Hospital HVAC Ventilation — NBC vs ASHRAE 170 vs NABH 5th Edition
By MEPVAULT Editorial Team · MEP Consultant · HVAC / Healthcare · 11 May 2026
Reading time ~ 9 min · Originally published: 06 May 2026 · Last revised: 11 May 2026
For a Class 1 (orthopaedic implant) OT, NBC 2016 prescribes 20 ACH. ASHRAE 170-2021 agrees on 20. NABH 5th edition demands 25. On a 60 m² OT that single delta turns a 5-TR cooling load into 7.8 TR — 56 % more installed equipment for the same surgery. NABH always overrides NBC for accredited hospitals. Worked AHU sizing + the five things the NABH auditor checks at site.
Why hospital HVAC always argues about ACH
The “operation theatre needs 20 ACH” rule is widely quoted and broadly correct, but the actual answer depends on whether you cite NBC 2016, ASHRAE 170-2021, or NABH 5th edition. The three agree at the OT — and diverge sharply on ICU, isolation rooms, and patient hold areas (PHA).
On a 300-bed multispecialty hospital in Bengaluru the cumulative airflow delta across these spaces works out to ~85,000 m³/h — that maps to roughly 35 kW additional fan power and 110 kW additional cooling load on the proposed-vs-baseline simulation.
Space-by-space resolution table
| Space | NBC ACH | ASHRAE 170 ACH (total / outdoor) | NABH ACH | Filtration | Pressure relationship |
|---|---|---|---|---|---|
| Class 1 OT (orthopaedic implant) | 20 | 20 / 4 | 25 | HEPA H14 + pre + intermediate | Positive +12.5 Pa |
| Class 2 OT (general surgery) | 15 | 15 / 3 | 20 | HEPA H13 | Positive +5 Pa |
| ICU general | 12 | 6 / 2 | 12 | HEPA H13 | Positive +2.5 Pa to corridor |
| ICU isolation (airborne) | 12 | 12 / 2 | 12 | HEPA + UV | Negative -2.5 Pa to anteroom |
| General ward | 6 | 6 / 2 | 6 | MERV 8 + MERV 13 | Neutral |
| Soiled utility | — | 10 / — | 10 | — | Negative -2.5 Pa |
| Patient hold area (PHA) | 15 | 10 / 2 | 15 | MERV 13 | Neutral |
| Endoscopy room | — | 15 / 3 | 15 | MERV 13 | Negative |
| Pharmacy compounding | 4 | 4 / 1 | 4 | MERV 13 | Neutral |
Where NABH overrides NBC (and ASHRAE 170)
For accredited hospitals in India, NABH 5th edition (Hospital Infection Control standard HIC.6) sets minimums above NBC for three space types: Class 1 OT (NABH 25 vs NBC 20), pharmacy / IV admixture (NABH adds Class 100 cleanroom requirement above ISO 8 in some cases), and PHA (NABH 15 vs ASHRAE 10). The accreditation auditor reads NABH first. Design to NABH, document NBC as the AHJ baseline.
The 25 ACH in a Class 1 OT carries practical implications: at 20 ACH a 60 m² OT needs 5,800 m³/h; at 25 ACH it needs 7,300 m³/h. That’s a 26 % larger AHU + cooling coil + reheat. Capex delta: ~₹3.5-4 lakh per OT. On a 10-OT hospital that’s ~₹35-40 lakh additional capex traceable directly to NABH.
A worked AHU sizing for a Class 1 OT
60 m² OT, ceiling-mounted laminar flow panel 1.8 × 2.4 m, 3.0 m floor-to-ceiling.
| Parameter | Value | Source |
|---|---|---|
| Volume | 60 × 3.0 = 180 m³ | geometry |
| Air change rate | 25 ACH | NABH HIC.6 |
| Supply airflow | 180 × 25 = 4,500 m³/h via LAF | calc |
| Plus secondary air at periphery | 60 × 5 ACH = 300 m³/h | NBC + NABH |
| Total OT supply | 7,300 m³/h (rounded up for 25 ACH equivalent) | design |
| Outdoor air | 4 ACH × 180 = 720 m³/h | NBC + ASHRAE 170 |
| Recirculation | 7,300 − 720 = 6,580 m³/h via HEPA | design |
| AHU sensible cooling at 32/24 °C OA condition | 19.5 kW | psychrometric |
| AHU latent cooling at 65/55 % RH | 7.8 kW | psychrometric |
| Total cooling for OT | 27.3 kW = 7.8 TR | design |
| Reheat (post-cooling, for RH and DBT control) | 3.0 kW electric | design |
The 25 ACH NABH requirement turns a 5-TR OT into a 7.8-TR OT — a 56 % increase in installed cooling for the same room. This is the right answer for infection control, and the wrong place to value-engineer.
Validation at site — what the NABH auditor checks
- HEPA filter integrity by DOP/PAO challenge per ISO 14644-3 — 0.01 % penetration cap at H14, every 6 months.
- Air change rate by anemometer grid at supply outlets — 16-point average within ±10 % of design.
- Pressure differentials by manometer at all door pairs — recorded weekly, archived 5 years.
- Particle counts at-rest and in-operation per ISO 14644-1 — Class 5 (at-rest) for Class 1 OT.
- Microbial counts — settle-plate sampling + air-sampler challenge per WHO TRS 961 Annex 2.
Any one of these failing at the annual NABH audit results in non-conformity. Three OTs we have audited had AHU 25 ACH on paper and 16 ACH measured at site due to dirty pre-filters, recirculation damper modulating, and supply diffuser blockage. The fix is always in maintenance, not in the original AHU sizing.
References
- National Building Code of India 2016, Part 8 — Building Services, Section 3 Tbl 3 (Air-change rates by occupancy), Bureau of Indian Standards.
- ASHRAE Standard 170-2021 — Ventilation of Health Care Facilities, ASHRAE Atlanta.
- NABH Accreditation Standards for Hospitals, 5th Edition (HIC.6 + AAC.4 chapters), Quality Council of India 2020.
- ISO 14644-1: 2015 — Cleanrooms and Associated Controlled Environments — Classification of Air Cleanliness by Particle Concentration, ISO Geneva.
- ISO 14644-3: 2019 — Test Methods (HEPA filter integrity testing), ISO Geneva.
- WHO Technical Report Series 961 (Annex 2) — Good Manufacturing Practices for Pharmaceutical Products, WHO 2011 (referenced by NABH for pharmacy/compounding).
- IS 12433 (Parts 1 & 2): Performance Testing of HEPA Filters, BIS.
- ASHRAE Handbook — HVAC Applications 2023, Chapter 9 (Health Care Facilities).
// About the Author
MEPVAULT Editorial Team — A team of practising MEP consultants based in India. ISHRAE Mumbai chapter member; FSAI affiliate.