J1 PROCooling load calculation: rules of thumb vs. a proper estimate
The components of a room cooling load (conduction, solar, people, lighting, equipment and fresh air), how to turn sensible load into supply airflow, and when a rule of thumb is acceptable.
"One ton per so many square metres" is quick, and it's fine for a first budget. But two rooms of the same area can differ in load by a factor of two, depending on glazing, orientation, occupancy and equipment. A proper estimate adds up each heat gain separately.
The components of a cooling load
| Gain | How it is estimated | Type |
|---|---|---|
| Walls and roof | U × A × temperature difference (adjusted for sun on the surface) | Sensible |
| Glazing, conduction | U × A × (outdoor − indoor) | Sensible |
| Glazing, solar | Area × solar gain for the orientation × shading coefficient | Sensible |
| People | Number × gain per person for the activity | Sensible + latent |
| Lighting | Installed W/m² × area | Sensible |
| Equipment | Nameplate or W/m² allowance × usage factor | Sensible (+ latent for kitchens) |
| Fresh air / infiltration | Airflow × (outdoor − indoor) for temperature and moisture | Sensible + latent |
In hot climates like the Gulf, west-facing glass and fresh air are usually the two largest items. That's why shading and heat recovery on the fresh air make such a difference.
Sensible, latent and total
Sensible load changes the air temperature. Latent load is moisture that the coil has to condense. Total load is their sum, and the sensible heat ratio (SHR) is sensible ÷ total. A low SHR (lots of moisture, as in meeting rooms or kitchens) needs a colder coil or dedicated fresh-air treatment.
From load to airflow
The supply airflow is set by the sensible load and the supply temperature difference:
Example: a room with 4.5 kW sensible load, a 24 °C room and 13 °C supply air (ΔT = 11 K) needs about 4,500 ÷ (1.23 × 11) ≈ 333 L/s.
When a rule of thumb is acceptable
- Early feasibility and plant-room space planning.
- Cross-checking a detailed calculation. A result far from the usual range for that building type points to a missing or doubled item.
Use a full room-by-room calculation for equipment selection, duct sizing and any tender or authority submission.
The J1 Pro Cooling Load calculator separates sensible and latent gains and gives the supply airflow and a report.
Guides are for preliminary design and learning. Always verify final designs against project specifications, local codes and manufacturer data.