**5. Conclusions**

This paper has studied the energy performance of innovative building envelopes (facade and internal partitions), such as water flow glazing (WFG), coupled with an energy management system, as well as the relationships with steady and transient parameters. The energy strategies varied from a free-floating temperature regime on sunny winter days to the air-to-water heat pump, air heat exchanger, and buffer tank in summer conditions. A simple logic energy management system received inputs from temperature and relative humidity sensors. It controlled the heat pump and the air heat exchanger to deliver heat or cold to the buffer tank. The results included actual indoor air and glazing temperatures, heating and cooling energy consumption, and the influence of WFG in the mean radiant temperature and comfort.

Water-Flow Glazing was evaluated as a component of a hydronic radiant heating and cooling system. It showed final energy-saving potential, provided thermal comfort, and may be considered a valid option for office retrofitting. On the hottest day of the year, when the temperature ranged from 18 to 40 ◦C and the peak solar radiation was above 700 W/m2, the energy system consumed 32 kWh (0.8 kWh/m2) and the WFG managed to keep the indoor air temperature between 25 and 27 ◦C. The contribution of the air heat exchanger was negligible over the year because it was set to work for cooling only when the difference between the tank top temperature and outdoor temperature (*T\_tank\_top* − *T\_ext*) was above 10 ◦C. It complicated the piping and the control logic and did not improve the energy performance.

Radiant panels improve the performance of air-to-water heat pumps. The energy efficiency ratio (EER) reached 3.62 when the water temperature was 18 ◦C, and the coefficient of performance (COP) was 4.5 when the water temperature was 35 ◦C in heating mode. Using WFG as a radiant cooling facade and indoor partitions effectively reduced the operative temperature to comfortable levels when the indoor air temperature was between 25 and 27.5 ◦C.

The Predicted Mean Vote (PMV) in summer conditions was between 0 and −0.5 in working hours, within the recommended values of ASHRAE-55 standard. The MRT ranged from 19.3 to 23 ◦C, and the indoor air temperature ranged from 25.2 to 29.1 ◦C. In winter conditions, the electronic control unit was set to operate in heating mode if the indoor air temperature was below 20 ◦C. Then, for 45% of the working hours, the predicted mean vote was below −0.5, out of the comfort range, so the occupants would describe their comfort conditions as "Slightly Cool" or "Cool". The control unit logic should be fixed to start operating the heating mode if the indoor temperature drops below 21 ◦C. On mild sunny winter days, when the outdoor temperature reached 17 ◦C in the afternoon, the heat pump cooled down the buffer tank, but the WFG failed to deliver enough cooling power. The predicted mean vote was above 0.5, and the conditions could be described as "Warm" and out of the comfort range for more than three hours. There were two conditions to activate WFG in the cooling mode; first, indoor air temperature should be above 25 ◦C, and second, the difference between indoor air temperature and the bottom tank temperature should be more than 10 ◦C.

Water-Flow Glazing was evaluated as a component of a hydronic radiant heating and cooling system. It showed final energy-saving potential, provided thermal comfort, and may be considered a valid option for office retrofitting. The system is limited by its high initial cost and the need for an energy management system integrated with the rest of the equipment, especially the ventilation system and the heat pump. The ventilation system is an essential aspect of comfort. Controlling the relative humidity is indispensable in radiant systems to avoid condensation issues. Therefore, a more advanced ventilation device could help optimize the whole system's performance. Including a heat recovery and variable airflow would reduce the sensible and latent thermal loads and control the dew point temperature. There were uncertainties with the air-to-water heat pump operation. Although the radiant WFG panels could improve the heat pump COP and EER, there were issues with the operating cycles that could affect its performance. The selected heat pump was oversized, and frequently started and stopped because it prematurely detected that it had reached the target temperature.

After the first year of monitoring, there are uncertainties, misfunctions, and system issues that must be addressed. Firstly, due to the complexity of the elements involved in human comfort, the control unit must integrate the ventilation device. The operation logic should be able to modify the water mass flow rate and ventilation air heat flow. Secondly, the devices must be adequately dimensioned to avoid misfunctions, especially the air-to-water heat pump. Further research must include heat pump electricity monitoring to compare the actual thermal and electricity consumption and assess energy performance more accurately. Finally, further research on the standardization of its manufacturing process and deployment is needed to bring down initial costs and payback periods. Another research line would be to integrate WFG into commercial building performance simulations.

**Author Contributions:** Conceptualization, B.M.S., F.d.A.G., and J.A.H.R.; methodology, B.M.S. and F.d.A.G.; software, J.A.H.R.; formal analysis, B.M.S. and F.d.A.G.; data curation, J.A.H.R.; writing—original draft preparation, J.A.H.R.; writing—review and editing, F.d.A.G. and B.L.A.; visualization, B.M.S., F.d.A.G., and B.L.A.; supervision, J.A.H.R. and B.L.A.; project administration, B.M.S.; funding acquisition, F.d.A.G. All authors have read and agreed to the published version of the manuscript.

**Funding:** This article has been funded by the KSC Faculty Development Grant (Keene State College, New Hamshire, USA).

**Acknowledgments:** This work was supported by program Horizon 2020-EU.3.3.1: Reducing energy consumption and carbon footprint by smart and sustainable use, project Ref. 680441 InDeWaG: Industrialized Development of Water Flow Glazing Systems.

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