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Apprentice answer

Why is it bad to neck down a VAV discharge immediately after the box

3rd YearVAVs, Fan Coils & Equipment ConnectionsCheck first

An immediate neck-down after a VAV can choke airflow, raise static, and cause heat/reheat or noise problems. Keep discharge transitions approved and preserve airflow area where possible.

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Plain-English answer

A VAV box needs the discharge to leave cleanly. If the duct necks down hard right off the box to dodge an obstacle, the box may not deliver design air, and reheat/performance can suffer.

The recovery is to verify the VAV submittal, required discharge arrangement, available ceiling space, and obstacle. Likely options are a longer/wider transition, flatter duct with equivalent area, box shift, obstacle coordination, or revised detail. Do not choke the box just because it clears the cable tray.

Field checklist

Ask Foreman

VAV [tag] is necked down immediately after discharge to clear [obstacle]. I checked available space and downstream run. Do you want a longer transition, flatter equivalent-area duct, box shift, or detailer direction?

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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