CAV VAV systems use either constant or cycling fan mode. For cycling fan mode supply air temperature control with zone reheat boxes used to trim zone temperatures to the system should maintain heating set point temperature. If the zone thermostat temperature temps are mostly in the dead band during the cooling and heating season. For constant fan mode, season then the zone supply air temperature will be impacted by is too cold, or the operation of the fan when no cooling or heating is needed, and the outside VAV box minimum stops are too high. You might try using a supply air system, if used.temperature set point reset strategy based on outdoor air temperature (see SetpointManager:OutdoorAirReset).