In a hot-humid Indian climate, the outdoor air load can be 50-70% latent (moisture). Traditional all-air HVAC systems handle both sensible and latent at the same coil — at deep dewpoint requirements (5-7 °C supply air dewpoint) — wasting reheat energy after dehumidification. A Dedicated Outdoor Air System (DOAS) decouples the two: a separate unit handles outdoor air with deep dehumidification; parallel sensible cooling handles internal sensible loads at higher chilled-water temperature.
The result: 25-35% reduction in peak chilled-water demand, 40-50% reduction in annual cooling energy, and IAQ improvements that are often the deciding factor for Indian green-building credits.
What “DOAS” actually does (at a system level)
In a conventional VAV system:
- One AHU mixes outdoor air with return air → cools both to ~12 °C → reheats to ~16 °C → delivers to zones
- Latent (moisture) handled at the same coil as sensible
- Reheat energy + deep cooling = energy waste
In a DOAS:
- A separate DOAS unit treats only outdoor air — deeply dehumidified to ~6-10 °C dewpoint
- Outdoor air supplied directly to zones (or to AHU mixing box at controlled rate)
- Parallel cooling system (chilled beam, fan-coil, in-duct cooling coil) handles internal sensible loads at warmer chilled water (~12 °C, not 6-7 °C)
- Both systems run independently; latent is fully handled by DOAS
Energy benefits:
- Higher chilled-water temperature → higher chiller COP → 15-25% pump + chiller energy savings
- No reheat needed (DOAS supplies cold dry air; sensible cooling separately)
- Outdoor air load reduced by ERV (75-85% sensible + 65-75% latent recovery typical)
Two DOAS architectures
Architecture A: DOAS direct to zone (typical for office/hospitality)
DOAS dehumidifies + supplies outdoor air directly to each zone via dedicated supply diffusers. Parallel sensible cooling (chilled beam, fan-coil, VAV with re-cool) maintains zone setpoint.
Pros: simplest in terms of ductwork; outdoor air bypass control of ventilation.
Cons: more complex zone-level integration; may oversize duct runs.
Architecture B: DOAS to AHU mixing box (typical for retrofits)
DOAS dehumidifies outdoor air; supplies to AHU mixing box at fixed temperature/dewpoint. AHU continues VAV operation to zones.
Pros: works with existing AHUs; minimum system reconfiguration.
Cons: AHU still acts as latent absorber to some degree; less benefit than direct-to-zone.
For Indian new construction, Architecture A is preferred. For retrofits, B.
DOAS unit selection
Step 1: Outdoor air design conditions
For Mumbai 1% summer: 36 °C dry-bulb, 28 °C wet-bulb (peak humid hour).
For Delhi 1% summer: 41 °C dry-bulb, 23 °C wet-bulb (drier).
For Bangalore 1% summer: 32 °C dry-bulb, 22 °C wet-bulb (mild).
For Mumbai DOAS sized at 36 °C / 28 °C wb input → typically supplies at 6 °C dewpoint = 8-10 °C dry-bulb.
Step 2: DOAS supply temperature
- Cold DOAS (8-10 °C dry-bulb): supplies fully cooled outdoor air; absorbs latent + provides some sensible cooling to zone. Used when DOAS is direct-to-zone.
- Neutral DOAS (22-24 °C dry-bulb): pre-cooled and de-humidified to neutral temperature; zone cooling handles only internal sensible. Used when zones have different temperature requirements.
For typical office buildings: cold DOAS at 8 °C dewpoint.
Step 3: Energy recovery
Almost universal in new DOAS for Indian climates. Enthalpy wheel typical (75% sensible + 70% latent recovery). Pre-conditions outdoor air using exhaust:
- Mumbai 36 °C / 28 °C → through ERV → exits at ~28 °C / 22 °C wb
- DOAS coil only needs to cool from 28 / 22 → 8 / 8 (saturated)
- 60% reduction in coil load vs no-ERV
Step 4: DOAS coil load
For Mumbai, 5,000 cfm DOAS:
- Without ERV: cooling load ~80 kW (23 TR)
- With ERV (75/70%): cooling load ~30 kW (8.5 TR)
- 60% reduction
DOAS unit is typically a packaged rooftop or split with:
- Pre-filter (MERV-8) + ERV wheel + cooling coil + post-filter (MERV-13) + supply fan
- Total static pressure: ~250-400 Pa (driven by ERV + filters)
- Capacity: 6-10 TR per 5,000 cfm of OA in Indian climate
Parallel sensible cooling
Three common approaches:
Chilled beam (active or passive)
Cooling coil suspended from ceiling; warm room air rises through fins, cooled by chilled water at 12-15 °C, falls back to room.
