5.1 Successful kitchen exhaust hood performance requires the complete capture and containment of the effluent plume along the hood's entire perimeter. Any effluent leakage moving beyond 3 in. from the hood face will be deemed as having escaped from the hood, even if it may appear to be have been drawn back into the hood. If effluent spills from the hood, hot and greasy kitchens may be the result and the cause of the performance failure needs to be determined and corrected. Oftentimes, the exhaust flow rate needs to be increased to achieve proper hood performance for particular field conditions. As a result, the supply air to the kitchen will need to be increased to maintain the air balance. However, drafty room conditions due to incorrectly placed supply diffusers, cross drafts from windows and doors, return and supply at opposite ends of the kitchen, etc. could also severely degrade hood performance. Incorrectly designed supply systems may not be corrected by increasing the exhaust rate and could be corrected in a much more efficient and economical manner, such as by replacing a 4-way diffuser with a 3-way diffuser directed away from the hood. Likewise, if the plume is strongly captured, the hood may be over-exhausting and reducing the exhaust rate could be considered, along with a corresponding reduction of room supply air to maintain the building's air balance.

5.2 An appropriate airflow balance ensures adequate replacement air for the necessary exhaust conditions and allows the desired air pressure distribution to be maintained.

5.3 Negative air pressure in the kitchen with respect to the adjacent indoor spaces ensures that the air flow is from these spaces into the kitchen so that odors and cooking effluent are contained within the kitchen. However, too great a pressure imbalance will severely degrade hood performance by creating a wind tunnel effect. Negative air pressure in the dining area with respect to the outside is usually an indication that the supply air rate is inadequate and as a result the exhaust air system is not performing as specified.