Kitchen vent hoods are a common culprit for over exhausting. Many large kitchens operate their exhausts all day long. As a result, they are continually evacuating conditioned air from the space. The hoods are needed only when cooking is producing heat and smoke that need to be exhausted. Commercial kitchen hoods typically vent significant quantities of air, so any reduction in their use has an immediate impact on the electric bill.
Modern vent hoods have a makeup air feature. They blow untreated outside air into the area just below the face of the hood so that not all of the air exhausted is conditioned air from the kitchen. This can
lower the volume of tempered air exhausted by 80 percent, and the venting performance of the hood is not adversely affected. Of course, just because the hood is very efficient doesn’t mean it can run all the time. If it’s carefully scheduled on and off, there will be considerable air-conditioning savings as well as electricity savings from the reduced fan operating hours.
Proper adjustment of the fan speed can also lead to a lowering of energy use. The airflow can be measured and the fan pulleys or sheaves adjusted to provide the ideal flow rate. Most food service exhaust hoods have manufacturer’s guidelines, or the applicable codes can be consulted to determine the proper volume rate of flow.
Laboratory hoods are another source of exhaust air losses. They’re available in the same energy-saving configuration as kitchen vent hoods, with makeup air features. It’s even more important to limit their exhaust operation, as labs are often kept colder than kitchen spaces. This means the HVAC system must spend a lot more energy treating the makeup air. Also, low-humidity conditions are often maintained in many labs. Dehumidification is achieved at a higher expense than even the usual air-conditioning cost.
Other exhaust fans vent air from restrooms. In larger buildings, several restrooms are often ducted together and exhausted by a single fan. The alternative is a small exhaust fan dedicated to each restroom. In the first arrangement the fan is often operating constantly, and as a result, it vents a steady volume of conditioned air from a building. In the second system, the fans can be interlocked with the room lights. A 5- or 10-min delay after the lights are turned off still reduces the operating time of the exhaust fan. This also works on a system exhausting several spaces via a single fan interlocked with the lights so that when any space is in use, the fan operates.
Another option is to put the exhaust fans serving noncritical areas on a time clock or to interlock them with the air handler. This restricts operation to the same schedule as the air handler, which is at least an improvement over constant operation. One advantage of this system, for buildings such as schools or libraries with high occupancy and therefore a high outside air requirement, is that running the exhaust constantly is necessary anyway, to maintain the fresh air supply and the overall building air balance.