HVAC
Healthcare and hospital HVAC ventilation field guide (ASHRAE 170)
Hold the room pressure relationship, deliver the air changes and filtration each space needs, keep humidity in range, and monitor and validate it all to a clinical standard.
Direct answer
Healthcare HVAC is the ventilation that performs infection control in a hospital, not comfort alone. It holds operating rooms positive so clean air flows out, isolation rooms negative so pathogens stay in, and delivers the air changes, filtration, and humidity each space needs. ASHRAE Standard 170 and the FGI Guidelines set these as requirements.
Key takeaways
- ANSI/ASHRAE/ASHE Standard 170 and the FGI Guidelines govern healthcare ventilation space by space, treating a hospital room's air as a clinical infection-control device.
- Operating rooms run positive pressure at about 20 air changes per hour with HEPA final filtration; airborne infection isolation (AII) rooms run negative at about 12 ACH.
- Minimum pressure differential is commonly at least 0.01 in. wc, with many facilities operating at 0.02 to 0.03 in. wc for margin; OR humidity band is roughly 20 to 60 percent.
- Patient-care areas use two filter banks in series: a MERV 7 or higher pre-filter and a MERV 14 or higher final filter; HEPA removes at least 99.97 percent of 0.3 micron particles.
- Validate, do not just balance: confirm pressure direction with airflow visualization, leak-test HEPA in place, monitor critical-room pressure continuously with a calibrated door alarm, and keep the records.
What healthcare HVAC is, and why a hospital room's air is a clinical device
Healthcare HVAC is the ventilation and air conditioning that performs infection control inside a hospital. A patient room's air is a clinical device, not a comfort system. The same equipment that in an office keeps people from feeling stuffy is, in a hospital, the thing that decides whether a tuberculosis patient's airborne pathogens reach the corridor or whether a surgical site stays clean. That is a different job with a different standard behind it.
The way it does the job comes down to a few controlled variables. The system holds a pressure relationship so air moves in the direction you want. It delivers a high enough air change rate to dilute what builds up in the space. It filters the supply to a level matched to how sick or how vulnerable the occupants are, and it keeps temperature and humidity inside a range that limits microbial growth and serves the clinical work. Miss one and the room stops protecting anyone.
ASHRAE Standard 170 and the FGI Guidelines set these as requirements, space by space, because the failure mode is a patient infection, not a comfort complaint. The companion Anvilfield guide on ventilation rate and outdoor air under ASHRAE 62.1 covers the commercial-building floor for fresh air, and the DOAS guide covers drying that air. This guide is the healthcare layer that sits on top of both.
The air is infection control, not comfort
Hold this framing above everything else in the guide. In a hospital, the HVAC is part of the infection control program. Pressure, air changes, and filtration are clinical controls that protect patients, and comfort is a distant secondary concern that you serve only after the protection is intact.
The split shows up the moment something has to give. In an office, if the air handler is struggling you let the space drift a couple of degrees and nobody is harmed. In an operating room or an isolation room, you do not trade away the pressure relationship or the air change rate to save energy or to make a thermostat happy, because the thing you would be trading is the patient's protection. A reversed isolation room pushes airborne pathogens into the corridor. An operating room that loses positive pressure pulls dirty corridor air toward the surgical field.
This is why the design, the monitoring, and the commissioning are held to a clinical standard rather than a comfort standard. The engineer of record, the infection control team, and the authority having jurisdiction treat these rooms as part of patient care. Write your work as if a patient outcome depends on it, because it can.
What is ASHRAE 170 and the FGI Guidelines?
ANSI/ASHRAE/ASHE Standard 170, Ventilation of Health Care Facilities, is the governing technical standard for healthcare ventilation in the United States, and the FGI Guidelines for Design and Construction are the broader facility code that adopts and references it. The FGI Guidelines pull ASHRAE 170 in as the ventilation requirement, so on most projects the two move together and a jurisdiction adopting FGI is adopting ASHRAE 170 with it.
The heart of the standard is a space-by-space table. For each room type, from an operating room to an airborne infection isolation room to a soiled utility room, the table fixes the pressure relationship, the minimum total air changes per hour, the minimum outdoor air changes, whether the space may recirculate, the filtration level, and the design temperature and humidity range. The table is the code. When a question comes up about what a space requires, you look it up there rather than reasoning from comfort norms.