Pros: extremely high cooling at low fan power; quiet; works at warmer chilled water (higher chiller COP).
Cons: must keep zone dewpoint below 12 °C (which DOAS does); condensation risk if DOAS fails.
Fan-coil unit (FCU) with re-cool
Conventional fan-coil with chilled water at 7-9 °C; re-cools internal air; DOAS handles latent and outside air.
Pros: simpler integration; works without DOAS-specific zone control.
Cons: still uses cold chilled water; less energy benefit than chilled beam.
Cooled-only DOAS (no parallel cooling)
DOAS supply cold enough to handle both latent + zone sensible. Used for low-sensible-load zones (corridors, lobbies, retail with low occupancy).
Pros: simplest; one system.
Cons: works only when outdoor airflow > zone sensible load demands, otherwise zone overheats.
Worked example: 1,000 m² office in Mumbai
Office: 1,000 m² floor, 100 occupants, internal sensible load 80 kW (people + lighting + plug load).
Conventional VAV all-air system
- Total cooling: outdoor air load (50 kW) + internal sensible (80 kW) + reheat for low-load zones (15 kW) = 145 kW
- Chilled water at 6.5 °C
- Chiller COP at 6.5 °C: 5.0
- Annual cooling energy: ~290,000 kWh
DOAS + chilled beam
- DOAS: 5,000 cfm OA = ~6.5 TR (~23 kW with ERV) at 8 °C dewpoint
- Chilled beam: 80 kW internal sensible at 12 °C chilled water
- Total cooling: ~103 kW (28% less than conventional)
- Chilled water at 12 °C; chiller COP at 12 °C: 6.5 (30% better than 6.5 °C)
- Annual cooling energy: ~130,000 kWh
Annual savings: 290,000 – 130,000 = 160,000 kWh = ~₹13 lakh/year at typical office tariff.
Capex difference: DOAS + chilled beam vs VAV all-air typically ₹8-12 lakh higher upfront for the 1,000 m² building. Payback ~6-9 years from energy savings.
Common DOAS design pitfalls
1. Sizing DOAS without ERV. Doubles cooling-coil load and operating cost; ERV is essentially mandatory for hot-humid climate DOAS.
2. Cold DOAS with chilled beams without dewpoint control. Below 12 °C zone dewpoint (DOAS guarantees this), chilled beam at 14 °C surface temp doesn’t condense. Above 12 °C, condensation. Verify DOAS dewpoint target.
3. DOAS supply ductwork unsized for cold air. 8 °C supply in 28 °C ambient = sweating ducts. Specify minimum 50 mm duct insulation.
4. No zone-level OA control. DOAS supplies fixed CFM to each zone; no DCV. Solution: add VAV box at each DOAS zone supply for occupancy-based reset.
5. Forgetting condensate management. DOAS coil produces continuous condensate (especially in monsoon). Drain pan, P-trap, freeze protection if outdoor unit.
Implementation in Indian projects
LEED v4.1 EAc1 and IGBC v3 EE both reward DOAS via the energy modelling baseline. ASHRAE 90.1 Appendix G allows DOAS as a baseline option for whole-building performance.
ECBC 2017 implicitly supports DOAS through the high-efficiency ventilation path and SAT requirements. NBC 2016 Pt 8 §5 doesn’t explicitly require DOAS but the multi-zone Vot calculation favours it for systems with high Zd zones.
For new commercial buildings in Mumbai/Chennai/Bangalore, DOAS with chilled beam is increasingly the default for buildings ≥ 5,000 m². Smaller buildings often use DOAS-to-AHU integration.
Quick checklist
- [ ] DOAS architecture chosen (direct-to-zone or AHU-integrated)
- [ ] Outdoor air design conditions for site (1% summer, 1% winter)
- [ ] DOAS supply state (cold vs neutral) per parallel cooling type
- [ ] Energy recovery wheel sized (target 70%+ sensible + 60%+ latent)
- [ ] Parallel sensible cooling (chilled beam / FCU / VRF) sized for internal load
- [ ] Chilled water temperature reset for parallel cooling at 12-15 °C
- [ ] Supply duct insulation ≥ 50 mm for cold DOAS
- [ ] Condensate management with backup drainage
- [ ] DCV at zone-level for OA reset
References: ASHRAE 62.1-2022; ASHRAE Handbook HVAC Sys & Eqp 2024 Ch 4 (Air Handling and Distribution); AHRI 1060-2018; ECBC 2017 §5.2; ISHRAE Handbook 2024.
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