# Modelling a fixed minimum air flow rate on AHUs/VAVs

In large buildings with central air handling unit systems, it's common to have an effective minimum air flow rate that exceeds zero. A typical minimum zone air terminal flow rate is 40% of its maximum. It's considered good practice to aim for a 30% turndown, as this significantly reduces central plant energy consumption. This adjustment will have observable effects on annual fan energy consumption and supply airflow rates in the Design Day HVAC Nodes reports.

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To model a 30% minimum airflow in an air handling unit system for each applicable Space, go to 'Space Sizing', select 'Cooling Supply Air', and adjust the 'Cooling Min. Air Flow Fraction' to 30%. Next, on the 'HVAC Schematic', select each applicable 'VAV Terminal. Go to 'Air Terminal Settings', then 'Sizing', and adjust the 'Constant Min. Air Flow Fraction' to 30%. Lastly, On the 'HVAC Schematic', select each applicable 'AHU Unit'. Go to 'Air Handling Unit Settings', then 'Setpoint Manager Settings', and adjust the 'Min. Turndown Ratio' to 30%.
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<figure><img src="https://content.gitbook.com/content/fKJnzF2TwHA3APPbiCpF/blobs/Pi1mznojXMRaU4mGnK0K/AHU.gif" alt=""><figcaption></figcaption></figure>