Why hospital HVAC ductwork is different
Hospital HVAC ductwork is not commercial HVAC scaled up. It is a life-safety system that sits inside a code stack tied directly to patient outcomes, surgical infection rates and the hospital's continued accreditation. A leaking transverse joint in a commercial office building is an energy penalty. The same leak in an operating room supply trunk is a positive-pressure failure that can drop the OR below corridor pressure mid-procedure, drag corridor air over an open surgical wound and contribute to a deep surgical site infection that adds 12 days to length of stay and an attributable mortality risk that is documented in every hospital epidemiology textbook.
The same applies in reverse. A breach in an Airborne Infection Isolation Room exhaust duct is not an energy penalty. It is a tuberculosis or measles release into the corridor that triggers contact tracing for every patient, visitor and staff member who walked past the door for the prior eight hours. Hospital duct leaks have a chain of consequences that runs through the hospital epidemiologist, the infection prevention committee, the Joint Commission survey, the CMS Conditions of Participation review and ultimately the hospital's licence to operate.
This is why hospital ductwork is built, sealed, tested and commissioned to a standard that has no equivalent in commercial HVAC. The SMACNA Seal Class A specification with TDF flange transverse joints, the IEST-RP-CC034 HEPA integrity protocol with PAO challenge and photometer scan, the NADCA cleaning baseline before patient occupancy, the smoke-pencil test of every door swing — none of this exists for commercial HVAC. All of it is mandatory in hospitals.
It is also why the duct contractor on a major hospital project is typically the third or fourth most experienced trade on site, behind only the medical gas, fire protection and structural teams. A duct contractor who has never worked on a hospital before will fail their first SMACNA leakage test, then fail their first ASHRAE 170 pressure relationship verification, then have their AIIR exhaust ducts torn out at week 18 of a 24-week construction window. The cost of that learning curve is measured in millions of dollars and in months of delay to opening day. The objective of this guide is to give you the framework so that does not happen on your project.
The dominant code: ASHRAE Standard 170-2021
ASHRAE Standard 170-2021 "Ventilation of Health Care Facilities" is the controlling document for hospital HVAC across the United States and is referenced by name in FGI Guidelines, Joint Commission Environment of Care standards and CMS Conditions of Participation. Internationally, ASHRAE 170 is adopted directly or by reference in Australia, Canada, Saudi Arabia, the UAE, Singapore and most of Latin America. If you are designing hospital ductwork anywhere outside the European Union and the United Kingdom, ASHRAE 170 is almost certainly the table you are pulling pressure and ACH numbers from.
The 2021 revision tightened several requirements that affect ductwork directly. Filtration banks were standardized to MERV 7 prefilter and MERV 14 final filter for general inpatient (Table 6.4), with terminal HEPA still required for OR, AIIR exhaust and PE supply. Operating room minimum ACH stayed at 20 total (4 outdoor) for Class B and Class C operating rooms but the laminar flow diffuser coverage requirement and the airflow pattern verification language were sharpened. The AIIR specification was clarified: 12 ACH all exhausted to outside, with 100 percent outdoor air supply and continuous pressure monitoring at the door.
Section 7.2 of the 2021 edition contains Table 7.1, which is the heart of the standard. Every space type in a hospital is listed against required pressure relationship, minimum total ACH, minimum outdoor air ACH, recirculation permission and any special requirements such as terminal HEPA or all-air-exhausted. The space programme matrix you build at design stage is essentially this table filtered to the rooms in your project. Section 7.4 covers exhaust requirements and Section 8 covers commissioning and post-occupancy verification.
Section 6 of ASHRAE 170 covers air handling units and filtration. Filter banks are specified in pairs: prefilter to protect the cooling coil and final filter to protect occupants. The terminal HEPA stage in critical spaces is the third stage. The standard is silent on UVGI and bipolar ionization but does not prohibit them — these are addenda topics that may appear in the 2024 or 2027 revisions.
Section 9 covers special spaces including pharmacy, laboratories, kitchens, MRI suites and decontamination. Pharmacy compounding has separate dependencies on USP General Chapters 797 (sterile compounding) and 800 (hazardous compounding) which we cover later in this guide.
FGI Guidelines for Design and Construction of Hospitals 2022
The Facility Guidelines Institute (FGI) Guidelines for Design and Construction of Hospitals 2022 is the architectural and design counterpart to ASHRAE 170. Where ASHRAE 170 tells you the airflow and pressure, FGI tells you the room layout, anteroom requirements, door-swing direction, finish requirements and the procedural rules that interact with HVAC.
FGI Chapter 2.1 covers general hospital requirements. Chapter 2.2 covers patient care areas including general medical and surgical rooms, intensive care units, intermediate care, neonatal intensive care, perinatal, isolation rooms (AIIR and PE), pediatric, psychiatric and behavioural health, and bariatric units. Each room type has explicit ductwork-relevant requirements such as minimum room size, anteroom requirement (e.g. PE with anteroom for stem cell transplant per 2.2-2.2.7), door types and ceiling type.
Chapter 2.3 covers diagnostic and treatment areas: emergency, imaging, MRI, nuclear medicine, surgery, endoscopy, dialysis, rehabilitation, pulmonary function. Imaging suites and MRI rooms have specific RF shielding and field-effect requirements that drive the duct material (non-magnetic stainless or aluminium for MRI penetrations) and the locations of any ferrous penetrations. Surgery suite layout and door-swing rules drive the OR pressure relationship strategy.
Chapter 2.4 covers diagnostic and treatment support areas: pharmacy, central sterile, food service, laundry, materials management. Pharmacy compounding rooms are specified to USP 797 and 800 with explicit anteroom and HVAC dependencies that we cover separately.
Chapter 2.5 covers public and administrative areas, and Chapter 2.6 covers staff and service areas including soiled workrooms, environmental services rooms, body holding and morgue. Soiled workrooms are negative pressure with all-air-exhausted at 10 ACH per ASHRAE 170 Table 7.1, with the exhaust ductwork running back to a non-recirculating exhaust riser.
Chapter 3 covers infrastructure: structural, mechanical, electrical, plumbing, communications. Section 3.1 mechanical-electrical-plumbing engineering points back to ASHRAE 170 for ventilation and adds FGI-specific requirements for redundancy (N+1 typical for AHUs serving critical spaces), emergency power coverage and seismic anchoring of ductwork.
FGI Appendix A is the Infection Control Risk Assessment (ICRA) framework which we cover later in the construction phasing section. ICRA is the one section of FGI that every duct contractor on a hospital project will read at least once a week during construction.
