Room load calculator
A room-by-room cooling and heating load with every watt attributed: conduction, solar, people, lighting, equipment, ventilation and latent. Fast enough to use before the building model exists, and explicit enough to argue with.
| Component | W | Share |
|---|---|---|
| Conduction through walls and roof | 653 | 12.7% |
| Solar through glazing | 1600 | 31.2% |
| People, lighting and equipment | 1330 | 25.9% |
| Ventilation and infiltration | 995 | 19.4% |
| Latent gain | 555 | 10.8% |
The sum is the coil load before duct gain and fan heat, which add roughly 3–8%.
Q = U·A·ΔT · Walls, roof and glazing separately.
Q_s = 1.2 · L/s · 1.005 · ΔT · Sensible heat in the outdoor air.
V = Q_s / (1.2 · 1.005 · ΔT_supply) · Litres per second at the coil.
A clear path from inputs to results.
Before the building model exists, someone has to answer "how much cooling does this room need?". This splits the load into conduction, solar, internal gain, ventilation and latent, shows every watt, and sizes the supply air.
How to use
- Enter the room dimensions and the envelope U-values.
- Add glazing, occupants, lighting, equipment and ventilation.
- Read the total load, the sensible/latent split and the supply airflow.
The formula
Reference: Steady-state cooling load; ASHRAE Fundamentals for the air properties.
Worked example
8 × 6 × 3 m office, six occupants
- U 0.35 wall, 0.25 roof, 8 m² glazing at SHGC 0.4, 500 W/m² solar, 35 °C outside, 24 °C inside.
- 1
Conduction about 0.9 kW, solar 1.6 kW, internal 1.2 kW, ventilation 0.7 kW, latent 0.5 kW.
About 4.9 kW, or 1.4 tons, with roughly 950 L/s of supply air at a 9 K difference.
Interpreting the result
A high latent share means the coil has to run colder, not just move more air. If the sensible heat ratio is below about 0.75, check the ventilation rate before upsizing the unit.
Assumptions & limitations
- Steady state: no thermal mass and no radiant time series.
- Single solar irradiance for all glazing, regardless of orientation.
- No duct gain or fan heat.
- Not a compliance calculation.
Frequently asked questions
Is this a Manual J?
No. Manual J is a specific residential procedure with its own tables; this is a transparent steady-state estimate.
How do I pick the supply ΔT?
8–11 K for a conventional overhead system; a chilled beam or displacement system runs smaller.
Why is my load lower than the contractor’s?
They usually add duct gain, fan heat and a safety margin, which together can add 15–25%.
What about heating?
The same envelope with the winter design temperature, shown alongside the cooling result.
Methodology last reviewed 2026-10-22. Results are informational and not financial or engineering advice — see our disclaimer.