🌡️ Boundary Conditions
Define thermal conditions on the exposed edges of your model — convective, fixed temperature, or adiabatic.
Overview
Boundary conditions (BCs) are applied to the outer edges of your geometry. They tell the solver what happens at each surface: is it exposed to an environment with convective heat transfer, held at a fixed temperature, or thermally insulated (adiabatic)?
All outer (exposed) edges must have a boundary condition assigned before the model can be solved. Interior edges between adjacent regions are handled automatically by the solver via continuity conditions.
BC Types
The most common BC type. Defines the ambient temperature (T) and the surface film resistance (h, film coefficient in W/m²·K). The flux at the boundary is: q = h × (T_surface − T_ambient). Used for interior and exterior surfaces exposed to air.
Prescribes a constant temperature at the boundary. The solver enforces the exact temperature at all nodes on this edge (Dirichlet condition). Useful for modeling surfaces in direct contact with an isothermal body.
No heat flow across this boundary (zero flux / Neumann condition). Used for symmetry planes, cut edges at mid-sections, or well-insulated surfaces where heat loss is negligible.
Convective BC Parameters
| Parameter | Symbol | Unit | Description |
|---|---|---|---|
| Film coefficient | h | W/(m²·K) | Combined convective + radiant surface film coefficient |
| Ambient temperature | T | °C | Temperature of the adjacent air / environment |
| Resistance | R = 1/h | m²·K/W | Derived surface resistance (shown for reference) |
Standard Presets
ThermX includes two sets of standard boundary condition presets that can be applied with one click:
CEN (European Standard)
Based on EN ISO 10211 and EN 673 standard conditions for building thermal analysis:
| Surface | h (W/m²·K) | Rsi/Rse (m²·K/W) | Temperature |
|---|---|---|---|
| Interior | 7.7 | Rsi = 0.13 | +20°C |
| Exterior | 25.0 | Rse = 0.04 | 0°C or −10°C |
NFRC (US Standard)
Based on NFRC 100 methodology for fenestration rating:
| Surface | h (W/m²·K) | Temperature |
|---|---|---|
| Interior | 8.3 | +21.1°C (70°F) |
| Exterior | 30.0 | −17.8°C (0°F) |
Use the CEN preset for European building certification (EN ISO 10211, EN ISO 10077). Use NFRC for North American window frame analysis.
Applying Boundary Conditions
- Switch to the Boundary Conditions tab in the right panel
- Click on an edge of your geometry (a line on the canvas) to select it
- For multiple edges: hold
Shiftand click additional edges (multi-edge selection) - Choose a BC type (Convective / Fixed Temperature / Adiabatic)
- For Convective: enter the film coefficient (h) and ambient temperature (T)
- Click Apply
You can also click a preset button (Apply CEN Interior, Apply CEN Exterior, etc.) to fill in values from the standard library.
Multi-Edge Selection
To apply the same BC to many edges efficiently:
- Hold
Shiftand click edges individually to add them to the selection - Or drag a selection box to select all edges within a region
- All selected edges are highlighted in the canvas; the BC panel shows the count
- Apply the BC once — it is assigned to all selected edges simultaneously
Color-Coded Edge Rendering
Boundary conditions are color-coded on the canvas for quick visual inspection:
- Blue — Convective (interior)
- Orange — Convective (exterior)
- Red — Fixed temperature
- Gray — Adiabatic / no BC assigned
An interactive legend is shown in the canvas overlay. Hover over any color in the legend to highlight all matching edges on the canvas.
BC Validation
Before solving, ThermX checks that all exposed outer edges have BCs assigned. Unassigned edges are highlighted and listed in the validation panel. Resolve all warnings before proceeding to the solver.