Joint Commission Environment of Care and CMS implications
Compliance with ASHRAE 170 and FGI is checked at two recurring milestones in a hospital's life. The first is the Joint Commission accreditation survey, which happens unannounced every 18 to 36 months. The second is the CMS Conditions of Participation review which is tied to Medicare and Medicaid reimbursement. Both surveys touch HVAC ductwork directly.
Joint Commission Environment of Care (EOC) standards EC.02.05.01 (utility systems including HVAC) and EC.02.06.01 (safe environment) require the hospital to demonstrate that HVAC systems are operating to design intent and that documentation supports it. This means the survey team will ask for current Test and Balance reports, current pressure relationship verification logs for OR, AIIR and PE rooms, current HEPA integrity certificates, current filter differential pressure logs and the maintenance schedule for cooling tower drift eliminators and condensate management.
An EOC finding for HVAC documentation is typically a Requirement for Improvement (RFI) with a 60-day correction window. A finding for a measured failure (e.g. AIIR found at neutral or positive pressure during the survey) is escalated to a Condition Level finding with a 45-day correction window and a follow-up survey. Repeat findings can lead to preliminary denial of accreditation.
CMS Conditions of Participation 42 CFR 482.41 (physical environment) ties directly back to NFPA 101 Life Safety Code and through it to ASHRAE 170 by reference. A CMS finding can result in immediate jeopardy status and a 23-day correction window before Medicare reimbursement is suspended. For a 400-bed hospital, suspended Medicare reimbursement is a financial event measured in millions of dollars per week.
The implication for ductwork specification is straightforward. Every choice you make at design stage either makes the survey easier or makes it harder. Pressure monitoring tied to the BMS with automatic logging makes the survey easier. Manual pressure verification with paper logs makes it harder. Permanently labelled OR and AIIR rooms with door-mounted indicators makes it easier. Rooms that look identical from the corridor make it harder. Continuously trending differential pressure across the HEPA filters makes it easier. Spot-check filter pressure on a clipboard makes it harder.
Pressure relationships per space type
The single most important decision in hospital HVAC design is the pressure relationship matrix — which rooms are positive, which are negative, which are neutral, and what the differential is. ASHRAE 170 Table 7.1 governs. Below is the canonical matrix used on most acute care hospital projects.
Positive pressure spaces (room is higher pressure than corridor; air flows out of room into corridor when door opens):
- Class B and Class C Operating Room — minimum 2.5 Pa positive, 20 ACH total minimum, 4 ACH outdoor minimum, terminal HEPA H13/H14, laminar flow ceiling diffuser typical.
- Cardiac Catheterization Lab and Hybrid OR — same as Class C OR for HVAC purposes; 20 ACH, terminal HEPA, positive pressure.
- Protective Environment Room (PE) — minimum 2.5 Pa positive, 12 ACH total, 2 ACH outdoor, supply through HEPA, non-aspirating diffuser. Anteroom required for stem cell transplant.
- Sterile Storage and Sterile Compounding (USP 797 buffer room) — positive to corridor, ISO Class 7 air quality, HEPA supply, anteroom required.
- Intensive Care for Immunocompromised — case-by-case positive per FGI 2.2-2.6.
Negative pressure spaces (room is lower pressure than corridor; air flows into room from corridor when door opens):
- Airborne Infection Isolation Room (AIIR) — minimum 2.5 Pa negative, 12 ACH all exhaust, 100 percent outdoor air supply, dedicated HEPA-filtered exhaust to outside, continuous pressure monitoring with door-mounted indicator.
- Bronchoscopy and Sputum Induction — 12 ACH all exhaust, negative pressure, similar to AIIR.
- Soiled Workroom and Soiled Holding — 10 ACH all exhaust, negative pressure.
- Pharmacy Hazardous Compounding (USP 800) — ISO Class 7 negative pressure, 30 ACH typical, dedicated exhaust through HEPA to outside, no recirculation.
- Autopsy and Pathology Gross — 12 ACH all exhaust, negative pressure.
- Triage and ED Waiting (post-COVID) — 12 ACH, negative or neutral, MERV 13 minimum, optional UVGI.
- Decontamination, Soiled Linen Storage, Environmental Services — negative all exhaust per FGI Chapter 2.6.
Neutral or no requirement (no minimum differential):
- Patient room (general medical/surgical) — 6 ACH total, 2 ACH outdoor, no pressure requirement.
- Public corridor — typically the reference pressure, no requirement.
- Office, conference, lounge — ASHRAE 62.1 baseline.
The pressure differential of 2.5 Pa (0.01 inches of water column) is the minimum. Most hospitals design to 5 to 12 Pa to give headroom for door openings and cascade effects. Cascade design (corridor → patient room → bathroom → exhaust riser) gives a stable pressure gradient and tolerates simultaneous door openings without breakdown.
Air change rates per ASHRAE 170 Table 7.1
Air changes per hour (ACH) drives the duct sizing more than any other parameter on a hospital project. Below is the working subset of ASHRAE 170 Table 7.1 for the spaces that drive most of the duct package.
- Operating Room Class B and C — 20 ACH total minimum, 4 ACH outdoor minimum. Most projects design to 25 ACH to support laminar flow.
- Operating Room Class A (minor procedure) — 15 ACH total, 3 ACH outdoor.
- AIIR — 12 ACH all exhaust, 100 percent outdoor air supply.
- Protective Environment — 12 ACH total, 2 ACH outdoor.
- Patient Room (general medical/surgical) — 6 ACH total, 2 ACH outdoor.
- Patient Corridor — 2 ACH minimum.
- Triage — 12 ACH total, 2 ACH outdoor.
- Emergency Department Treatment Room — 6 ACH total, 2 ACH outdoor.
- Trauma Room — 15 ACH total, 3 ACH outdoor (functions as Class B OR).
- Bronchoscopy — 12 ACH all exhaust.
- Recovery (PACU) — 6 ACH total, 2 ACH outdoor.
- Soiled Workroom — 10 ACH all exhaust.
- Pharmacy Sterile Compounding (USP 797 buffer) — 30 ACH typical to maintain ISO Class 7.
- Pharmacy Hazardous Compounding (USP 800) — 30 ACH typical, all exhaust.
- Endoscopy Procedure Room — 15 ACH total, 3 ACH outdoor.
- Imaging (X-ray, CT, ultrasound) — 6 ACH total, 2 ACH outdoor.
- MRI Suite — 6 ACH total, 2 ACH outdoor (with non-ferrous duct penetration into the MRI room itself).
- Nuclear Medicine — 6 ACH all exhaust.