Treat the specific numbers in this guide as the current values from recent editions and verify them against the edition the jurisdiction has actually adopted, because ASHRAE 170 is revised on a cycle and the FGI Guidelines are published on their own schedule. The adopted edition, any state amendments, and the authority having jurisdiction control the requirement. The engineer of record and infection control own the call on any space the table does not cleanly cover.
The pressure relationships: positive versus negative
The pressure relationship is the core of healthcare ventilation, and it is decided by one question: what are you protecting? Positive pressure protects the patient from the room by pushing clean air out through the door gaps so dirtier air from adjacent spaces cannot drift in. Negative pressure protects everyone outside the room from the patient by pulling air in under the door so the patient's airborne pathogens stay contained.
Get the direction backward and the room does the opposite of its job. A positive room that goes negative pulls contaminated corridor air toward a vulnerable patient. A negative room that goes positive pushes infectious air out into the corridor. Either reversal is a patient-safety event, which is why the relationship is monitored continuously in the spaces that matter most and not just set once at balancing.
ASHRAE 170 expresses the relationship as a required direction and a minimum differential, commonly at least 0.01 in. wc, with many facilities designing and operating at 0.02 to 0.03 in. wc to keep margin against door swings and system swings. Some spaces are required to be neutral or simply have no defined relationship. The required direction for any given room type comes from the ASHRAE 170 table and the engineer, not from a rule of thumb.
| Relationship | What it protects | Example spaces |
|---|---|---|
| Positive | The patient, from the room | Operating rooms, protective environment rooms |
| Negative | Everyone else, from the patient | Airborne infection isolation, bronchoscopy, soiled utility |
| Neutral or none | No directional requirement | Many general patient and support spaces |
Why are operating rooms positive pressure?
Operating rooms and protective environment rooms are positive because the patient inside is the one at risk, so you push clean air out and keep dirtier air from coming in. In an operating room the open surgical site is the vulnerability, and any airborne particle that settles on it is a potential infection. In a protective environment room the patient is immunocompromised, a bone marrow or stem cell transplant recipient for example, and even ordinary environmental organisms that a healthy person shrugs off can cause serious infection.
So the air handler supplies more conditioned air to these rooms than it exhausts, and the surplus leaks out through the door gaps and around the seals, always moving from clean to less clean. The patient sits in the cleanest air in the building, and nothing flows toward them from the corridor.
Positive pressure is paired with the rest of the package to make it work. High air change rates dilute and sweep the space, HEPA-level filtration cleans the supply, and tight humidity control limits growth. ASHRAE 170 sets the required direction and differential for these rooms; the engineer of record and infection control confirm the specific values for the project and the edition adopted.
What is a negative pressure isolation room?
A negative pressure isolation room, formally an airborne infection isolation (AII) room, is a patient room held at lower pressure than the corridor so air flows in under the door and the patient's airborne pathogens cannot escape to the rest of the floor. It is the containment space for diseases that spread through the air, such as tuberculosis, measles, and varicella.
The room exhausts more air than it supplies, which is what creates the inward pull. That exhaust is sent directly outdoors to a safe discharge point, or it passes through HEPA filtration before any of it is recirculated, so the captured pathogens are removed from the building rather than mixed back into it. The pressure is monitored continuously while an infectious patient is present, with a visible indicator at the door so staff can confirm the direction before they enter.
Do not confuse an AII room with a protective environment room. They look similar and they are opposites. AII is negative to protect the corridor from the patient. PE is positive to protect the patient from the corridor. A room that is plumbed and controlled for one cannot simply be flipped to the other without the engineer reworking the airflow, the exhaust, and the controls.
How many air changes per hour does an operating room need?
An operating room needs a minimum of about 20 total air changes per hour under ASHRAE 170, with a portion of that, commonly at least 4 air changes, coming from outdoor air. An airborne infection isolation room needs about 12 total air changes per hour. The air change rate is the dilution rate: how many times per hour the room's full volume of air is replaced, sweeping out contaminants and shortening the time any airborne particle lingers.
Higher acuity means more air changes. A general patient room sits much lower, often in the 4 to 6 range, while procedure and surgical spaces and isolation rooms run high because the consequence of a lingering particle is a clinical one. The total rate and the outdoor-air portion are two separate numbers in the table, and a space can recirculate part of its air through filters to make the total while bringing in less outdoor air.
