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Apprentice answer

How many duct diameters of straight pipe do I need before the inlet of a VAV box

2nd YearVAVs, Fan Coils & Equipment ConnectionsCheck first

You need a minimum of 3 to 5 straight, uninterrupted duct diameters of straight round pipe before entering the VAV box inlet. * If you place an elbow or a sharp transition directly on the inlet collar, the air will spin turbulently, causing the flow sensor tubes inside the VAV inlet to read incorrectly, ruining the system balance.

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Plain-English answer

You need a minimum of 3 to 5 straight, uninterrupted duct diameters of straight round pipe before entering the VAV box inlet. * If you place an elbow or a sharp transition directly on the inlet collar, the air will spin turbulently, causing the flow sensor tubes inside the VAV inlet to read incorrectly, ruining the system balance.

Field checklist

Ask Foreman

Hey boss, I’m checking vavs & fan coils: How many duct diameters of straight pipe do I need before the inlet of a VAV box? I found the likely issue and want to verify the next step before I lock it in. Do you want me to adjust it now or check the drawing/detail first?

Verify before acting

Use this as training guidance. The foreman, approved drawings, project specs, manufacturer installation instructions, employer safety policy, and AHJ/code requirements always control the final answer.

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