These devices are often installed to vent through a permanent opening. Modern devices have backdraft dampers that close automatically to prevent reverse flow through the unit, from the conditioned space outward. Many manufacturers market retrofit kits that can be installed to incorporate a damper into existing units. These can be money savers, especially for exhaust and vent penetrations. Operate exhaust fans only when necessary Kitchen vent hoods are a common example of over exhausting.
Many large kitchens operate their exhausts all day long.
They are constantly evacuating conditioned air from the space. The hoods are actually only needed when cooking is producing heat and smoke that needs to be exhausted. Commercial kitchen hoods typically vent huge quantities of air, so any reduction in their use has an immediate impact on utility bills. Proper adjustment of the fan speed can also lead to a lowering of energy use. The air flow can be measured and the fan pulleys or sheaves adjusted to provide the ideal flow rate.