These figures are the current minimums from recent editions and they are minimums, not targets to undercut. The exact total and outdoor-air values for each space, and which spaces may recirculate, come from the ASHRAE 170 table for the adopted edition. The engineer of record sets the design rates; commissioning proves the room actually achieves them.
| Space | Pressure | Approx. minimum total ACH |
|---|---|---|
| Operating room | Positive | ~20 |
| Protective environment room | Positive | ~12 |
| Airborne infection isolation (AII) | Negative | ~12 |
| General patient room | No requirement | ~4 to 6 |
| Airborne isolation anteroom | Per design | ~10 |
Filtration by space, and the two filter banks
Filtration in healthcare is matched to the space, and the standard pattern for patient care and surgical areas is two filter banks in series. The first bank is a pre-filter, commonly MERV 7 or higher, that catches the gross load and protects the expensive filter behind it. The second bank is a higher-efficiency final filter, commonly MERV 14 or higher, that does the real cleaning of the supply air. The pre-filter is there as much to extend the final filter's life as to clean the air.
Where the patient is most vulnerable, the requirement steps up to HEPA. Operating rooms, protective environment rooms, and similar spaces use HEPA final filtration, which removes at least 99.97 percent of particles at 0.3 micron. AII rooms that recirculate any of their exhaust use HEPA on that recirculated path so pathogens are not carried back into the building.
Filters only protect the space if they stay sealed and get changed on condition, not on a guess. Banks at MERV 13 and above are monitored with a differential pressure gauge across the bank, and you change the filter when the pressure drop says it is loaded, watching for the bypass that happens when a filter is loose in its frame or the gasket has failed. The exact MERV and HEPA requirement for each space is set by ASHRAE 170 and the engineer; the companion Anvilfield air filtration and MERV guide covers the filter classes in depth.
Temperature and humidity ranges
Healthcare spaces hold temperature and humidity inside tight clinical ranges because both affect infection risk, not just comfort. Operating rooms commonly run a temperature range around 68 to 75 degrees F with the surgical team able to ask for the room to go outside it for a specific procedure, and a relative humidity band that recent editions set roughly between 20 and 60 percent.
Both ends of the humidity range earn their place. The low end exists because too dry an environment raises static electricity risk and can affect patients and some equipment, which is why the floor was relaxed from older higher values in recent editions. The high end matters more for infection: above roughly 60 percent relative humidity, mold and microbial growth become a real risk on surfaces and inside the system, so the ceiling is held to keep the environment hostile to growth.
Holding a humidity band that tight, regardless of the outdoor weather, the heat the surgical lights and equipment throw off, and the doors opening, is a real equipment problem. It usually takes a system that can dehumidify hard and then reheat, which is part of why healthcare HVAC is energy-intensive. The required ranges for each space come from ASHRAE 170 for the adopted edition; confirm them with the engineer and infection control.
Outdoor air, recirculation, and all-outdoor-air spaces
Every healthcare space has a minimum outdoor air requirement, and some spaces are required to run all outdoor air with no recirculation at all. The outdoor air is the dilution that the ASHRAE 62.1 ventilation rate procedure handles for commercial buildings, and the Anvilfield ventilation rate and outdoor air guide covers that floor. Healthcare layers its own, usually higher, minimums on top through ASHRAE 170.
The table gives two numbers per space: a total air change rate and an outdoor-air change rate. The gap between them is what the space is allowed to make up with filtered, recirculated air. A space that may not recirculate has to make its entire air change rate from outdoor air, which is expensive to condition and is reserved for spaces where recirculation would carry contaminants the wrong way.
Drying all that outdoor air is a separate equipment question, and on many healthcare projects a dedicated outdoor air system handles it. The Anvilfield DOAS guide covers how a DOAS dries the ventilation air to a low dewpoint and lets a parallel system carry the room sensible load, which is a common fit for the high outdoor-air rates healthcare demands. Whether a given space recirculates, and how much outdoor air it takes, is fixed by ASHRAE 170 and the engineer, not chosen for energy convenience.
The operating room, space by space
The operating room is the most demanding space in the building and it stacks every control at once. It is positive to all adjoining spaces, runs roughly 20 air changes per hour, uses HEPA-level final filtration, and holds a tight temperature and humidity band. The supply is delivered through a large laminar diffuser array centered over the surgical table, sized to wash filtered air straight down over the sterile field and the team before it migrates to the low returns at the room perimeter.
That diffuser array is the part crews get wrong on a retrofit. The array has a required coverage over the table and a face velocity that produces a low-turbulence downward flow, and if you crowd it with new lights or booms, or undersize it, you break the clean wash that the whole room depends on. The returns belong low on the walls so the air sweeps down and out, not high where it would short-circuit.
