*2.4. Nanofluid Loading Procedure*

The volume of heat transfer fluid within each HVAC thermal line was 1460 L. As the final step in preparing the building for experimental testing, the concentration of propylene glycol within the HVAC system (left and right wings of the building) was measured: it was the same in both systems and equal to 30% vol.

Therefore, the nanofluid was loaded within the HVAC-1 system (left wing) only, up to reach a nanoparticle concentration of 2% vol, leaving the right wing of the building (HVAC-2) loaded with the conventional water–propylene glycol heat transfer fluid: this approach allowed comparison of the performance of two identical systems, working with different heat transfer fluids at the same time.

**Figure 4.** HVAC system with data sensors positions and flowchart of test methodology.
