A technician performs duct leakage diagnostics and finds that of the system's total 300 CFM of duct leakage, roughly 220 CFM occurs on the return side, with the return duct routed through a 140°F attic. What is the most likely operational consequence during cooling season?
- AHot, unconditioned attic air is drawn into the return airstream, increasing cooling load and reducing delivered capacity
- BThe refrigerant charge will automatically adjust to compensate for the extra airflow
- CThe compressor will short-cycle due to excess refrigerant subcooling
- DThe system will run more efficiently because attic air adds beneficial static pressure
Show answer & explanationAnswer & explanation
Correct answer: A. Hot, unconditioned attic air is drawn into the return airstream, increasing cooling load and reducing delivered capacity
Because the return duct is under negative pressure relative to its surroundings, leaks on the return side draw hot attic air directly into the airstream before it reaches the coil. This raises the entering air temperature, increases the sensible and total load on the equipment, and reduces the effective delivered cooling capacity and efficiency.
Why the other options are wrong
- B. Refrigerant charge does not automatically compensate for airflow changes.
- C. Short-cycling from subcooling is unrelated to return duct leakage location.
- D. Attic air infiltration degrades, not improves, system performance.
Return Duct Leakage Impact
Return-side duct leaks in hot or unconditioned spaces draw outside air into the system under negative pressure, increasing cooling load and reducing delivered capacity, unlike supply-side leaks which simply waste conditioned air.
- Return ducts operate under negative pressure
- Leaks there pull in unconditioned air, not lose conditioned air
- Return leaks in hot attics significantly raise cooling load
Memory trick: Return sucks in the heat; supply just leaks it out.