All too often we are called in to evaluate problem restaurants where the hood is not capturing the heat or smoke from the cooking surfaces. One of the first things to look at when diagnosing a capture hood is the possibility of cross current interference. Typically this is caused by improper placement of supply registers for the a/c unit that are too close and above the plane of the hood.
As the cold air comes out of the supply registers, which are not balanced for velocity, the air now enters the capture zone of the hood. This causes tremendous turbulence and the cold dense air can push the radiant heat and smoke outside of the hood, causing extreme comfort and IAQ issues for the restaurant employees, as well as a huge waste of energy for the restaurant owner.
The supply registers should be a minimum distance of 10 feet from the hood and, most critical for side wall supply registers, be at an even plane with the hood. Unfortunately, this is not the norm. Consider changing the type of ceiling supply registers that will throw the air away from the capture hood, a two-way versus a four-way or side wall register that will allow you to direct the air away from the hood. This will reduce the turbulence and go a long way toward keeping the radiant heat and smoke from the cooking surfaces in the containment area of the hood on which you are working.
What if the restaurant cooking area is located in the dining area? Cross currents can push smoke and heat outside the hood and cause a host of odor and comfort issues for the patrons, and a nightmare for the restaurant. Supply a/c registers placement is extremely critical.
So if you are working in a retrofit application, remember, cross currents caused by the a/c unit could have a major impact on air quality, utility expense, and hood performance.