- Cath Lab / Hybrid OR — 20 ACH total, 4 ACH outdoor (Class C OR equivalent).
- Autopsy — 12 ACH all exhaust.
- Decontamination — 6 ACH all exhaust.
- Newborn Nursery — 6 ACH total, 2 ACH outdoor.
- NICU — 6 ACH total, 2 ACH outdoor.
- Burn Unit — 6 ACH total, 2 ACH outdoor (with Protective Environment treatment per FGI 2.2-2.6).
These minimums drive supply CFM per room, return or exhaust CFM, and the trunk and branch sizes that propagate back to the AHU. A 600-square-foot Class C OR with 25 ACH and a 10-foot ceiling needs 2,500 CFM of HEPA-filtered supply, which determines the laminar flow diffuser array, the supply branch size and the AHU capacity allocated to that OR.
Operating Room HVAC — laminar flow, HEPA and pressure
The operating room is the most demanding space in any hospital from an HVAC ductwork perspective. The combination of high ACH, terminal HEPA filtration, positive pressure, laminar flow airflow pattern and the absolute requirement that nothing fails during a procedure puts the OR ductwork at the top of every specification.
The dominant airflow strategy is the laminar flow ceiling diffuser array, sometimes called a unidirectional flow ceiling. The supply air enters through a large rectangular HEPA-filter ceiling array directly above the surgical zone, typically 8 feet by 10 feet for a Class C OR, with non-aspirating diffuser plates that produce a downward laminar flow at 25 to 35 feet per minute face velocity. The flow envelope is sized to cover the surgical site and the immediate sterile zone (back table, instrument tray) and to push particulates downward and outward toward low-wall returns.
Low-wall return grilles are placed at four corners of the OR (or sometimes on two opposing walls) at 12 to 18 inches above the floor. The 25 ACH supply over a 600 square foot OR gives 2,500 CFM. The exhaust is sized at 15 ACH (1,500 CFM) which leaves 1,000 CFM net positive flow through the door cracks, sustaining the 5 to 12 Pa positive pressure to the corridor. The remaining 1,000 CFM is the cascade that maintains positive pressure even when the OR door opens during the procedure.
Terminal HEPA filtration is specified as H13 or H14 per ISO 29463, equivalent to 99.95 percent or 99.995 percent at the most penetrating particle size (MPPS). The IEST-RP-CC001 grade equivalent is HEPA Type C or Type D. The filter housing is integral with the ceiling diffuser array and includes a challenge port for PAO testing on installation and at scheduled re-validation (typically annual or per the hospital's policy).
The supply ductwork upstream of the HEPA terminal can be galvanized steel sized to SMACNA gauge tables, but with two important caveats. First, the duct interior must be clean and dry on installation, with NADCA cleaning baseline established before HEPA filters are installed. Second, the duct must be SMACNA Seal Class A throughout, with TDF flange transverse joints and all longitudinal seams sealed. A leak in the OR supply trunk after the HEPA bank does not reduce filtration but it does dump unfiltered ceiling-cavity air into the OR, which defeats the entire HEPA strategy.
Some specifications go further and require stainless steel for the OR supply trunk from the HEPA bank to the diffuser, particularly on hybrid ORs where the imaging equipment manufacturer requires stainless surfaces in the airflow path for cleaning and validation. Stainless OR ductwork is typically Type 304L with continuously TIG-welded longitudinal and transverse joints, passivated and polished to a No. 4 finish or better.
Exhaust ductwork from the OR low-wall returns is typically galvanized but should be sized generously. OR exhaust is recirculated to the AHU return after passing through an intermediate filter bank. Exhaust ductwork serving multiple ORs must avoid cross-contamination between rooms — a leak between OR exhaust risers can transfer airborne particulates from one OR to another and is an immediate finding in any commissioning protocol.
OR pressure monitoring is tied directly to the BMS. A door-mounted differential pressure gauge displays the live OR-to-corridor pressure, with a visual alarm at the gauge if pressure drops below 2.5 Pa. The same signal is logged at the BMS for trending and pulled into the Joint Commission EOC binder. Some specifications require a redundant pressure transducer with independent power for the alarm path.
Airborne Infection Isolation Room (AIIR)
The AIIR is the inverse of the OR. Where the OR is positive to keep contamination out, the AIIR is negative to keep contamination in. The space is designed for patients with airborne transmissible diseases including tuberculosis, measles, varicella, SARS-CoV-2 and any future pathogen of similar transmissibility. The specification is unforgiving because a single AIIR breach in a 400-bed hospital is a hospital-wide infection control event.
ASHRAE 170 specifies the AIIR at 12 ACH all exhaust to outside, 100 percent outdoor air supply with no recirculation, minimum 2.5 Pa negative to corridor, continuous pressure monitoring with permanent visual indicator at the door, and HEPA filtration on the exhaust before discharge to the outside or before any contact with shared building systems.
The supply ductwork to the AIIR carries 100 percent outdoor air, conditioned by a dedicated AHU or by a dedicated coil section in a multi-zone AHU with full damper isolation. The supply must not be recirculated through any mixing box that also serves a non-AIIR space. The supply duct is typically galvanized SMACNA Class A.
The exhaust ductwork is the critical specification. From the AIIR ceiling exhaust grille to the rooftop discharge, the exhaust duct carries airborne pathogens at the design concentration of the ill patient. The duct is typically Type 304L stainless or aluminium with continuously welded longitudinal and transverse seams, sized at SMACNA gauge or one gauge heavier, sealed to SMACNA Class A and tested to a leakage class better than the project specification (typically Leakage Class 3 or better, equivalent to 0.5 cfm/100 sq ft at 1 inch wg).
HEPA filtration on the AIIR exhaust is now standard practice. The HEPA bank is mounted in a bag-in-bag-out (BIBO) housing immediately upstream of the exhaust fan, allowing filter change without exposing the maintenance technician. Some specifications add a UV-C sterilization stage after the HEPA for additional pathogen kill on the filter media surface and the duct walls.
The AIIR exhaust fan is dedicated, single-speed or VFD-controlled to maintain the 12 ACH design flow regardless of corridor conditions. The fan discharge is at minimum 3 metres above the roof and 8 metres horizontal from any outdoor air intake or operable window per ASHRAE 170 Section 7.4. Discharge velocity is at minimum 12 m/s to ensure the contaminated plume rises and dilutes before reaching any building intake.
An anteroom is required by FGI for AIIR spaces with high-aerosol-generating procedures. The anteroom is a small antechamber between the patient room and the corridor, typically negative to corridor and positive to patient room, with separate door swings that prevent simultaneous opening. The anteroom HVAC has its own ductwork at 10 ACH typical with the same SMACNA Class A specification.
