*3.2. September 2020 Results*

Figures 7 and 8 show the experimental results in terms of mean hourly COP and average daily COP obtained in September 2020. In this period, the HVAC machines worked as chillers in cooling mode.

**Figure 7.** Hourly COP comparison between HVAC-1 and HVAC-2 (7 September 2020–25 September 2020).

**Figure 8.** Average daily COP comparison and daily COP ratio between HVAC-1 and HVAC-2 (7 September 2020–25 September 2020).

It can be seen that increments in terms of COP were recorded during the entire period of experimentation, with lower values in the first week (average weekly increment equal to 6.7%), middle values in the second week (average weekly increment equal to 7.7%), and maximum values in the final week (average weekly increment equal to 11.2%), with a mean value of 8.9% over the entire period.

Nanoparticle concentration was measured before the data acquisition, in September. In fact, before the test period, the system was stopped from April to September. In that period, nanoparticle sedimentation occurred in the system. Density measured before the test campaign was 1065 g/L, therefore concentration was 1.7% in volume. Total re-dispersion of the nanoparticles was achieved after 29 h of the pumps being in operation, and the density was again 1079 g/L, therefore the concentration was again 2% (Figure 9).

**Figure 9.** Nanofluid dispersion during weight measurements in lab. (**a**) Density = 1065 g/L before pumps switched on. Concentration (1.7%); (**b**) Density = 1079 g/L after 29 h of running pumps. Concentration (2.0%).
