Conclusions and Recommendations
For proper use of the simulation tools and to get reliable results, the following procedure is suggested:
• The purpose of the simulation is defined precisely and accurately (what do we want to know?). Parameters and requirements are specified for the performance assessment.
• Program and input are chosen according to the problem and the answers looked for.
• The problem resolution is gradually increased (e. g., from a single-zone to multizone approach, from design air change rate to thermally driven natural airflows, from simple energy-demand calculation to integrated performance assessment, considering energy, thermal and visual comfort, and indoor air quality).
• Building and systems are modeled in accordance with this purpose. Geometry input may be established via CAD, elements then linked to database entities for properties, etc.
Detailed knowledge about the code and the underlying physical phenomena is necessary for the definition of an appropriate building modelization and for the input definition.
• Results are extracted and prepared as appropriate.
• A log file is maintained which documents the assumptions and decisions made in the evolution of the project.
• Quality-assurance procedures have to be established for the checking of both input and results: checks of energy balances, plausibility tests, and comparison with steady-state calculations and with results from similar cases. These checks are demanding and time consuming and thus prone to be omitted but are mandatory for reliable simulations.
• Models and results are properly archived.
Usually, a precise definition of the input parameters in absolute terms is very difficult. Nevertheless, in many cases this does not cause crucial problems because relative results may be sufficient to compare different design options. The influence on the results of uncertainties in input should be evaluated whenever possible by using sensitivity analysis techniques.
11.3.7.3 Modeling and Dimensioning of HVAC System and Components
The HVAC system is quite often not simulated but dimensioned on the space load results of the simulation by using simpler tools. However, aspects of control and the related potential for optimization are then neglected.
Thermal Comfort
For a person at a certain location in a room, direct radiation from internal heat sources may significantly affect the thermal comfort level. However, in the codes, room (or operative) temperatures are calculated on the basis of the room air and the wall surface temperatures only (both calculated considering the internal heat source, however).
Due to the methods and limitations outlined in Section 11.3.3, in thermal comfort analysis, draft risk evaluations cannot be performed using this type of room model. Analysis of air temperature stratification and thermal comfort for the occupant zone can be achieved only by using multi-air-node room models.
Room Airflow Pattern
Due to the assumption of well mixed air, detailed spacial zone air temperature distributions and ventilation efficiency studies cannot be performed. For this, CFD methods have to be applied (see Section 11.2).
However, with CFD, configurations with mostly known or at least steady- state boundary conditions and surface temperatures are calculated. In cases where the dynamic behavior of the building masses and the changing driving forces for the natural ventilation are of importance, thermal modeling and combined thermal and ventilation modeling must be applied (see Section 11.5).
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