Heat
One fully mixed zone with effective thermal storage. Outdoor conduction, solar gains, internal gains and signed ventilation enter an energy balance. It does not calculate surface or operative temperatures.
Synthetic weather · no login
Method
003 · A room, a day, five decisions
How much can the room do on its own?
Compare two room designs. See what shade, storage and ventilation change—and what they do not. Free educational calculations in your browser. No account or upload.
Start with one design question
Keep an A. Try a B.
Name your alternatives, copy one into the other, then change a single setting. Both use the same synthetic day. Save a study to continue later; export a brief to discuss the trade-offs.
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Illustrative climateSynthetic day · 1.3° N
Hot afternoons, warm nights, diffuse brightness, and useful southeast breezes.
02 · Schematic section
4:00 PM · Facade SE
Synthetic 24-hour cycleExplore every hour, including night
4:00 PM
Same weather · separate trade-offs
What changed between A and B?
Always B minus A, whichever you are editing
Why the model can change
Transparent by design
A thinking instrument, not an engineering certificate.
Solar gain
A fixed +20° solar declination and local solar time determine facade exposure. A top-mounted overhang shades direct beam; diffuse light is not shaded. Solar gain is heat through the window, not daylight quality.
Air
A simple wind/stack estimate drives controlled purge ventilation, plus 0.35 ACH infiltration. Arrows are illustrative. Neither indoor air velocity nor humidity effects are calculated.
Use it early
Three synthetic days are teaching scenarios, not weather records. The 22–28°C band is illustrative; 08:00–20:00 is the occupied interval. Real decisions require site weather, calibrated models and qualified review.
Assumptions, equations & model limits
C dT/dt = H (Tout − T) + Qsolar + Qinternal. Each 10-minute interval uses the exact constant-forcing solution. Days repeat until the full temperature trace and opening decisions settle, or 60 cycles are reached.
Floor area 36 m²; volume 108 m³; four external walls; one 18 m² glazed facade; window height 2.4 m; adiabatic floor. Wall / roof / glass conductance: 0.35 / 0.25 / 1.8 W/m²K. Glass solar transmission 0.5. Effective thermal storage 80–260 kJ/m²K. Internal gains 4 W/m² occupied, 1 W/m² otherwise.
Maximum effective opening 0.5 m², wind coefficient 0.25, stack discharge coefficient 0.6, stack height 1.5 m, air density 1.2 kg/m³, heat capacity 1005 J/kgK. Two proportional opening ramps reach full opening at room temperature 25.5°C and an indoor–outdoor difference of 1°C. This is an illustrative implementation inspired by standard zone heat balances and wind/stack equations, not EnergyPlus or a validated substitute.
No HVAC, moisture/latent heat, adjacent buildings, detailed envelope solar absorption, daylight distribution, or human comfort model. Control switching and the single-node approximation introduce uncertainty.
Method background: EnergyPlus ventilation reference ↗. Source assumptions are included in every exported brief.
Both alternatives are editable. Initial A is an illustrative west-facing, unshaded starting point, not a measured building. Solar / Heat / Air change the drawing emphasis, not the calculation. Nothing is autosaved; save study JSON before leaving. Text briefs cannot be reopened as studies.