Method and assumptions
A simplified ASHRAE / Carrier E-20 style cooling load estimate, in SI units. Everything below is what the tool actually uses — no hidden factors.
Read this first
This is a handbook-level estimate, not a full design calculation and not a radiant time series (RTS) model. It must be checked and approved by a qualified HVAC engineer before any equipment is ordered or any document is issued. The numbers here are typical practice values for about 10° north latitude (Kerala / South India / the Gulf).
1. Design conditions
Two outdoor values and two indoor values drive the whole calculation. In the app these come from the country / city list, and you can type over them.
| Condition | Default | Where it is used |
|---|---|---|
| Outdoor dry bulb (DB) | 35 °C (Kochi) | Temperature difference across walls, glass and roof; sensible fresh-air load. |
| Outdoor wet bulb (WB) | 28 °C (Kochi) | Outdoor humidity ratio, and so the latent load on fresh and infiltrating air. |
| Indoor dry bulb | 24 °C | Comfort set point; ΔT = outdoor − indoor. |
| Indoor relative humidity | 50 % | Indoor humidity ratio; the latent load is proportional to (outdoor − indoor humidity ratio). |
Other built-in cities include Thiruvananthapuram 34/27.5, Palakkad 38/26, Chennai 38/28, Bengaluru 34/22, Delhi 43/24, Dubai 46/29, Abu Dhabi 46/29, Riyadh 46/22, Doha 46/28, Muscat 46/29 and Kuwait City 48/24. Always check the values against your local code.
2. The components
For each room the heat gain is built from these pieces, then summed.
| Component | How it is calculated | Notes |
|---|---|---|
| Glass — solar | glass m² × solar factor for the orientation × shading coefficient | Peak solar gain, storage effect already included. W orientation is the worst. |
| Glass — conduction | glass m² × U glass × ΔT | Outdoor − indoor dry bulb difference. |
| External wall | net wall m² (gross − glass) × U wall × wall ETD | ETD (equivalent temperature difference, the sol-air value) depends on orientation. |
| Roof | area × U roof × roof ETD | Applied only to rooms you tick as Roof — normally the top floor. |
| Partition | partition m² × U partition × (ΔT − 3 K) | Wall to a non-air-conditioned space. The 3 K is the usual allowance for the cooler side. |
| People — sensible | people × sensible W per person (space type) | 75 W for office / conference, 70 W for homes and classrooms, 210 W for a gym. |
| People — latent | people × latent W per person (space type) | 55 W for office, 35 W for a bedroom, 80 W in a restaurant, 315 W for a gym. |
| Lighting | area × lighting W/m² | 6 to 20 W/m² depending on the space type. |
| Equipment / power | area × equipment W/m² | 15 W/m² for an office, 300 W/m² for a server room. |
| Infiltration — sensible | 1.23 × air (L/s) × ΔT | Air from the air-change rate: ACH × volume. |
| Infiltration — latent | 3010 × air (L/s) × ΔW | ΔW is the difference in humidity ratio (kg/kg) between outdoor and indoor. |
| Outdoor / fresh air (ASHRAE 62.1) | (people × L/s per person) + (area × L/s per m²), then the same 1.23 and 3010 formulas | The ventilation air is the biggest single block in hot, humid climates. It is counted on top of the room load. |
| Safety factor | sensible and latent room load × (1 + safety %) | Applied to the room load only, before fresh air is added. |
3. The factors and units actually used
These are the only conversion numbers in the engine. Everything else is built on top of them.
- 1 TR = 3517 W. The tonne of refrigeration is the metric tonne (3.517 kW), not the 12,000 Btu/h ton. Total load in watts is divided by 3517 to get TR.
- Sensible: 1.23 × L/s × ΔT watts. The constant 1.23 is ρ × cp for air (about 1.2 kg/m³ × 1.005 kJ/kg·K, with the L/s-to-m³/s factor folded in).
- Latent: 3010 × L/s × ΔW watts, where ΔW is the humidity ratio difference in kg/kg.
- Area: 1 m² = 10.7639 ft². Used for the ft² column and for the ft²/TR density check.
- Airflow is shown in L/s (litres per second). 1 L/s = 2.11888 CFM = 3.6 m³/h, if your schedule or tender is in another unit.
- Supply air: room sensible heat divided by (1.23 × supply ΔT), so the figure depends on the supply temperature difference you set.
- Humidity ratio is worked out at sea level (101.325 kPa) from the dry bulb / wet bulb pair outdoors and the dry bulb / RH pair indoors, so the latent load follows the local climate.
- Peak solar gain through glass (W/m²): N 110 · NE 300 · E 440 · SE 300 · S 130 · SW 350 · W 470 · NW 300. West is the worst side; the storage effect is already inside these figures.
- Wall ETD (K) for a 230 mm plastered brick wall: N 7 · NE 10 · E 12 · SE 11 · S 9 · SW 13 · W 15 · NW 12.
4. Default assumptions
These are the starting values. Every one of them is adjustable in the calculator under Construction and system assumptions, and every one is printed on the report so a checker can see what was used.
| Assumption | Default | Basis |
|---|---|---|
| Wall U value | 2.0 W/m²K | 230 mm brick wall, plastered both sides. |
| Glass U value | 5.8 W/m²K | Single clear glass. |
| Shading coefficient (SC) | 0.6 | Clear glass with internal blinds. One value covers all shading. |
| Roof U value | 2.0 W/m²K | RCC slab with waterproofing / weathering course. |
| Roof equivalent ΔT (ETD) | 22 K | Sol-air value for an exposed roof in this climate. |
| Partition U value | 2.2 W/m²K | Partition wall to a non-air-conditioned space. |
| Infiltration | 0.5 air changes per hour | Closed building, doors mostly shut. A simple estimate. |
| Safety factor | 10 % | Added to the room sensible and latent load. |
| Supply air ΔT | 11 K | Room air minus supply air, used to size the supply airflow (L/s). |
| Glazing, when glass area is blank | 30 % of the exposed wall | A starting window-to-wall ratio until you measure the drawing. |
If a room has a length and a width, one long side × height is taken as the exposed wall, and the glass is 30 % of that. If you can measure the real glass and wall areas, type them in — they change the answer more than any other field.
5. What the method leaves out
Being honest about the gaps is part of the method. All of the following are outside the calculation and must be added or checked by an engineer when the equipment is selected.
- ETD / CLTD is a simplification. A single peak equivalent temperature difference stands in for the whole day's heat flow through a wall or roof. It does not model thermal mass, the real peak hour, or how the load shifts into the evening. A full radiant time series or a proper hourly model will give different numbers.
- One shading coefficient covers overhangs, fins, blinds, neighbouring buildings and reflected radiation.
- No duct heat gain, no duct leakage and no fan heat.
- No diversity factor on people, lighting or equipment — everything is at its stated value at the same time.
- Infiltration is a simple air-change estimate; door openings, stack effect and window leakage are not modelled.
- Toilets, staircases and shafts are assumed to be unventilated and are excluded from the totals by default.
- Altitude, part-load behaviour, redundancy and controls are not part of the load at all.
Requirement
Every value produced by LoadLens must be verified by a qualified HVAC engineer against the actual architectural drawing, the real construction build-up and the ventilation code that applies to the project (for example ASHRAE 62.1, NBC India or ECBC) before it is used for equipment selection.
Want to check the arithmetic yourself? The whole engine is one readable file, and the Help page shows where each number appears in the report.