CSLB C-20 HVAC ContractorVentilation and Air DistributionHard
A commercial rooftop unit supplies a constant 4,000 CFM to a zone. Per ASHRAE 62.1, the required outdoor air ventilation rate for the space is calculated at 800 CFM. What minimum outdoor air damper position, expressed as a percentage of total supply airflow, should the technician set to meet this requirement?
- A32%
- B15%
- C8%
- D20%
Show answer & explanationAnswer & explanation
Correct answer: D. 20%
Minimum OA percentage = required OA CFM / total supply CFM = 800 / 4,000 = 0.20, or 20%. The damper must be set to admit at least this fraction of outdoor air continuously during occupied operation to satisfy ASHRAE 62.1.
Why the other options are wrong
- A. 32% oversupplies outdoor air beyond the calculated requirement.
- B. 15% would supply only 600 CFM, still under the requirement.
- C. 8% would only supply 320 CFM, insufficient outdoor air.
Minimum OA Damper Position
The minimum outdoor air damper setting needed to meet ventilation code requirements, calculated as required outdoor air CFM divided by total system supply CFM.
- OA% = required OA CFM / total supply CFM
- Damper must maintain at least this minimum position during occupied hours
- Economizers can exceed minimum OA% for free cooling but not go below it
Memory trick: Divide the fresh need by the total feed to find the damper's minimum speed.