What do I check when a branch line has zero CFM but the dampers are open?
Zero CFM with open dampers usually means a real blockage, closed/tripped device, wrong connection, or disconnected path. Trace the branch before blaming the balancer.
Do not assume controls or TAB are wrong until the physical air path is checked.
Check
Confirm the balancer reading and which exact branch/zone is zero.
Check that the unit is running and main/zone dampers are actually open, not just handles turned.
Trace from main tap to VAV/damper/flex/diffuser for disconnection, crushed duct, tripped damper, or blockage.
Use camera/mirror/register access before cutting holes.
Document what you found and coordinate correction with TAB/foreman.
Steps
Confirm the balancer reading and which exact branch/zone is zero.
Check that the unit is running and main/zone dampers are actually open, not just handles turned.
Trace from main tap to VAV/damper/flex/diffuser for disconnection, crushed duct, tripped damper, or blockage.
Use camera/mirror/register access before cutting holes.
Document what you found and coordinate correction with TAB/foreman.
Say this to your foreman
Zone/branch [tag] reads zero CFM even with dampers open. I traced from the main to [point] and checked for crushed duct/tripped damper/blockage. Do you want me to open access at [location] or have TAB/controls verify next?