# BEST--Building Energy-Saving Tool for Sustainable Residential Buildings
## 2 Materials and Methods
### BEST Tool Description
#### 2.1.1 Useful Energy Demand for Heating and Cooling Heat Balance  
The heat balance calculation algorithm implemented in the spreadsheet used in this study is more detailed than that prescribed by the UNI TS 11300 standards [41]. The development of a more refined method primarily arises from two limitations of the UNI TS 11300 standard:  
* The inability to adequately assess intermittent use of the heating system;
* The assumption that the external temperature remains constant, equal to the 24 h average temperature. The dynamic approach entails setting up an energy balance over a maximum time interval of one hour, which the procedure automatically reduces (multi-rate logic) based on the rate of temperature variation. This ensures a higher calculation density during periods of rapid change while optimizing computational efficiency during more stable periods. The user can specify the maximum number of time steps per hour, which consequently defines the minimum thermal balance period. The energy balance accounts for the various thermal exchanges affecting the building, including heat transfer, ventilation, internal gains, and solar gains. These exchanges are classified as either \"sensible\" or \"latent\", as the method includes a detailed psychometric analysis of air transformations. For both winter and summer seasons, the heat balance equation takes the following form:  
\[Q_{bil,sens}=Q_{tr+sol,\rho}+Q_{ve,sens}+Q_{int,sens}+Q_{sol,t}+Q_{core}+Q_{ restart} \tag{1}\]  
\[Q_{bil,lat}=Q_{ve,lat}+Q_{int,lat} \tag{2}\]  
During the winter season, if the heat balance Equations (1) and (2) yield a positive result, it indicates that the incoming heat exceeds the outgoing heat, resulting in a null thermal load. Conversely, if the balance is negative, a thermal load must be met through the HVAC systems. The procedure simulates each emission system as an air-based system, with appropriate values for recirculated and outdoor airflows. These values vary depending on the type of emission system, the temperature of the heat transfer fluid, the recirculation rate, the presence of heat recovery devices, the type of fans used, and other relevant parameters. For each step, based on the installed emission system, the procedure determines the supply air state required to meet the thermal demands established by Equations (1) and (2). If the system cannot control relative humidity (e.g., radiators, fan coil units, radiant floor heating, etc. ), the supply air state will compensate only for the sensible heat component. Otherwise, it will also account for humidity correction, either through humidification or dehumidification. Once the supply air state is determined, the procedure constructs the actual psychrometric transformation performed by the HVAC systems, considering both ambient conditions and the specific characteristics of the installed equipment. The thermal and cooling energy required for this transformation is then calculated, representing the net winter heating demand \(Q_{H,nd}\) or summer cooling demand \(Q_{C,nd}\). The psychrometric transformations performed follow a pattern similar to those illustrated in Figure 2.  
The psychrometric analysis integrated into the BEST tool is based on a detailed thermodynamic approach, grounded in the properties of moist air as defined in the ASHRAE technical handbooks (particularly the ASHRAE Handbook--Fundamentals). The psychrometric transformations are not resolved through approximate empirical correlations but through precise modeling that takes into account fundamental parameters such as dry bulb temperature, dew point temperature, relative humidity, absolute humidity, specific enthalpy, and specific volume. In the thermal balance calculation, the procedure implements an hourly iterative resolution of the state point of the supply air, based on the sensible and latent loads to be compensated. It clearly distinguishes between systems capable of regulating humidity (Air Handling Units with air treatment) and those that operate solely on the sensible load.