NASCLA Accredited Commercial General ContractorSpecialties and EquipmentHard
A contractor is installing a new commercial kitchen with a high-efficiency exhaust hood system. During the final inspection, the inspector notes that the exhaust fan is operating at a significantly lower CFM than specified in the design documents, leading to inadequate grease removal. Which of the following is the most likely cause of this issue?
- AThe makeup air unit is providing too much conditioned air.
- BThe fire suppression system within the hood is partially activated.
- CThe kitchen's ambient temperature is too low, reducing air density.
- DThe exhaust ductwork system has excessive static pressure due to improper sizing or obstructions.
Show answer & explanationAnswer & explanation
Correct answer: D. The exhaust ductwork system has excessive static pressure due to improper sizing or obstructions.
Excessive static pressure in the ductwork, caused by undersized ducts, too many elbows, or obstructions (like a dirty filter or damper issues), directly reduces the airflow (CFM) a fan can deliver, leading to inadequate exhaust.
Why the other options are wrong
- A. Too much makeup air would generally aid exhaust, not hinder it, assuming proper balance.
- B. A partially activated fire suppression system would likely be a more obvious issue with alarms or discharge, and while it might obstruct flow, it's less likely than ductwork issues for a 'lower CFM' observation.
- C. While air density changes with temperature, the effect on CFM for typical kitchen temperatures is usually minor compared to static pressure issues.
HVAC Static Pressure
The resistance to airflow in a ductwork system, measured in inches of water gauge (in. w.g.). High static pressure can significantly reduce the actual airflow (CFM) delivered by a fan.
- Caused by friction from duct walls, elbows, filters, and dampers.
- Fans have performance curves showing CFM vs. static pressure.
- Excessive static pressure leads to reduced airflow and increased energy consumption.
Memory trick: Low airflow? 'Static' pressure is 'Sticking' it up!