The Sensible Heat Ratio (SHR) of a space cooling load is a single number that tells you whether your AHU will actually maintain humidity. Indian designs frequently miss this. SHR = 0.85 looks fine on a calc sheet. SHR = 0.65 means you have a different design problem entirely. This article walks through how SHR shifts across Indian climates, why warm-humid zones are unforgiving, and what the AHU coil has to look like to keep up.
SHR by Indian climate zone
| Climate zone | Typical Indian commercial SHR | Driver |
|---|---|---|
| Hot & Dry (Ahmedabad, Jaipur) | 0.85-0.92 | Low OA humidity (4-5 g/kg); latent load dominated by people only |
| Composite (Delhi, Pune) | 0.78-0.86 | Monsoon transition pulls SHR low for 4 months/yr |
| Warm & Humid (Mumbai, Chennai) | 0.62-0.75 | OA at 18-22 g/kg dominates latent budget |
| Temperate (Bengaluru, Hyderabad) | 0.78-0.85 | Moderate OA humidity, mild peak |
| Cold (Shimla, Srinagar) | 0.85-0.95 | Cooling rare; when needed, sensible-dominated |
SHR below 0.70 means latent load equals or exceeds sensible. Standard 4-row AHU coil at 7°C/13°C chilled water cannot reliably hit those numbers without operational compromise.
What changes between sensible and latent removal
A cooling coil removes sensible by lowering DBT (air-side) and latent by lowering humidity ratio (mass-side). For the same coil:
- Adding rows (4 → 6 → 8) primarily improves latent removal
- Lowering chilled water temperature (7°C → 5.5°C) primarily improves latent removal
- Slowing face velocity (2.5 m/s → 2.0 m/s) improves both, latent more
- Adding reheat (downstream) increases coil sensible demand without reducing latent — useful for SHR matching at part-load
A coil “designed for” SHR 0.85 cannot deliver SHR 0.65. The physical apparatus dewpoint (ADP) of the coil determines how dry the leaving air can be. If room SHR < coil SHR, room humidity rises until equilibrium — which is to say, until your tenant complains.
Why Indian warm-humid is unforgiving
Mumbai 4-PM design condition: 33°C DBT, 28°C WBT, ~22 g/kg humidity ratio. Indoor target: 24°C / 50 % RH ≈ 9 g/kg. The coil has to drop 13 g/kg of moisture from the OA stream alone, and a person at 70 W sensible + 70 W latent contributes another 0.04 g/s/person to the latent load.
For a 100 m² office at 30 occupants:
- Sensible from people: 30 × 70 = 2.1 kW
- Latent from people: 30 × 70 = 2.1 kW
- Sensible from OA at 11 L/s/p × 30 = 330 L/s = 0.40 kg/s of OA, ΔDBT 9°C: 0.40 × 1.006 × 9 = 3.6 kW
- Latent from OA: 0.40 × 2501 × (0.022 – 0.009) = 13.0 kW
Total: sensible ≈ 5.7 kW, latent ≈ 15.1 kW, SHR ≈ 0.27 — for the OA + people load alone, before any envelope or equipment. This is why DOAS (Dedicated Outside Air System) makes sense in warm-humid India: pre-condition the OA separately, leave the terminal AHU to deal with sensible-dominated room loads.
DOAS as the response
DOAS architecture in Indian warm-humid:
- DOAS unit delivers neutral or cold-dry OA at 14°C / 50 % RH (~8 g/kg)
- Terminal unit (FCU, chilled beam, or VRF indoor) handles only sensible room load
- Coil sizing: DOAS coil designed for low SHR (~0.4); terminal at high SHR (~0.95)
- Energy benefit: 15-25 % lower fan energy, 8-12 % lower chiller demand vs mixed-air AHU
DOAS is now standard practice at 5-star hotels (Marriott, Hilton, IHG India SOPs all specify it for new builds in Mumbai/Chennai). Office and retail are catching up.
How MEPVAULT Psychrometric Analyzer helps
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For warm-humid design:
1. Set OA condition (Mumbai: 33°C / 28°C WBT)
2. Set return-air condition (24°C / 50 %)
3. Pick “mixing” mode → mixed air state at 30 % OA fraction
4. Switch to “cool_dehumid” mode → compute coil duty + SHR for any leaving DBT/RH
5. Iterate leaving condition until SHR matches your room SHR target
From the Field — Engineer’s Notebook
A Mumbai BPO floor commissioned in 2024 had a recurring complaint: 22°C DBT but “feels sticky.” Investigation: the AHU coil was sized for design SHR 0.78, but the actual room SHR (after we ran the load with realistic OA mass flow) was 0.62. Coil leaving DBT was 13°C as designed, but leaving RH was 95 % — the coil simply could not produce drier air with 4 rows at 7°C water. We retrofitted with 6-row coils + post-cool reheat for part-load humidity control, and 24-month follow-up showed RH stable at 52-55 %. Lesson: in warm-humid India, design SHR and coil SHR must be matched at the calc stage, not discovered six months in.
5 common mistakes
1. Designing for SHR ≥ 0.85 when actual room SHR is 0.65-0.75. Run the latent load explicitly; don’t assume default SHR.
2. Using 7°C chilled water in warm-humid without checking ADP. A 7°C water gives ADP ~10°C → leaving air saturated at 10°C → 7.5 g/kg. If you need 9 g/kg with reheat, OK. If you need 8 g/kg, you need 5.5°C water.
3. Single AHU mixed-air system in warm-humid hotels. DOAS is now standard.
4. Ignoring the OA flow’s full latent contribution. OA is the dominant latent driver in warm-humid, often 40-60 % of total Q_latent.
5. Sizing reheat as an afterthought. In warm-humid + ECBC compliance, reheat ≤ ASHRAE 90.1 §6.5.2.1 limits — must be specified at design, not added during commissioning.
Designer’s checklist
- [ ] Climate zone confirmed (warm-humid is most SHR-sensitive)
- [ ] OA latent contribution computed explicitly (not bundled with envelope load)
- [ ] Room SHR documented in cooling load report
- [ ] Coil ADP sized to deliver target leaving humidity ratio
- [ ] DOAS architecture evaluated for hotel + office in warm-humid zones
- [ ] Reheat strategy planned at design, not commissioning
- [ ] Part-load SHR verified at 50 %, 75 % design
- [ ] Coil rows specified explicitly (6-row baseline for warm-humid mixed-air systems)
- [ ] Drain pan trap depth verified per IS 8543
Pairs with: Psychrometric Analyzer, Cooling Load to AHU Selection