EngineerHub.orgChoose the level of detail
Basic — area/volume rule of thumb with simple multipliers.Advanced — steady-state heat transfer plus ventilation and internal gains.
Inputs
The basic model starts from 125 W/m² at a 2.5 m ceiling, then applies the selected factors. It is intentionally approximate.
Envelope
Cooling gains / allowance
Wall area is calculated from the room perimeter and height, less the entered window area. Cooling mode adds solar, people and equipment gains. Latent moisture loads are not calculated.
Estimated load
| Component | Load | Share |
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Background
Engineering basis, assumptions, interpretation and practical limitations.
Transmission: Q = U × A × ΔT
Ventilation: Q ≈ 0.33 × ACH × Volume × ΔT
For cooling, the model also adds window solar gain, 75 W sensible heat per occupant, and equipment or lighting heat. For heating, these internal gains are not credited, which is conservative for this simple screening model.
- Use design outdoor temperatures, not a typical daily average.
- Do not choose equipment only from the rounded result; check available modulation range and humidity control.
- For multi-room homes, calculate each room or use a proper whole-building load method.
- Cooling capacity does not directly equal electrical input power.
Basic mode is intentionally a rule-of-thumb screen. It starts from 125 W/m² at a 2.5 m ceiling and applies simple climate, insulation and solar-exposure multipliers, then adds sensible occupants and equipment gains in cooling mode.
Use it for rough early sizing only, not final equipment selection.
Advanced mode includes steady-state transmission through walls, windows, roof/ceiling and floor, plus ventilation/infiltration, solar gain through glazing, sensible occupant gains, equipment/lighting gains and a user-selected design allowance.
It does not calculate latent moisture loads, detailed solar geometry, duct heat gain/loss, thermal bridges, adjacent-zone interactions, equipment part-load behavior or local code compliance.
Frequently Asked Questions
Practical questions about Basic versus Advanced mode, sizing accuracy and equipment selection.
Basic mode uses a transparent room-size rule of thumb. Advanced mode estimates transmission, ventilation, solar and internal gains from envelope and design-condition inputs.
No. It is a preliminary screening calculator. Equipment selection should use a recognized room-by-room or building load method and local design weather data.
Oversized cooling equipment may cycle frequently, control humidity poorly and operate inefficiently.