**4. Conclusions**

In this work, we investigated the performances of two full-scale HVAC systems, installed at the educational building "Corpo O" in the Campus of University of Salento, Lecce, Italy, working with a conventional water–glycol mixture and with Al2O3-nanofluid. In particular, the nanofluid was composed of water–glycol and 2% vol aluminum oxide (Al2O3) nanoparticles, having a controlled size distribution between 100 nm and 600 nm and a controlled stable suspension during the operation in the system. Long term stability of the nanofluid caused reliable and consistent thermal performance over a wide temperature range with limited effects on viscosity.

The results obtained in three experimental campaigns allowed both to quantify the performance increase due to the use of nanofluid instead of the conventional water and glycol mixture:


Although the relationship between the HVAC system performance and the use of nanofluids needs to be better investigated, the results of this work suggest that nanofluids can significantly improve the performance of air conditioning systems.

**Author Contributions:** Conceptualization, M.M. and A.d.R.; methodology, M.M., F.M. and G.C.; validation, M.M. and G.C.; writing—original draft preparation, M.M. and F.M.; writing—review and editing, M.M. and G.C.; supervision, A.d.R.; project administration, A.d.R.; funding acquisition, A.d.R. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
