Design VAV always with fully fresh air. Do not use recirculation. Today there are a lot of good way,s for heatrecovery with wheel or cross heater exchanger. Here in the Netherlands it is not allowed to use recirculation on a AHU. The requierd minimal ventilation rate is 1,5 ACH most VAV system are here induction VAV so we can control from 20% to 100% primairy air.
In case that you have a exsiting system with recirculation we place a Co@ sensor which will control the minimum setting.