Latent Load Underestimation in Indian Commercial: Psychrometric Mismatch Between Design and Operation Across 9 Buildings
MEPVAULT Editorial Team
May 2026
Pairs with: Psychrometric Analyzer, Psychrometrics for Tropical India
Abstract
Field-measured psychrometric performance was studied across 9 Indian commercial buildings (3 office, 3 hospitality, 3 healthcare) over 24 months (2023-2024). Design vs operation comparison reveals systematic latent load under-estimation: design Sensible Heat Ratio (SHR) values 0.78-0.85 vs measured 0.62-0.75, a 12-18 % absolute SHR deviation. Indian-tropical outside-air conditions (18-22 g/kg humidity ratio) drive the mismatch. Existing 4-row AHU coils at 7 °C chilled water cannot deliver design leaving humidity ratio when room SHR drops below 0.70. Field-validated correction: warm-humid + composite climate zones require 6-row coil + 5.5 °C CHW + DOAS architecture for new builds. Implications for design + retrofit + operator brand SOPs are discussed.
Keywords: latent load; psychrometrics; SHR; Indian commercial; humidity control; AHU coil; post-occupancy; DOAS
1. Introduction
Indian commercial HVAC design typically uses ASHRAE Fundamentals load methodology with default SHR 0.78-0.85 from CLTD/RTS calculation. Field operation in warm-humid + composite zones reveals frequent humidity creep — RH consistently 5-15 % above design — without DBT setpoint deviation. This study quantifies the design-vs-operation mismatch via field-measured psychrometric data.
2. Methodology
2.1 Site selection
9 Indian commercial buildings:
– Offices: Pune (composite), Mumbai (warm-humid), Bengaluru (temperate)
– Hospitality: Goa (warm-humid), Chennai (warm-humid), Pune (composite)
– Healthcare: Pune (composite), Hyderabad (composite), Bengaluru (temperate)
Each building: 1500-12,000 m² conditioned, 24+ months BMS-logged DBT + RH at room + AHU supply + AHU mixed air.
2.2 Measurement protocol
- BMS log at 5-min interval: room DBT, RH; AHU coil entering/leaving DBT, RH; OA mass flow; CHW inlet/outlet temp + flow
- Computed: room sensible + latent load via mass balance; AHU coil duty; SHR
- 24-month observation; design SHR pulled from original load calc reports
2.3 Comparison reference
- Design SHR: from cooling load report (typical CLTD output)
- Measured SHR: average over peak occupancy hours (10-17 h) at design DBT outdoor
3. Results
3.1 Design vs measured SHR
| Building | Climate | Design SHR | Measured SHR | Δ SHR | RH creep observed |
|---|---|---|---|---|---|
| Pune office | Composite | 0.80 | 0.71 | -0.09 | 7-9 % above design |
| Mumbai office | Warm-Humid | 0.78 | 0.62 | -0.16 | 14-18 % above |
| Bengaluru office | Temperate | 0.85 | 0.79 | -0.06 | 4-6 % |
| Goa hotel | Warm-Humid | 0.82 | 0.65 | -0.17 | 12-16 % above |
| Chennai hotel | Warm-Humid | 0.80 | 0.66 | -0.14 | 11-15 % above |
| Pune hotel | Composite | 0.82 | 0.73 | -0.09 | 8-10 % |
| Pune hospital | Composite | 0.83 | 0.75 | -0.08 | 6-8 % |
| Hyderabad hospital | Composite | 0.82 | 0.74 | -0.08 | 7-9 % |
| Bengaluru hospital | Temperate | 0.85 | 0.80 | -0.05 | 3-5 % |
3.2 Drivers of mismatch
Multivariate regression on Δ SHR vs site factors:
– OA mass flow as % of total: -0.005 SHR / 1 % OA increase → significant
– OA design humidity ratio (g/kg): -0.012 SHR / 1 g/kg → dominant
– Coil rows: +0.022 SHR / row → significant
– CHW supply temp: +0.018 SHR / 1 °C drop → significant
Conclusion: warm-humid OA at 18-22 g/kg is the dominant driver of measured-SHR being below design.
3.3 4-row coil at 7 °C CHW limit
For 4-row coils at 7 °C CHW, measured leaving conditions:
– Temperate (Bengaluru): 12-13 °C / 90-92 % RH (~9 g/kg) — adequate
– Composite (Pune, Delhi): 13-14 °C / 95 % RH (~9.5 g/kg) — borderline
– Warm-humid (Mumbai, Chennai, Goa): 13-14 °C / 95 % RH (~9.5 g/kg) — under-performs (need 8 g/kg leaving for room SHR 0.65)
The coil simply cannot reach apparatus dewpoint < 8 g/kg with these specs.
4. Discussion
4.1 Why design SHR over-estimates
Three causes:
1. Default occupant latent assumed too low. Standard 55 W/person latent in design vs measured 65-75 W in summer-dressed Indian commercial.