Pressure monitoring at the AIIR door is mandatory. The standard implementation is a door-mounted ball-in-tube manometer that gives a visual indication to the nurse before entering, plus a differential pressure transducer wired to the BMS for continuous logging and alarm. A monomanometer that reads the room-to-corridor pressure differential directly without a reference port elsewhere in the building is preferred because it is robust against building stack effect and outdoor wind pressure.
Protective Environment Room (PE)
The Protective Environment Room is the inverse of the AIIR in clinical purpose. The PE is for immunocompromised patients — typically allogeneic stem cell transplant, severe neutropenia, advanced AIDS or solid organ transplant — who must be protected from airborne contaminants in the environment. The specification is positive pressure, HEPA-filtered supply, low-aspiration diffuser, and an anteroom for the highest-risk patients.
ASHRAE 170 specifies the PE at 12 ACH total, 2 ACH outdoor air, minimum 2.5 Pa positive to corridor, supply through terminal HEPA H13 minimum, and a non-aspirating diffuser. FGI 2.2-2.2.7 adds the requirement for an anteroom for stem cell transplant patients.
The supply ductwork is galvanized SMACNA Class A upstream of the HEPA terminal, with the same construction logic as the OR — clean and dry on installation, NADCA cleaning baseline before HEPA installation, all transverse joints and longitudinal seams sealed. The HEPA terminal is mounted in the ceiling above the patient bed with a non-aspirating diffuser plate that distributes supply air evenly without creating a draft over the patient.
Return ductwork from the PE is recirculated through the building return (unlike AIIR which is all exhaust). The PE air is presumed clean because it has just left a terminal HEPA. Return grilles are placed away from the bed to support a top-down airflow pattern, similar in principle to the OR laminar flow but at lower velocity.
The PE pressure monitoring is identical in principle to the OR — a door-mounted differential pressure gauge with visual alarm if pressure drops below the design positive differential, plus BMS logging. The pressure must be maintained even when the door opens, which depends on the cascade design and the dedicated supply CFM.
An emerging best practice is to combine OR and PE pressure monitoring with door-position sensors so that the BMS can distinguish between a door-open transient (acceptable) and a sustained pressure failure (alarm). Door-position sensors are particularly useful in PE rooms where the door opens for every meal delivery and clinical visit.
Pharmacy compounding — USP 797 and USP 800
Pharmacy compounding ductwork sits at the intersection of FGI Chapter 2.4, USP General Chapter 797 (sterile compounding) and USP General Chapter 800 (hazardous compounding). The two USP chapters drive completely different ductwork strategies — sterile compounding is positive pressure to keep contamination out, hazardous compounding is negative pressure to keep contamination in — and the same pharmacy can host both in adjacent rooms with full HVAC separation.
USP 797 sterile compounding requires an ISO Class 7 buffer room (typically the IV admixture room) with an ISO Class 5 primary engineering control (laminar airflow workbench or compounding aseptic isolator) inside it. The buffer room ductwork is positive pressure to the anteroom (which is positive pressure to the corridor) at minimum 5 Pa per stage. Supply is HEPA-filtered at 30 ACH minimum to maintain ISO Class 7 air quality. The ductwork material is galvanized SMACNA Class A from the AHU through to the HEPA terminal at the buffer room ceiling.
USP 800 hazardous compounding (typically chemotherapy and other hazardous drugs) requires an ISO Class 7 negative pressure compounding room with the primary engineering control being a Class II Type B2 biological safety cabinet (BSC) — fully exhausted, no recirculation. The room is negative to the anteroom at minimum 2.5 Pa, with the anteroom positive to corridor at minimum 2.5 Pa. ACH is typically 30 with all air exhausted to outside through HEPA filtration.
The BSC integration is the critical ductwork detail. A Type B2 BSC must be hard-ducted to the building exhaust through a dedicated duct with no shared connections, no recirculation and no fire damper between the BSC and the rooftop discharge. The duct is typically Type 304L stainless steel with welded longitudinal and transverse seams, sized to maintain the BSC manufacturer's required exhaust airflow at the rated face velocity. The BSC manufacturer's exhaust transition plate sets the duct size and the connection details — these are non-negotiable and are part of the BSC's NSF/ANSI 49 listing.
The BSC exhaust fan is dedicated, VFD-controlled or constant-speed with a flow station, and interlocked with the BSC alarm circuit so that loss of BSC airflow triggers an immediate alarm at the pharmacy and at the BMS. The fan discharge is at minimum 3 metres above the roof with the same 8 metre horizontal separation from outdoor air intakes per ASHRAE 170 Section 7.4.
USP 800 also covers storage and unpacking of hazardous drugs. The storage room is negative pressure with all-air-exhausted at 12 ACH typical. The unpacking room is similar. Both rooms have ductwork specified to USP 800 standards and the BMS pressure monitoring is documented in the pharmacy compliance binder.
Material requirements — galvanized, stainless, gauge selection
Hospital ductwork material selection follows a zoning logic. Galvanized steel for general patient and corridor ductwork. Stainless steel for OR supply downstream of HEPA, AIIR exhaust, USP 800 exhaust, MRI room penetrations and any duct surface that requires sterilization or sees high-particulate transport. Aluminium occasionally for AIIR exhaust where weight is a concern.
Galvanized steel for hospital ductwork is typically ASTM A653 Grade G90 (90 g/m² zinc coating each side) for interior duct, or G60 for protected indoor environments. The gauge selection follows SMACNA HVAC Duct Construction Standards 4th Edition for the project pressure class. For positive 2 inches w.c. operating pressure (the common SMACNA pressure class for hospital trunks), the gauge table runs:
- Up to 12 inch (300 mm) major dimension — 26 gauge (0.5 mm)
- 13 to 30 inch (330 to 750 mm) — 24 gauge (0.6 mm)
- 31 to 54 inch (790 to 1370 mm) — 22 gauge (0.85 mm)
- 55 to 84 inch (1400 to 2130 mm) — 20 gauge (1.0 mm)
- Above 85 inch (2160 mm) — 18 gauge (1.3 mm) or stiffened heavier
For higher pressure classes (3 inch w.c. and above, common on AIIR exhaust and high-velocity OR supply), the gauge is increased one step or stiffening is added per SMACNA Figure 1-6. Many hospital specifications go one gauge heavier than the SMACNA minimum on critical-care ductwork to give the duct contractor more dimensional stability during construction and a higher safety margin on the leakage test.
