An effective commercial kitchen ventilation (CKV) system requires balance air balance that is. And as the designer, installer or operator of the kitchen ventilation system, you may be the first person called upon to perform your own “balancing act” when the exhaust hood doesn’t work.

Unlike a cooking appliance, which can be isolated for troubleshooting, the exhaust hood (a non-functioning sheet metal box by itself) is only one component of the ventilation system. To further complicate things, the CKV system is a subsystem of the overall building heating, ventilating and air-conditioning (HVAC) system.

Fortunately, there is no “magic” to the relationship between an exhaust hood and its requirement for replacement or makeup air (MUA). The physics are simple: air that exits the building (through exhaust hoods and fans) must be replaced with outside air that enters the building (intentionally or otherwise).

The essence of air balance: “air in” = “air out!” Background If the replacement air doesn’t come in, that means it doesn’t go out the exhaust hood and problems begin. Not only will the building pressure become too “negative,” the hood may not capture and contain (C&C) cooking effluents due to reduced exhaust flow. We have all experienced the “can’t-open-the-door” syndrome because the exhaust fan is sucking too hard on the inside of the restaurant.

The mechanical design may call for 8000 cubic feet per minute (cfm) of air to be exhausted through the hood. But if only 6000 cfm of outdoor air is able to squeeze in through closed dampers on rooftop units and undesirable pathways in the building envelope, then only 6000 cfm is available to be exhausted through the hood. The exhaust fan creates more suction (negative pressure) in an unsuccessful attempt to pull more air through the hood. There is no piece of equipment that generates more controversy within the food service equipment supply and design community than the exhaust hood in all its styles and makeup air combinations. The idea that by not installing a dedicated

makeup air supply, the operator is going to save money (in both first cost and operating cost) is short sighted. It may be okay if, by design, all of the makeup air can be provided through the rooftop HVAC units (this strategy has been adopted successfully by several leading quick-service chains). However, in full-service and institutional kitchens with larger exhaust requirements, it may not be practical to supply 100% of the replacement (makeup) air through the building HVAC system.

The solution is to specify an independent makeup air (MUA) supply. But, once MUA has been added to the system, the challenge becomes introducing this air into the kitchen without disrupting the ability of the hood to capture and/or without causing discomfort for the kitchen staff. Kitchens are not large and dumping 7000 cfm of MUA, for example, in front of a cook line does not go as smoothly in practice as it does on the air balance schedule! This design guide presents strategies that can minimize the impact that the makeup air introduction will have on hood performance, kitchen environment, and energy consumption.

An exhaust fan in the ceiling could easily remove the heat produced by cooking equipment. But mix in smoke, volatile organic compounds, grease particles and vapor from cooking, a means to capture and contain the effluent is needed to avoid health and fire hazards. While an exhaust hood serves that purpose, the key question is always: what is the appropriate exhaust rate? The answer always depends on the type (and use) of the cooking equipment under the hood, the style and geometry of the hood itself, and how the makeup air (conditioned or otherwise) is introduced into the kitchen. Cooking appliances are categorized as light-, medium-, heavy-, and extra heavy-duty, depending on the strength of the thermal plume and the quantity of grease and smoke produced. The strength of the thermal plume is a major factor in determining the exhaust rate. By their nature, these thermal plumes are very turbulent and different cooking processes have different “surge” characteristics. For example, the plume from hamburger cooking is strongest when flipping the burgers. Ovens and pressure fryers may have very little plume until they are opened to remove food product. Open flame, non-thermostatically controlled appliances, such as charbroilers and open top ranges, exhibit strong steady plumes. Thermostatically controlled appliances, such as griddles and fryers have weaker plumes that fluctuate in sequence with thermostat cycling (particularly gas-fired equipment). As the plume rises by natural convection, it is captured by the hood and removed by the suction