Car park carbon monoxide exposure - are you at risk of non-conformance?
Having performed many peer reviews over the years of third-party CFD modelling Performance Solution (PS) reports for car park ventilation systems, it’s surprising how many practitioners continually assess to the wrong carbon monoxide (CO) assessment criteria, or alternatively manipulate their modelling results so that a “compliant” solution can be obtained.
First let’s provide some context. In addition to the individual states’ Building Acts that legislate and regulate state building developments, all building work must also comply with the National Construction Code (NCC) which includes the Building Code of Australia (BCA) in Volumes 1 & 2 of the NCC. This is a performance-based code and provides the minimum required levels for the safety, health, amenity, accessibility and sustainability of certain buildings (including car parks).
To comply with the NCC, a solution must achieve compliance with the Governing Requirements and the Performance Requirements; the Performance Requirements are the only NCC technical provisions that must be satisfied. In the case of car parks (Class 7a buildings), clauses FP4.3 and FP4.4(a) are the relevant Performance Requirements and compliance with these clauses is verified when “carbon monoxide exposure levels do not exceed the limits specified in Table FV4.2.” These maximum exposure levels are:
100 ppm - not to be exceeded
90 ppm - 15 minutes (total exposure duration per day)
60 ppm - 1 hour (total exposure duration per day)
30 ppm - 8 hours (total exposure duration per day)
OK, that’s it for the formalities…
Now practically all PS designs we have come across are based on a peak 1-hour scenario in terms of calculating the CO generation rates, and therefore the CO concentration results. Given this 1-hour simulated scenario, you would think it makes sense to assess against the 1-hour maximum exposure concentration of 60 ppm, right? Apparently not…
Time and time again, we have seen results based on a steady-state simulation of the 1-hour scenario but with maximum CO concentrations well over the 60 ppm limit - in one case, more than half of the car park floor area showed CO concentrations > 60 ppm. But because the floor-area-average 1-hour average CO concentration was slightly under 60 ppm, and because the maximum CO was < 90 ppm, this was argued to be compliant. Using the 15-minute exposure criterion of 90 ppm for a 1-hour average simulation is akin to comparing apples and oranges - the best way to assess against the 15-minute 90 ppm criterion is to perform a transient (not steady-state) simulation and compute rolling time-average concentrations over 15-minute durations. Not to mention the fact that taking a floor-area-average of the 1-hour average CO concentrations is meaningless.
To illustrate the latter point, consider a hypothetical scenario at home where you are busy downstairs preparing dinner in the kitchen, and your kids are upstairs doing their homework. Unbeknownst to you, there is a leaky valve with your gas cooktop which causes you to be exposed to natural gas for a significant duration of time. Upstairs, however, everything is normal.
Taking the floor-average gas concentration would show a very low spatial-average level since the majority of the house would have zero gas concentration, which completely ignores the fact that the local gas concentration (in the kitchen) is dangerously high. YOU end up in hospital on a ventilator, but everyone else in the house is fine because they were upstairs. But hey, the floor-average result said there would be no problems!
Yet these are the arguments that some practitioners make, completely missing the point of what gas exposure actually means, or how it should be reported.
We have also seen PS designs that refer exclusively to documents other than the NCC, for example AS1668.2 which, being an Australian Standard, is (on its own) a voluntary document and there is no requirement to comply with it. Not to mention other documents such as Safework Australia guidelines (for OH&S workplace purposes) which have higher limits of, for example, a CO concentration of 100 ppm for a maximum exposure duration of 30 minutes. Just to re-iterate, no reference to these documents are made in the Performance Requirements of the NCC and therefore the use of them is moot.
Hopefully you find this information useful next time you receive a PS result or report that attempts to justify high CO concentrations…