Goord morning, 

I have a question regarding the minimum rate ventilation for enclosed parkings. For our project, we have an exhaust system for the enclosed parking that has a contaminant sensors control. This system has the capacity to fullfilled the minimum rate for parkings of .75 CFM/sqft on a peak demand. It also has VFD to decrease the energy use of the system, when the appropiate contaminant concentrations levels are maintained.  The sequence of operation desired by the client is the following: 
- For the daytime schedule, the exhaust system will only work when the contaminat sensors detect an increase over 25 ppm. If any sensor doesn´t detect any increase of concetration of contaminants, the exhaust system will remain turned off. This way the project will save the most energy possible.

- For the night schedule, the exhaust system will remain turned off from 8:00 pm to 7:00 am. 

Is this sequence of operation possible? Or it is mandatory to maintain a minimum ventilation rate even though the system has a contaminant control to indicate when the concentration arises?