When a rooftop unit is equipped with an economizer, the system must also include an effective method for allowing excess air to escape the building in order to avoid the space over-pressurization. This effective means is known as a Power Exhaust. The basic system is to using a barometric relief damper in lieu of the power exhaust, but typically it is not capable of removing enough building return air to provide the desired building space pressure. Therefore, a Power Exhaust can be added to an economizer to adequately relieve building pressure.
For constant volume rooftop unit, a constant volume power exhaust can be used but for the VAV rooftop units the VAV power exhaust need to be used which the electrical motor can be equipped with a Variable Frequency Drive (VFD) and interlock with the economizer to change the exhaust air volume in respect with the Outside Air volume supplying to the space by economizer.