Door discipline is part of the system, not a separate housekeeping issue. Every door opening collapses the pressure relationship for a moment and stirs the room, so OR traffic and the number of door swings during a case are tracked as an infection control metric. The HVAC sets up the clean field; the staff either protect it or defeat it. The specific airflow, filtration, and humidity values for the room come from ASHRAE 170 and the engineer of record.
The airborne infection isolation room, space by space
The AII room is the containment space, and its whole design points inward and out the exhaust. It is negative to the corridor and to any anteroom, runs about 12 air changes per hour, and exhausts directly outdoors or through HEPA before any recirculation. The supply diffuser is placed near the door and the exhaust grille is placed low behind or beside the patient bed, so clean air enters at the doorway, moves across the room, and is pulled out past the patient and away from anyone entering.
That airflow geometry is the part that gets undone in the field. Swap the supply and exhaust locations, or let a return get blocked, and you can create a path that carries the patient's exhaled air back toward the door and the staff. The room can read negative on the gauge and still have a bad internal flow pattern, which is why smoke or airflow visualization is part of validating one of these rooms, not just a pressure reading.
The pressure is monitored continuously while an infectious patient is in the room, with a visible local indicator at the door and an alarm if the relationship is lost. Many AII rooms include an anteroom as an airlock. The required differential, air change rate, and exhaust path come from ASHRAE 170, the engineer, and infection control.
Anterooms and the pressure cascade
An anteroom is a small airlock between an isolation room and the corridor that holds an intermediate pressure, so the pressure steps down or up in stages rather than all at once across a single door. It gives staff a place to gown and don respirators, and it keeps the relationship more stable because two doors are rarely open at the same time.
The direction of the cascade follows what you are protecting. For an AII room, the corridor is higher than the anteroom, and the anteroom is higher than the isolation room, so air always moves toward the contained patient and out the room exhaust. For a protective environment room, the cascade runs the other way, with the patient room highest. A combined PE and AII room, used for a patient who is both infectious and immunocompromised, requires an anteroom precisely because you cannot satisfy both protections across a single door.
Whether a given isolation room requires an anteroom, and the differential at each step, is set by ASHRAE 170 and the FGI Guidelines for the adopted edition and confirmed by infection control. Do not assume an anteroom is optional; on some room types it is part of the requirement.
How is room pressure monitored?
Critical-space pressure is monitored continuously with a permanently installed pressure monitor at the room, showing a local visual indicator of the direction and alarming when the relationship is lost. The point is that staff can confirm the room is doing its job before they take a patient in or before they enter a contained room, without trusting that it was set correctly at the last balancing.
A monitor reads the differential between the room and a reference space, usually the corridor or anteroom, and displays it at the door. Operating rooms and isolation rooms get this treatment because a reversal there is a patient-safety event. The monitor itself is a clinical instrument: it has to be calibrated on a schedule, and a monitor that drifts and reads positive when the room is actually negative is worse than no monitor, because it tells everyone the room is safe when it is not.
Continuous monitoring only protects patients if someone owns the alarm and the log. A field operations record on a platform like FieldOS keeps the per-room pressure checks, the alarm events, and the monitor calibration dates in one place, so a lost relationship is caught and corrected rather than living quietly on a display nobody logged. Verify the monitoring and alarm requirements for each space against ASHRAE 170 and the facility's infection control program.
The systems: air handlers, terminals, and reheat
Healthcare air handlers are built for tight control and for cleaning. A typical critical-space air handler carries the two filter banks in series, a cooling coil sized to dehumidify hard, a heating coil, and humidification, with terminal units at the zones for final temperature trim. The zone reheat is what lets the system dry the air down at the coil and then warm it back to the room setpoint without overcooling, which is how you hold a tight humidity band and a comfortable temperature at the same time.
Controls carry more weight here than in a commercial building because the sequences enforce the infection control. The pressure relationships are maintained by tracking supply and exhaust airflow at the room, and the control logic has to hold the offset between them through load swings, filter loading, and door events. A control problem in a hospital is not a comfort complaint; it can be a reversed room.
Construction matters as much as the schedule of equipment. Ductwork serving critical spaces is built and sealed to a tighter leakage class so the air goes where the design sends it, and the access for filter changes and coil cleaning is designed in, because a coil you cannot reach is a coil that grows biofilm. The specific equipment, filtration, and control requirements come from the engineer of record and ASHRAE 170.
