**Abbreviations**

The following abbreviations are used in this manuscript:


#### **References**


**Marco Milanese \*, Francesco Micali, Gianpiero Colangelo and Arturo de Risi**

Department of Engineering for Innovation, University of Salento, 73100 Lecce, Italy; francesco.micali@unisalento.it (F.M.); gianpiero.colangelo@unisalento.it (G.C.); arturo.derisi@unisalento.it (A.d.R.)

**\*** Correspondence: marco.milanese@unisalento.it; Tel.: +39-0832-299-438

**Abstract:** Nowadays, energy saving is considered a key issue worldwide, as it brings a variety of benefits: reducing greenhouse gas emissions and the demand for energy imports and lowering costs on a household and economy-wide level. Researchers and building designers are looking to optimize building efficiency by means of new energy technologies. Changes can also be made in existing buildings to reduce the energy consumption of air conditioning systems, even during operational conditions without dramatically modifying the system layout and have as low an impact as possible on the cost of the modification. These may include the usage of new heat transfer fluids based on nanofluids. In this work, an extended experimental campaign (from February 2020 to March 2021) has been carried out on the HVAC system of an educational building in the Campus of University of Salento, Lecce, Italy. The scope of the investigation was comparing the COP for the two HVAC systems (one with nanofluid and the other one without) operating concurrently during winter and summer: simultaneous measurements on the two HVAC systems show that the coefficient of performance (COP) with nanofluid increased on average by 9.8% in winter and 8.9% in summer, with average daily peaks of about 15%. Furthermore, the comparison between the performance of the same HVAC system, working in different comparable periods with and without nanofluids, shows a mean increase in COP equal to about 13%.

**Keywords:** heat transfer fluid; nanofluid; heating; ventilation and air conditioning system; experimental test; coefficient of performance
