Picking the right cooling-load method is the first design decision that ripples through every downstream choice — chiller plant, AHU coil rows, duct main static, and the final sqm/TR ratio your client measures you against. Three methods dominate Indian practice: CLTD/CLF/SCL (the workhorse), RTS (modern ASHRAE default), and HBM (the rigorous baseline). Each makes different assumptions, and each lands at a different number for the same building.
What changes between the three
CLTD (Cooling Load Temperature Difference) / CLF (Cooling Load Factor) / SCL (Solar Cooling Load) — the method most Indian consultants learned first. Pre-tabulated CLTD/SCL/CLF values absorb the time-lag between heat gain entering a wall/glass surface and that heat showing up as a cooling load on the conditioning system. Fast to calculate by hand, lookup-table heavy, accuracy modest. Tables are referenced to a Latitude 40°N standard and need correction for Indian latitudes (8°N to 35°N). The ASHRAE Fundamentals chapter still ships the tables for design use, but flags them as legacy.
RTS (Radiant Time Series) — the post-2001 ASHRAE default. RTS replaces the static CLTD lookup with two time-series convolutions: the CTS (Conduction Time Series — how heat passes through the construction) and the RTS (Radiant Time Series — how that heat is then released into the space air). RTS handles thermally massive constructions (like 230 mm brick + plaster, the Indian standard exterior wall) more accurately than CLTD. It also explicitly separates radiant from convective gains so you don’t double-count.
HBM (Heat Balance Method) — the rigorous engine inside EnergyPlus, OpenStudio, eQUEST, and Trane TRACE 3D Plus. Solves the energy balance simultaneously at every interior + exterior surface every timestep. Produces the most physically defensible answer but requires hourly weather data, surface-by-surface envelope inputs, and trained software hands. Used for compliance modelling (ASHRAE 90.1 App G, ECBC PRM, LEED EAp2), not first-pass design.
Where the numbers diverge
Run the same Indian commercial floor — say a 1500 m² Pune office, double-glazed reflective on the west façade, 230 mm brick on east/south, RCC slab over cooled space — through all three:
| Method | Peak load (TR) | Sqm/TR |
|---|---|---|
| CLTD/CLF/SCL (hand calc, latitude-corrected) | 96 | 15.6 |
| RTS (HAP 5.1) | 90 | 16.7 |
| HBM (HAP heat balance / TRACE 3D Plus) | 87 | 17.2 |
The 9 % spread between CLTD and HBM matters at chiller selection: a 100 TR vs 90 TR screw chiller is one frame size + 12-15 % on opex over 15 years. CLTD over-estimates because the legacy SCL tables don’t fully credit thermal mass, and the latitude correction for low-Indian-latitude west sun is rough.
When each method is the right choice
CLTD — quick design-development sizing, BOQ extraction, sanity-checking a third-party load calc. Don’t ship a tender on CLTD alone for a project above ~500 TR.
RTS (via HAP) — production design for any project where you’d run HAP anyway. ASHRAE 62.1 VRP, 90.1 fan kW check, building-load report — RTS is the default. India consultancy practice has converged on HAP RTS as the production tool.
HBM — ECBC compliance, LEED EA, IGBC EE, GRIHA Criterion 14. Required for any whole-building performance path. Also: thermally massive constructions (rammed earth, hollow concrete block, exposed slab cooling) where CLTD/RTS approximations break down.
How MEPVAULT’s Cooling Load Calculator handles it
Open Cooling Load Calculator →
The calculator runs a CLTD method with Indian-latitude-corrected SCL tables (built in for hot-dry, warm-humid, composite, temperate, and cold ECBC zones). It produces a peak load in TR + kW, a sqm/TR figure, sensible heat ratio, and a component breakdown chart so you can see exactly where the load is coming from. The method is documented; the lookup tables are server-side so they cannot be reverse-engineered into a competing Excel.
For projects above 500 TR or any compliance path, run the same building through HAP after this calculator gives you the first-pass envelope and the chiller frame. The two should agree within 5 %; if they don’t, walk through the gain-by-gain output and find which assumption diverged.
From the Field — Engineer’s Notebook
On the Yashvant Mall expansion in Pune (2024 retrofit), the original 1998 design was sized at 14.0 sqm/TR using a manual CLTD calc. The new tenant brought in higher equipment density (retail-electronics with display-zone backwall LEDs running 18 hours/day), and we re-sized at 16.8 sqm/TR using HAP RTS. The old plant was actually sufficient — the original CLTD over-estimated by 16 %, and the higher new-tenant equipment load was absorbed within the existing capacity. We saved the client an entire chiller frame replacement by running RTS instead of trusting the legacy CLTD output.
5 common mistakes
1. Using CLTD tables without latitude correction. ASHRAE published CLTD at Latitude 40°N. Indian projects are 8-35°N. Apply the latitude correction factor (typically 0.85-0.95 multiplier for west walls in Indian latitudes) or your CLTD numbers will run high by 8-12%.
2. Mixing latent and sensible at the room level. People + outside air both have latent components. CLTD/RTS handle sensible cleanly; latent is computed separately. Combining them at the space level (instead of at the AHU level) leads to over-sized coils.
3. Not separating top floor from middle floor. Roof CLTD on Indian summer is 31-35°C — top floor will run 30 % higher TR/m² than mid floors. Size each floor separately or you’ll oversize the entire stack.
4. Treating glass solar gain at the design hour without diversity. West-facing glass peaks at 15:00; building peak is 14:00-16:00. East glass peaks 09:00. Use SCL/CLF at the design building peak hour, not the orientation-peak hour.
5. Ignoring infiltration-driven latent in warm-humid zones. Mumbai/Chennai/Kolkata: outside air at 30°C / 80 % RH carries 20-22 g/kg humidity. Even 0.3 ACH infiltration can add 8-12 % to total load and shift SHR by 0.05.
Designer’s checklist
- [ ] Climate zone confirmed against ECBC 2017 § 3.2 (hot-dry / warm-humid / composite / temperate / cold)
- [ ] Outdoor design DBT/WBT cross-checked against ISHRAE Handbook for the actual city, not the climate zone average
- [ ] Indoor design 24°C / 50 % RH for office, 22°C / 55 % for hospitality, 21°C / 50 % for healthcare
- [ ] Latitude correction applied to CLTD if using legacy tables
- [ ] Separate top floor calc, with roof CLTD applied
- [ ] OA per ASHRAE 62.1 Table 6-1 (Indian default 8.5 L/s/person for office)
- [ ] Infiltration ACH 0.5 minimum for tested commercial; 1.0 for retail with frequent door open
- [ ] SHR reported alongside total TR — coil selection driven by SHR
- [ ] HAP RTS run for sanity check on any project above 500 TR
- [ ] Component breakdown chart reviewed with client architect (orientation, glazing area, SHGC are the design levers)
Pairs with: Cooling Load Calculator, Research Paper 021 — CLTD vs RTS vs HBM Validation, India