Many buildings exhaust more air than the outside air requirements of the occupants. This is especially true for facilities that exhaust from laboratories, kitchens, or laundries. Any air that’s exhausted beyond the minimum cubic feet per minute of outside air equates o outside air that must be artificially introduced to the system and tempered accordingly. An energy-saving feature is to recirculate outside air after passing the volume through an activated carbon filter. This device removes odors by absorption. The filter element can be replenished periodically as performance falls off, although, if the air is contaminated with toxic or caustic fumes, it should never be recycled. The safest exhaust air systems for returning air to the conditioned space are exhausts from
laundries and dining rooms.
To reintroduce the exhaust air to the building, it must be ducted to the return side of the nearest air-handling unit. There it can mix completely with air returning from the space plus the proper amount of outside air to continue the conditioning cycle. It’s imperative that the air is ducted back to the same unit serving the exhausted zone, to maintain proper building system balance.
The quantity of exhaust air that’s returned can vary, according to when the exhaust fan is operating. In larger buildings, the fan often operates all the time, so the volume of returned air necessary to maintain air balance in the building can be determined while ensuring adequate air quality to the occupants.
Smaller facilities often interlock exhaust fans to the restroom lights, with a short delay in the circuit to keep the fan running long enough after occupancy terminates to circulate a full room volume of air out of the space. These facilities are usually designed to introduce a fixed amount of outside air to the air handlers at all times, so additional air must be supplied to make up any that is exhausted. Although, exhaust air from restrooms should never be recirculated.
One problem that may occur is that a reduction in exhaust volume may cause an increase in building pressurization, with a reduction in outside air intake at the air handler, especially in newer buildings with a low envelope permeability to infiltration and exfiltration. If this occurs, the exhaust volume will have to be partially vented to the atmosphere, gravity dampers will have to be installed to relieve the pressure buildup so that the outside air quantity is not diminished by the recirculation of exhaust air, or a differential pressure sensor will have to be installed in the system to modulate outside and relief air dampers.
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Excerpt from Energy Conservation Projects.