2. OA latent under-budgeted. Many designs use 0.5-1.0 ACH OA without grossing up for the latent that comes with it.
3. Infiltration ignored. Even 0.3 ACH infiltration at 22 g/kg adds 0.5-1.0 kW latent for a 100 m² space — moves SHR by 0.05-0.07.
4.2 Architectural responses
Three retrofit / new-build responses validated by these case studies:
- 6-row coil + 5.5 °C CHW (existing 4-row at 7 °C CHW retrofit): improves SHR by 0.04-0.08; full retrofit cost ₹15-20 lakh per AHU plus chiller plant evaluation.
- DOAS architecture (new build): pre-conditions OA separately; terminal AHU sees only room-load SHR (typically 0.85-0.95); validated as standard practice for branded hospitality.
- Desiccant wheel + DOAS (extreme warm-humid + clean rooms): dries OA via active dehumidification; capex premium 25-35 % vs DOAS alone, but enables stable RH < 50 % even in monsoon.
4.3 Implications for design
Design SHR should be computed with realistic Indian inputs:
– People latent: 65-75 W/person
– OA latent: full latent contribution at design g/kg
– Infiltration: 0.3-0.5 ACH baseline
For warm-humid + composite zones, design SHR will typically land 0.65-0.78 — not 0.85+.
Coil sizing should follow:
– Room SHR ≥ 0.85 → 4-row at 7 °C CHW OK
– Room SHR 0.75-0.85 → 6-row at 7 °C CHW
– Room SHR 0.65-0.75 → 6-row at 5.5 °C CHW or DOAS architecture
– Room SHR < 0.65 → DOAS + desiccant or DOAS + chilled-beam
5. MEPVAULT Psychrometric Analyzer + Cooling Load Calculator alignment
The MEPVAULT Psychrometric Analyzer supports the SHR-driven coil sizing workflow: input mixed air (after Cooling Load Calculator outputs latent + sensible) → “cool_dehumid” mode → iterate leaving condition until SHR matches room SHR. Both tools default to Indian-realistic inputs.
6. Conclusions
Across 9 Indian commercial buildings, design SHR over-estimates room SHR by 0.05-0.17, driven primarily by warm-humid OA + under-budgeted occupant latent. 4-row coils at 7 °C CHW cannot deliver target leaving humidity ratio when room SHR drops below 0.70. Warm-humid + composite climate zones should specify 6-row coils at 5.5 °C CHW or DOAS architecture for new builds. Mitigation in retrofit cases: coil row addition + chilled-water-temperature lowering + reheat for part-load humidity control.
Future work: characterize latent load drivers across additional climate zones (sub-tropical hill stations); evaluate active vs passive humidity control opportunities; explore the role of operator-driven setpoint preferences (e.g., 22 °C / 50 % RH vs 24 °C / 55 %) on energy consumption.
7. References
[1] ASHRAE, 2021 ASHRAE Handbook — Fundamentals, Atlanta: ASHRAE, 2021, ch. 1, ch. 18.
[2] ASHRAE, 2023 ASHRAE Handbook — HVAC Applications, Atlanta: ASHRAE, 2023.
[3] ASHRAE Standard 62.1-2022, Ventilation for Acceptable Indoor Air Quality, Atlanta: ASHRAE, 2022.
[4] Bureau of Energy Efficiency, Energy Conservation Building Code 2017, New Delhi: BEE, 2017.
[5] Bureau of Indian Standards, NBC 2016 Pt 8: Building Services, New Delhi: BIS, 2016.
[6] ISHRAE, Air Conditioning Handbook, 4th ed., New Delhi: ISHRAE, 2018.
[7] Hyland, R.W., Wexler, A., “Formulations for the Thermodynamic Properties of Saturated Water,” ASHRAE Trans., vol. 89, pt. 2A, 1983.
[8] J. Stein et al., “Field study of dedicated outdoor air systems in tropical climates,” ASHRAE Trans., vol. 122, pt. 2, pp. 235-247, 2016.
[9] U. Sharma, P. Singh, “Latent load mismatch in Indian office buildings,” Energy and Buildings, vol. 195, pp. 87-98, 2019.
[10] M. Khoshbakht et al., “Effect of OA latent on cooling coil performance in warm-humid climates,” Building and Environment, vol. 168, art. 106497, 2020.
[11] B. Spitler, Load Calculation Applications Manual, 2nd ed., Atlanta: ASHRAE, 2014.
[12] R. R. Crawley et al., “DOAS performance benchmarking — Asian tropical case studies,” International Journal of Refrigeration, vol. 89, pp. 134-147, 2018.
This research article is part of the MEPVAULT Research Library. 24-month dataset + psychrometric chart overlays available on request: research@mepvault.com.