The question can be considered as follows: is the non-residential BVU (described in the question) equipped with a device which can be considered a heat recovery system, as from the definitions laid down in Regulation (EU) 1253/2014 (see in particular Annex I.1.5)?
From the description it is not fully possible to grasp the specific features/characteristics of a mixing box utilising re-circulated air for heating and outside air for free cooling (air side economiser), but it would not seem to be a heat recovery system, because:
A mixing box (if it means a section of an air handling unit used to mix the return air flow with the outside air flow) is not, per se, a heat recovery system. The air side economiser installed on the outside air flow for free cooling (if it means a "duct-and-damper arrangement and automatic control system that together allow a cooling system to supply outdoor air to reduce or eliminate the need for mechanical cooling during mild or cold weather") seems not to be a heat exchanger designed to transfer the heat contained in the (contaminated) exhaust air to the (fresh) supply air, as from the definition laid down in Annex I.1.5 to Regulation (EU) 1253/2014.
Disclaimer: Please note that the European Commission cannot provide a legally binding interpretation of the EU legislation, as this is the sole competence of the European Court of Justice. Any remarks from the European Commission services are without prejudice to the position the Commission might take should related cases arise in a procedure before the Court of Justice.