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

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.

['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.']

Stop if

• 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.

Check

• A bad discharge connection can choke airflow, increase noise/static, affect heat performance, and create TAB/startup problems.

Steps

• Confirm VAV tag, discharge size, downstream duct size, and obstacle causing the neck-down.
• Check VAV manufacturer/submittal and project detail for discharge requirements.
• Look for reheat coil, noise, high static, or TAB concerns downstream.
• Measure space for a longer transition or wide/flat duct shape.
• Ask before changing size/shape at equipment discharge.

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