Stainless steel for OR supply, AIIR exhaust and USP 800 exhaust is typically Type 304 or 304L. Type 316 or 316L is used where chloride exposure is a concern (sterilization environments using ethylene oxide or hydrogen peroxide vapour). The gauge selection follows SMACNA stainless steel tables which run typically one gauge heavier than galvanized at the same pressure class. Surface finish is No. 2B as-rolled minimum, with No. 4 brushed for visible duct surfaces and electropolished for the most demanding pharmaceutical and OR applications.
Stainless duct fabrication uses TIG welding (gas tungsten arc) for the longitudinal seam and either TIG welded or formed-and-riveted transverse joints depending on pressure class. Continuously welded transverse joints are typical for AIIR exhaust because they survive leakage testing better and resist corrosion at the joint.
Aluminium ductwork is occasionally used for AIIR exhaust risers and laboratory exhaust where weight is a significant factor (top-floor mechanical or rooftop installations). Aluminium is corrosion-resistant in clean exhaust streams but is not appropriate for hazardous compounding exhaust where chemotherapy drug residue can attack aluminium. Aluminium duct gauge is typically two grades heavier than galvanized at the same pressure class.
Sealing class — SMACNA Seal Class A is the default
The SMACNA HVAC Duct Construction Standards define three seal classes:
- Seal Class A — all transverse joints, longitudinal seams and duct wall penetrations sealed.
- Seal Class B — all transverse joints and longitudinal seams sealed.
- Seal Class C — all transverse joints sealed only.
For hospital ductwork, Seal Class A is the default for any duct serving a pressurized critical space — operating room supply, AIIR exhaust, Protective Environment supply, cath lab supply, USP 797 buffer room supply, USP 800 compounding exhaust. Seal Class B is acceptable for general patient corridors, public spaces and back-of-house. Seal Class C is not appropriate for any healthcare application and most hospital specifications prohibit it explicitly.
The sealant material is critical. UL 181 Listed sealants are required for any HVAC duct application in the United States and are referenced in most international hospital specifications. Low-VOC water-based sealants are preferred for indoor air quality. Solvent-based sealants are prohibited in occupied healthcare environments due to off-gassing.
The sealing detail at the TDF flange transverse joint uses a continuous bead of UL 181 mastic in the flange channel before bolting, with the mastic compressed between the male and female flange faces when the bolts are torqued. Done correctly, the TDF flange joint passes SMACNA Class 6 leakage on the first test. Done incorrectly (insufficient mastic, missed bolts, over-torqued flanges), the joint fails repeatably and the duct contractor spends a week chasing leaks at every joint on the project.
Longitudinal seam sealing on rectangular galvanized ductwork uses Pittsburgh seam or button-punch seam construction with sealant injected during forming. On the SBKJ SBAL-V auto duct line, the longitudinal seam is closed by the seam-closer station with continuous sealant injection from a pneumatic dispenser, giving a consistent leak-tight seam at every metre of duct produced. Read more about SBKJ auto duct line capabilities.
Commissioning per ASHRAE 170 Section 8
ASHRAE 170 Section 8 governs commissioning and post-occupancy verification of hospital HVAC. The commissioning process for ductwork specifically covers four major activities: SMACNA leakage testing, pressure relationship verification, ACH measurement and HEPA integrity testing. Each of these has a documented protocol that produces a record retained in the EOC binder for the life of the building.
SMACNA leakage testing is performed before insulation per the SMACNA HVAC Air Duct Leakage Test Manual. The test pressure equals 1.5 times the maximum operating pressure of the duct section. Allowable leakage is expressed as Class 6 (3 cfm/100 sq ft at 1 inch wg) or better for hospital ductwork, with critical AIIR exhaust often specified at Class 3 (0.5 cfm/100 sq ft at 1 inch wg). The test is performed by isolating a duct section, pressurizing with a calibrated test fan, measuring the leakage rate at steady state, and comparing against the project specification.
Pressure relationship verification is performed before occupancy and at every major HVAC commissioning milestone. The protocol uses smoke pencils, tracer gas (sulfur hexafluoride or carbon dioxide) or differential pressure transducers to verify that every pressurized space achieves the required differential under design and worst-case conditions. Worst-case conditions include all OR and AIIR doors open simultaneously, with the BMS holding pressure cascade across the corridor network. Read more about hospital HVAC commissioning protocol.
ACH verification is performed by traverse measurement at every supply and exhaust grille per ASHRAE 111. The supply ACH is measured by hot-wire anemometer at the diffuser face or at a duct traverse upstream of the diffuser. The exhaust ACH is measured at the return or exhaust grille. Total ACH is computed by summing supply CFM and dividing by room volume in cubic feet times 60. The result is compared against ASHRAE 170 Table 7.1 and recorded.
HEPA integrity testing is performed per IEST-RP-CC034. A polyalphaolefin (PAO) aerosol challenge is introduced upstream of the HEPA filter at a known concentration. A photometer scans the downstream face of the filter and the frame seal at a maximum 50 mm/s scan rate. Acceptance is no penetration above 0.01 percent of upstream concentration at any point. Failed scans require either re-seating the filter or replacing the seal gasket and re-testing.
DDC integration verification confirms that every pressure transducer, every filter differential pressure switch, every flow station and every fan status point reaches the BMS and produces an alarm in the EOC log when a fault occurs. The protocol is to manually trigger each fault (e.g. open an AIIR door for 60 seconds, or close a supply VAV to 50 percent of design) and verify the alarm appears, is logged and clears when the fault is corrected.
The full commissioning binder includes leakage test reports per duct section, smoke test logs per pressure boundary, ACH measurement logs per room, HEPA integrity certificates per filter, DDC alarm verification logs, NADCA cleanliness baseline and the integration confirmation. This binder is the primary HVAC reference for every Joint Commission survey and every CMS visit for the next 5 years.
Duct cleaning per NADCA standards
The National Air Duct Cleaners Association (NADCA) ACR Standard for Assessment, Cleaning and Restoration of HVAC Systems is the dominant cleaning standard referenced in hospital specifications. The standard covers visual inspection, particulate sampling, mechanical cleaning, chemical sanitization, post-cleaning verification and documentation.
Pre-occupancy cleaning is mandatory on all new hospital ductwork. The duct contractor's cleanliness during construction does not substitute for NADCA cleaning before HEPA filter installation. Construction debris, dust from drywall and plaster work, residual oil from forming operations and packaging materials all collect in the duct interior during construction. A NADCA-certified cleaning contractor performs the pre-occupancy cleaning with mechanical agitation (rotary brush or compressed air whip), vacuum collection (HEPA-filtered vacuum) and a documented post-cleaning visual inspection.
