DCV control

If you have a normal VAV system with DDC control, exception b does not apply. Exception b could apply if you had a multi-zone system where each space with a thermostat had its own stand-alone control to modulate its damper, but did not have a VAV box controller communicating with a central DDC system. Yes, unless any of the other exceptions apply (a, c, or d), DCV is required in an auditorium fed by a multizone DDC VAV system that fits the parameters of 6.4.3.9. of ASHRAE 90.1-2007. A controls contractor I worked with recently said it normally works best to let the VAV box be controlled by temperature and use the DCV carbon dioxide sensor to control the outdoor air damper at the air handling unit. It can get tricky to satisfy both 90.1 and 62.1 while maintaining comfort conditions in the whole system, so the HVAC engineer should definitely look at the control sequence carefully. Depending on the envelope loads, etc. , it's possible the VAV zone damper for the auditorium would need to be forced open above minimum in some cases if the temperature is satisfied but the CO2 sensor is reading high. Hopefully there is plenty of heating capacity in the AHU and also reheat at the VAV box.