CSLB C-20 HVAC ContractorPlanning and LayoutMedium
A technician installs flexible supply duct in a joist bay but does not fully extend it, leaving it compressed to about 85% of its rated stretched length. What is the most significant effect on system performance?
- ANo measurable change since flex duct friction is independent of extension
- BIncreased duct friction loss, reducing airflow delivered to the register
- CReduced duct friction loss, increasing airflow to the register
- DDecreased static pressure requirement at the blower
Show answer & explanationAnswer & explanation
Correct answer: B. Increased duct friction loss, reducing airflow delivered to the register
Compressed flexible duct bunches its corrugated inner core, dramatically increasing surface roughness and effective friction rate. This raises static pressure loss along that run and reduces delivered airflow compared to Manual D design assumptions based on fully extended duct.
Why the other options are wrong
- A. Flex duct friction is highly sensitive to how fully it is extended.
- C. This is the opposite of what compression does to friction loss.
- D. Compression increases, not decreases, the static pressure requirement.
Flex Duct Compression Effect
Flexible duct must be fully extended during installation because compression increases internal corrugation exposure, raising friction loss and reducing system airflow.
- Compressed flex duct increases friction rate significantly
- Manual D friction charts assume fully stretched duct
- Sagging or kinks compound the airflow reduction
Memory trick: Squished flex duct chokes the airflow.