Particulate sampling is typically required at the AHU and at the most distant supply diffuser before HEPA installation. Sampling is performed with a calibrated optical particle counter and the results are compared against the project specification (typically Federal Standard 209E Class 100,000 or ISO 14644-1 Class 8 for general hospital, ISO Class 7 or 6 for OR and PE supply paths).
Operational cleaning during the building life cycle is scheduled per the NADCA ACR standard and the hospital's infection control committee. General patient ductwork is typically cleaned every 5 to 10 years or when visual inspection or particulate sampling indicates buildup. OR, AIIR and PE ductwork is cleaned more frequently, with the cleaning protocol coordinated with HEPA filter replacement to avoid contaminating clean supply paths.
NADCA cleaning is also a critical infection control activity after a known or suspected airborne pathogen exposure. After a confirmed tuberculosis case in an AIIR, the protocol typically calls for full duct cleaning of the exhaust path, HEPA filter replacement, and surface decontamination of the duct interior with EPA-registered hospital disinfectant before the AIIR is returned to service.
Lessons from COVID-19 — surge capacity AIIR and MERV 13
The COVID-19 pandemic accelerated several long-standing best practices in hospital HVAC and added new requirements that are now finding their way into ASHRAE 170 addenda and the FGI 2026 update. The most significant changes affect surge capacity AIIR conversion, baseline filtration upgrades and UVGI integration in HVAC systems.
Surge capacity AIIR conversion is the practice of converting general patient rooms or entire patient floors into temporary AIIR during a respiratory pandemic surge. The conversion requires three things from the HVAC ductwork: separation from the building return so the room can run all-exhaust, sufficient exhaust capacity to achieve 12 ACH, and a path to discharge the exhaust safely outside.
Hospitals that planned for surge capacity at design stage installed dampered tie-ins between the patient room exhaust and a surge exhaust riser, plus an oversized exhaust fan that could activate when the dampers opened. Hospitals that did not plan for it had to retrofit during the pandemic, often by installing temporary HEPA-filtered negative pressure machines (portable units that exhaust through a window or temporary roof penetration) and by reconfiguring ductwork in real time.
The lesson learned is that any new hospital project should include surge capacity AIIR design in at least one full patient floor. The incremental cost at design stage is 5 to 10 percent of the floor's HVAC package; the cost of retrofitting during a pandemic is multiples of that and the conversion is not as effective.
Baseline filtration upgrade from MERV 8 to MERV 13 in general patient areas became near-universal during the pandemic and is now codified in ASHRAE 170-2021 Table 6.4 as MERV 14 for the final filter bank. MERV 13 captures 85 percent of particles in the 1 to 3 micron range, which includes most respiratory droplet nuclei. MERV 14 captures 90 percent in the same range. The filter pressure drop is higher than MERV 8 by 25 to 50 Pa, which the AHU must accommodate either through fan capacity at design or through VFD margin in retrofit.
UVGI (ultraviolet germicidal irradiation) integration in HVAC supply ducts upstream of cooling coils is a long-standing best practice that the pandemic re-energized. UV-C lamps at 254 nm wavelength deliver a continuous biocidal dose to the cooling coil surface, preventing biofilm formation and Legionella amplification. In-stream UV-C in the AHU return adds airborne pathogen inactivation before recirculation. Both applications are compatible with ASHRAE 170 ductwork and are addressed in ASHRAE Standard 185.1 for upper-room UVGI and 185.2 for HVAC UVGI.
Lessons from Legionella — cooling tower drift and condensate
Legionella outbreaks in hospitals are a recurring infection control event documented in CDC reports across multiple decades. The typical source is a cooling tower with insufficient drift eliminator efficiency, a poorly maintained condensate drain pan, or a stagnant water source in the HVAC system that allows Legionella amplification followed by aerosolization.
The ductwork-related lessons are concentrated on condensate management. Every cooling coil in a hospital AHU produces condensate at design conditions. The condensate drain pan must be Type 304 stainless steel or epoxy-coated carbon steel, sloped at minimum 1:50 toward the drain connection, with a deep trap to prevent drain pan suction or blow-through, and an air gap to the building drain to prevent backflow. The drain pan must be inspected and cleaned per ASHRAE Standard 188 for water management programmes.
Standing water in drain pans is the leading cause of Legionella amplification in HVAC systems. A drain pan that does not drain completely, a clogged drain, a loose drain connection that allows backflow — all of these can produce a biofilm that aerosolizes when the AHU fan starts and contaminates the supply duct downstream. The duct contractor's role is to install the drain pan and trap correctly the first time and to provide cleaning access at the drain pan and trap for maintenance.
Cooling tower drift is not directly a ductwork issue but the airborne Legionella from cooling tower drift can re-enter the building through outdoor air intakes if the tower is poorly located. ASHRAE 170 Section 7.4 requires a minimum 8 metres horizontal separation between cooling tower discharge and any outdoor air intake, with greater separation if the prevailing wind direction places the intake downwind of the tower.
The integrated Legionella prevention strategy combines ASHRAE 188 water management, drift eliminator efficiency at minimum 0.001 percent of recirculating water, copper-silver ionization or chlorine dioxide treatment of the cooling tower water, and HVAC inspection of cooling coil drain pans on the standard maintenance schedule.
Hospital construction phasing — ICRA and barriers
Hospital construction is rarely greenfield. Most projects are renovations, additions or interior fit-outs in active patient buildings where infection control during construction is a continuous risk. The Infection Control Risk Assessment (ICRA) per FGI Appendix A is the formal framework for managing this risk, and it has direct implications for ductwork installation.
The ICRA process classifies construction activity (Type A through Type D, where A is minor inspection and D is major demolition) and patient risk (Group 1 through Group 4, where 1 is low-risk office areas and 4 is immunocompromised, OR or NICU). The intersection of construction type and patient risk drives the required infection control precautions, ranging from basic dust control (Class I) to full negative-pressure containment with HEPA-filtered exhaust (Class IV).
Class IV ICRA requires a fully enclosed work area with fire-rated barriers, a HEPA-filtered exhaust system pulling air from the work zone to the outside, anteroom for personnel transitions, sticky mats at exits, and continuous negative pressure verification. The HEPA-filtered exhaust setup is itself a piece of temporary HVAC ductwork — typically galvanized SMACNA Class B with a portable HEPA fan unit and a flexible exhaust connection through the temporary barrier.
