Reduced airflow has been measured at the registers served by one duct branch. The filter and blower operation have already been checked and are satisfactory. Which next step most directly investigates a problem in that branch before replacing equipment?
- ACheck the air filter.
- Inspect the ductwork for leaks and obstructions.
- CMeasure the airflow at the registers.
- DReplace the furnace fan motor.
Where to look it up
https://www.energy.gov/sites/default/files/2013/11/f5/hvac_guide.pdf
Why B is correct
Inspect the affected branch for disconnected joints, leakage, crushed sections or other obstructions, then use appropriate airflow or leakage measurements to confirm the fault. The question already establishes that the filter and blower were checked and reduced register flow measured. Replacing the fan motor before finding evidence of a motor fault is not justified. This is a diagnostic scenario, not a universal first-step sequence prescribed by the IRC. Source: https://www.energy.gov/sites/default/files/2013/11/f5/hvac_guide.pdf