A properly designed and sized hood system will ensure that effluents and convective heat are captured and exhausted. However, it is not enough to guarantee the kitchen space temperature is comfortable. The radiation load from appliances, people, lights, kitchen shell, solar load, and potential heat and moisture from untreated make-up air are to be handled by the kitchen air conditioning system.

It is recommended that a negative air balace be maintained in the kitchen; The amount of air exhausted from the kitchen should be at least 10 % higher than the supply air flow into the kitchen. This will guarantee that the odours from the kitchen do not spread to the adjacent spaces.

Traditional mixing systems supply air at high velocity at the ceiling level. The goal is to thoroughly mix room air with supply air. Impurities and heat are diluted and mixed within the zone.

Thermal Displacement Ventilation is based on natural air movement. Low velocity supply air is introduced through specially designed displacement diffusers. Heat and impurities are stratified toward the ceiling. Displacement ventilation provides continually replenished fresh supply air.

Research has shown that if mixing diffusers are located close to the hood, the high velocity air interrupts the cooking plume, drawing some of it out of the hood and further increasing the heat load on the space.

According to the new VDI 1999, mixing ventilation systems require 15 to 20% higher exhaust rates than displacement systems for the same efficiency.