I am a Clean Room Specialists and am having some issues with a dynamic system which is being controlled by Price vav's at which are attempting to maintain a negative pressure and constant flow in a space. I have noticed at times that the DP varies although flows are being maintained and duct static pressures are maintained.

The system varies from -.06(ideal) to -.09(life safety concern) however like I said the flow rates have negligible change no more than about 35cfm on a system that flowing 1200cfm. When the system pressure gets upwards of -.09 we have door closing issues. Any suggestions or information would be helpful.