First of all, thanks a lot for your answer. Probably I understood wrong, but it doesn't seem what I wanted to model.

This system is placed within the AHU and serves multiple zones. This system is used in place of a heat recovery, between the outside air flow (coming into the AHU) and the exhaust air flow (going out from the AHU); it doesn't only recover heat but it is a heat pump air to air that works with very advantageous conditions. The problem is that it works in the AHU, not directly in the indoor zones.