CSLB C-20 HVAC ContractorPlanning and LayoutMedium
A supply trunk duct leaves the plenum carrying 1,600 CFM. The first branch takeoff delivers 400 CFM to a bedroom register. According to Manual D trunk duct design principles, what CFM must the remaining downstream trunk duct be sized to carry?
- A400 CFM
- B1,200 CFM
- C2,000 CFM
- D1,600 CFM
Show answer & explanationAnswer & explanation
Correct answer: B. 1,200 CFM
As air is removed from the trunk at each branch takeoff, the remaining trunk duct only needs to carry the leftover airflow. Subtracting the 400 CFM branch from the initial 1,600 CFM leaves 1,200 CFM for the downstream trunk section, allowing it to be reduced in size.
Why the other options are wrong
- A. This is only the branch CFM, not the remaining trunk airflow.
- C. This adds airflow instead of subtracting it, which is not physically correct.
- D. This ignores the air already removed by the first branch takeoff.
Trunk Duct Reduction
In Manual D design, trunk duct size decreases progressively downstream as each branch takeoff removes airflow, allowing smaller duct sizes further from the plenum.
- Trunk CFM = total minus all upstream branch takeoffs
- Reducing trunk size downstream saves material and maintains velocity
- Oversized downstream trunk can cause excessive noise/turbulence
Memory trick: Every branch takes a bite out of the trunk's CFM.