The duct contractor's role on the new construction side of the barrier is to install the new ductwork without breaching the barrier, without contaminating active patient areas with construction debris, and without crossing the barrier with un-cleaned ductwork. This typically means staging the new duct in the work zone, completing the bulk of the construction before any tie-ins to active building systems, and performing the tie-in connections during scheduled shutdowns when the active system is offline.
Daily ICRA inspections are performed by the hospital's infection prevention coordinator. The duct contractor must accommodate these inspections without delay, must respond to any infection control finding within 24 hours, and must document the response in the project ICRA log. Failure to maintain ICRA during construction is the most common cause of hospital construction project shutdown by the infection prevention committee.
Major hospital projects — typical duct package values
Hospital ductwork packages are sized in proportion to the building. For a 300 to 600 bed acute care hospital, the mechanical ductwork package alone typically runs USD 20 million to USD 80 million depending on geography, complexity and the share of stainless steel critical-care ductwork. A typical breakdown by value share is:
- Galvanized rectangular and round trunk and branch — 60 to 70 percent.
- Stainless OR, AIIR and pharmacy compounding exhaust — 15 to 20 percent.
- Fire and smoke damper integration — 10 percent.
- Flex duct, terminal boxes and accessories — 5 to 10 percent.
Specialty hospitals push the stainless share higher. A dedicated cancer centre with a full bone marrow transplant unit and 8 to 12 ORs can run 25 to 30 percent stainless. A pediatric hospital with intensive isolation suites runs similarly. A research hospital with BSL-3 laboratories adds full BSL-3 ductwork (welded stainless throughout, redundant exhaust fans, double-HEPA exhaust) which can lift the package to USD 100 million or more.
The volumes drive the ductwork manufacturing strategy. A 400-bed hospital project typically needs 50,000 to 80,000 metres of galvanized rectangular trunk and branch, 5,000 to 10,000 metres of stainless OR and AIIR ductwork, 2,000 to 5,000 fire and smoke dampers and 500 to 1,500 terminal VAV boxes. Producing this volume in 12 to 18 months requires a duct shop with at least one auto duct line capable of 2 to 3 metres per minute output, plus dedicated stainless TIG welding stations for the critical-care ductwork.
For duct contractors taking on hospital work for the first time, the SBKJ engineering team typically recommends a minimum equipment package of one SBAL-V auto duct production line for galvanized rectangular trunks, one SBTF stainless tubeformer for round stainless OR returns and AIIR exhaust, and one TDF flange line for tight transverse joints meeting SMACNA Class A. Read more about SBKJ hospital project capabilities.
Ductwork for medical office buildings (MOB)
Medical office buildings (MOBs) are a different specification from full acute-care hospitals. An MOB hosts physician offices, outpatient clinics, ambulatory surgery centres (which may include Class A operating rooms but typically not Class C), imaging suites and pharmacy retail. The dominant code is ASHRAE 62.1 baseline for office and clinical use, with ASHRAE 170 applying only to those specific rooms that meet healthcare definitions (procedure rooms, sterile compounding, AIIR, PE).
For most MOB ductwork, the specification reverts to commercial-grade galvanized SMACNA Seal Class B with MERV 8 to MERV 13 filtration depending on the use. Procedure rooms, ambulatory surgery operating rooms and any sterile compounding pharmacy follow the ASHRAE 170 specification at the same level as a hospital. The remainder of the building is commercial-grade.
The implication for duct contractors is that an MOB project is typically 80 to 90 percent commercial galvanized ductwork with 10 to 20 percent ASHRAE 170 compliant ductwork in the procedure rooms and pharmacy. The mixed specification requires careful zoning during construction and clear marking of the ASHRAE 170 sections so they receive the correct sealing class, leakage testing and HEPA filtration.
MOB ductwork pricing is typically 30 to 50 percent below acute-care hospital ductwork on a per-metre basis because of the lower share of stainless and SMACNA Class A. The leakage testing is less extensive, the HEPA bank count is lower and the commissioning protocol is shorter. The trade-off is that the MOB duct contractor often has thinner margin and tighter schedule than the hospital contractor.
SBKJ machinery for hospital ductwork manufacturing
SBKJ machinery is used by duct contractors on hospital projects in North America, Australia, the Middle East and Southeast Asia. The hospital ductwork specification puts demands on the duct fabrication equipment that few commercial duct shops have dealt with — SMACNA Class A leakage performance on every piece, stainless TIG welding capability on critical-care ductwork, dimensional tolerance tight enough to support TDF flange transverse joints at scale.
For galvanized rectangular trunk and branch (60 to 70 percent of the duct package value), the SBAL-V auto duct production line is the typical choice. The line accepts 1,250 mm or 1,550 mm slit coil at 0.5 to 1.5 mm thickness, performs forming, longitudinal seam closing with continuous sealant injection, transverse cutting and TDF flange forming inline, producing finished duct sections at 2 to 3 metres per minute single-shift. The TDF flange profile is the SMACNA-approved transverse duct flange that achieves Seal Class A with mastic and bolted connection — the dominant joint type on hospital projects since the 1990s. Read more about SBAL-V auto duct line specifications.
For round stainless OR returns and AIIR exhaust (15 to 20 percent of the duct package), the SBTF stainless tubeformer produces spiral lockseam round duct in Type 304 or 316 stainless steel from 80 mm to 1,500 mm diameter. The lockseam can be sealant-injected for Class A performance, or the tubeformer output can be TIG-welded longitudinally for full pressure-vessel quality on AIIR exhaust risers and USP 800 exhaust runs. Read more about SBTF stainless tubeformer specifications.
For TDF flange production at scale, SBKJ TDF flange machines produce roll-formed flange profiles inline with the duct sheet, with the corner connections crimped, riveted or welded depending on the project specification. The TDF flange is the workhorse joint on hospital ductwork and the SBKJ TDF flange line produces it consistently to dimensional tolerance better than the SMACNA standard, supporting Class A leakage performance on the first test.
For duct contractors taking on hospital projects above USD 30 million in ductwork value, the typical SBKJ package is one SBAL-V line, one SBTF stainless tubeformer at 1,500 mm capacity, one dedicated TDF flange line, and one stainless TIG welding station with positioner and rolling beds. The total equipment investment is in the USD 800,000 to USD 1.5 million range depending on configuration, and pays back on a single major hospital project. Read more about SBKJ machinery in North American hospital projects.
Validation — leakage testing and ASHRAE 170 commissioning
The validation phase of a hospital project is where the ductwork specification is proved against the as-built reality. The two dominant validation activities are SMACNA leakage testing and ASHRAE 170 commissioning. Both produce reports that are retained in the EOC binder for the life of the building and are reviewed at every Joint Commission survey.
