Part 1: Demand Control Ventilation Does Not Work in Older Buildings

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[@fixithvac9360]

How do you design the air handlers in Australlia? Do you control the outside air dampers by percentage, or you actually measure how many CFMs are going in? How do you control the exhaust fans?

[@bryceanderson17]

On expensive premium grade commercial offices we could have volume sensors on the outside air, if they don't get value engineered out.
However, for this discussion, where we are adding CO2 control to an older building, the AHU's wouldn't have volume sensors already, so they generally would not be added as part of a demand control ventilation project. Dampers then controlled as a percentage.
The return air fans track the speed of the AHU supply air fans to maintain the correct outside air balance and building positive pressure. However, this is very complicated as the RAF and SAF usually have different characteristics (axial RAF with EC motor SAF) so you can't simply run the RAF 10% slower than the SAF.