant high humidity levels in Darwin, as the make-up air for the tower is taken from the air conditioning exhaust air. Thermal energy is taken from the water (supplied and returned to coils) by evaporation into the low wet bulb exhaust air from the air handlers (op theatre). This air, after traversing the cooling tower, is rejected from the tower to outdoors.

ASHRAE has found the effectiveness of such a system to be 58%.

For this particular system, and allowing for 4,000 hours of operation annually, I calculated the saving to be 460,000kWh per year.

Of course, a simpler version of the runaround would see a coil in exhaust, a coil in intake, and piped recirculation. Introducing the cooling tower and circuitry introduces additional, but well understood, maintenance requirements, and this plant has proven to be beneficial in the hot humid climate of Darwin.

The challenge

As you can see, finding the perfect balance between CO₂, humidity and energy use always presents a challenge.

The calculations I’ve presented in this article are simple ones, but they’re worth considering when we design systems and develop Australian Standards. Rising atmospheric carbon dioxide levels may not be lethal – at least to humans – but they add another layer of complexity to the already significant challenge of creating indoor spaces that prioritise occupant comfort and wellbeing.

An abundance of fresh air in our environment is a gift we Australians value. It’s our duty as system designers to protect that in the face of increasing environmental and technical challenges.

Reference 1. McNall, Preston, (1988), ‘The HVAC Engineer and Indoor Air Quality’. Heating/Piping/ Air Conditioning, Feb 1988, pp 65-70.