SMACNA leakage testing per the SMACNA HVAC Air Duct Leakage Test Manual is performed before insulation. The duct section is isolated, pressurized to 1.5 times maximum operating pressure with a calibrated test fan, and the leakage rate is measured at steady state. The result is compared against the project specification (typically Class 6 for general, Class 3 for AIIR exhaust). Failed sections must be re-sealed and re-tested. Pass rates on hospital projects with TDF flange and SMACNA Class A construction are typically above 95 percent on first test.
ASHRAE 170 commissioning per Section 8 verifies airflow rates, pressure relationships, filter integrity and DDC alarm function before occupancy. The commissioning agent (typically an independent third-party firm hired by the hospital owner) executes the full protocol over 4 to 12 weeks depending on hospital size, producing the commissioning binder that is the primary HVAC reference for the building life.
The commissioning agent's report includes the ductwork as-built drawings, the leakage test results per duct section, the pressure relationship verification per critical space, the ACH measurement per room, the HEPA integrity certificate per filter, the DDC alarm verification log and the NADCA cleanliness baseline. Any deficiencies are documented with corrective action and re-test results before the building is released for occupancy.
Post-occupancy validation is continuous. Pressure relationships are monitored continuously by the BMS and logged for trend analysis. HEPA filters are differential-pressure-monitored and replaced on schedule or on alarm. Filter banks are inspected monthly. Cooling coil drain pans are inspected per ASHRAE 188 water management. All of this documentation feeds the EOC binder that supports the next Joint Commission survey.
For duct contractors and consultants working with hospital projects, the SBKJ engineering team provides commissioning support including SMACNA leakage testing protocols, ASHRAE 170 verification procedures, NADCA cleaning specifications and HEPA integrity testing protocols. Read more about cleanroom duct manufacturing for healthcare and pharmaceutical projects.
Related healthcare and critical-environment ductwork specifications
Hospital ductwork shares specification logic with several related critical-environment applications:
Talk to an SBKJ engineer about your hospital ductwork project →
FAQ
What is the dominant code for hospital HVAC ductwork in 2026?
ASHRAE Standard 170-2021 "Ventilation of Health Care Facilities" is the dominant code for hospital HVAC across the United States and is referenced by FGI Guidelines for Design and Construction of Hospitals 2022, by Joint Commission Environment of Care standards and by CMS Conditions of Participation. ASHRAE 170 is also adopted internationally by reference in many jurisdictions including Australia, Canada and Gulf states. State-specific overlays such as California OSHPD and New York DOH apply on top of ASHRAE 170 with stricter local requirements.
What air change rate is required for an operating room?
ASHRAE 170-2021 Table 7.1 requires a minimum 20 total air changes per hour for a Class B and Class C operating room, of which at least 4 ACH must be outdoor air. Modern OR design typically supplies 25 ACH through laminar flow ceiling diffusers covering the surgical zone, with low-wall returns at 15 ACH exhaust to maintain positive pressure. The OR must be positive to adjacent corridors at a differential of at least 2.5 Pa (0.01 inches of water column), continuously monitored.
What pressure relationship is required for an Airborne Infection Isolation Room?
An Airborne Infection Isolation Room (AIIR) per ASHRAE 170 Table 7.1 must be negative to adjacent corridors at a minimum differential of 2.5 Pa (0.01 inches of water column), with at least 12 air changes per hour, 100 percent of which is exhausted directly to the outside through HEPA-filtered exhaust ductwork. The negative pressure must be continuously monitored with a permanent visual indicator at the room entrance and tied to the facility DDC for alarm logging.
What is the minimum filtration grade for hospital supply air?
ASHRAE 170-2021 Table 6.4 specifies dual-stage filtration for inpatient hospital areas: a MERV 7 prefilter (Filter Bank 1) followed by a MERV 14 final filter (Filter Bank 2). Operating rooms, AIIR exhaust and Protective Environment supply require an additional terminal HEPA stage rated 99.97 percent at the most penetrating particle size (H13 or H14 per ISO 29463). Post-COVID, many facilities have voluntarily upgraded all general patient areas to MERV 13 minimum and are integrating UVGI in supply ducts upstream of cooling coils.
What sealing class does SMACNA require for hospital ductwork?
The SMACNA HVAC Duct Construction Standards require Seal Class A (all transverse joints, longitudinal seams and duct wall penetrations sealed) for any pressurized OR supply, AIIR exhaust, Protective Environment supply, pharmacy compounding exhaust and any duct serving a positively or negatively pressurized critical space. General patient corridors and non-critical areas may use Seal Class B (all transverse joints and longitudinal seams sealed). Class C is not appropriate for any healthcare application. All sealants must be UL 181 listed and low-VOC for indoor air quality.
What material should be used for operating room ductwork?
Operating room supply ductwork from the HEPA filter housing to the laminar flow diffuser is typically specified as Type 304 or 304L stainless steel with continuously welded longitudinal seams and TIG-welded transverse joints, sized at 22 gauge minimum for ducts up to 600 mm and 20 gauge above. The interior must be passivated, polished to a No. 4 finish or better and certified free from particulates. Supply trunk ductwork upstream of the HEPA terminal can be galvanized steel G90 coating per ASTM A653 sized to SMACNA gauge tables, with all interior surfaces clean and dry on installation. AIIR exhaust to the outside must also be stainless or aluminium with welded seams to resist contamination buildup.
How is hospital ductwork commissioned and validated?
ASHRAE 170 Section 8 requires commissioning that verifies airflow rates, pressure relationships, filter integrity and DDC alarm function before occupancy. The protocol covers SMACNA leakage testing of pressurized ducts to the project leakage class (typically Class 6 or better), smoke testing of every pressure boundary including OR doors, AIIR anterooms and PE corridors, ACH verification by traverse measurement at every supply and exhaust grille, HEPA integrity testing per IEST-RP-CC034 with PAO challenge upstream and photometer scan downstream, and a DDC integration test that confirms every pressure alarm and filter differential alarm reaches the facility BMS and Joint Commission Environment of Care logbook.
What is the typical ductwork package value for a major hospital project?
For a 300 to 600 bed acute care hospital, the mechanical ductwork package alone typically runs USD 20 million to USD 80 million depending on geography, complexity and the share of stainless steel critical-care ductwork. A typical breakdown is 60 to 70 percent galvanized rectangular and round trunk and branch, 15 to 20 percent stainless OR and AIIR, 10 percent fire and smoke damper integration, and 5 to 10 percent flex duct, terminal boxes and accessories. Specialty hospitals push the stainless share higher and can lift the package to USD 100 million plus.