Redundancy for critical spaces
Critical healthcare spaces are designed so the air does not stop when a piece of equipment fails or comes down for service. The common approach is N+1 redundancy, an extra unit or extra capacity beyond what the load needs, so a failed fan, a failed unit, or a planned maintenance shutdown does not drop the air changes or collapse the pressure relationship in an operating room or an isolation room.
The reason is practical. A hospital cannot evacuate an occupied surgical suite or an isolation room because a single air handler tripped, and these systems run continuously, so they need maintainability without a full shutdown. N+1, redundant fans, and the ability to isolate and service one path while another carries the space are how that gets built.
How much redundancy a given space gets is a design and code decision tied to how the space is used and to the project requirements, and it is also a life-cycle cost decision, since redundancy is expensive to build and to run. The engineer of record sets the redundancy level against the FGI Guidelines, the project program, and the owner's risk tolerance.
ICRA: protecting patients during construction
An infection control risk assessment (ICRA) is the formal process of figuring out how construction, renovation, or even maintenance work will affect the air and water around patients, and what containment is needed to keep that work from harming them. It is required by the FGI Guidelines for construction in healthcare facilities, and it is led by a team that includes infection control, not just the contractor.
The reason it exists is the dust. Disturbing ceilings, walls, and old construction releases Aspergillus and other spores, and a meaningful share of hospital Aspergillus outbreaks have been traced to nearby renovation or demolition. An immunocompromised patient down the hall can be infected by spores that drift from a poorly contained work area, which is why this is treated as a patient-safety control and not a jobsite courtesy.
On the HVAC side, the work area is held under negative pressure relative to the occupied space, with the airflow cascading from the patient areas into the containment and out, often through HEPA-filtered negative-air machines. The barriers are sealed, the pressure is monitored, and the area stays negative for the duration. The ASHE ICRA process, the CDC environmental infection control guidance, and the facility's infection control team set the class of precautions; confirm the specific containment with them before the first ceiling tile comes out.
Commissioning and validation, not just balancing
A healthcare room is not done when it is balanced; it is done when it is validated. Commissioning a critical space means proving, with measurements, that the room actually holds its pressure relationship, makes its air changes, achieves its filtration, and stays in its temperature and humidity band under real operating conditions. A general commercial test-and-balance does not clear that bar, because it was never aimed at the clinical performance.
The validation has specific pieces. The pressure relationship is measured and the direction confirmed, often with airflow visualization to prove the internal flow pattern, not just the gauge reading. The total and outdoor air changes are computed from measured diffuser airflow. The filter installation is leak-tested in place where HEPA is used. Operating rooms commonly get a particle count certification, and the rooms are recertified on a schedule, often annually or after any significant HVAC change.
Treat the validation as the clinical proof the room is fit to use, and keep the certificate. Commissioning is performed by people with healthcare experience, working from ASHRAE 170, the FGI Guidelines, and the project requirements, with infection control and the authority having jurisdiction signing off. A room that was balanced but never validated is a room nobody has proven is safe.
Other spaces: pharmacy, lab, and sterile processing
Several support spaces have their own rules layered on top of, or alongside, ASHRAE 170. The compounding pharmacy is the sharpest example. Sterile compounding under USP General Chapter 797 happens inside an ISO Class 5 primary engineering control sitting in an ISO 7 buffer room, with the non-hazardous buffer held positive at roughly 0.02 in. wc or more to keep contamination out of the clean space.
Hazardous drug compounding under USP General Chapter 800 flips the pressure for the same reason healthcare flips it everywhere: to protect the people outside the space. The hazardous buffer room is held negative, commonly 0.01 to 0.03 in. wc, so the drug aerosols stay contained, and the containment primary engineering control is exhausted to keep staff exposure down. A pharmacy that is positive where it should be negative is a staff exposure problem.
Laboratories, sterile processing departments, and other support spaces each have a pressure direction, an air change rate, and a recirculation rule in the ASHRAE 170 table tied to whether they handle clean or soiled material. Soiled and decontamination spaces are negative; clean and sterile storage spaces are positive. The USP chapters govern the pharmacy specifics; the engineer and infection control reconcile them with ASHRAE 170 for the adopted edition.
Legionella and the building water interface
Healthcare HVAC touches the building water in a few places that carry Legionella risk, and a hospital is exactly where that risk matters most because the patients are vulnerable. Cooling towers, evaporative equipment, and the humidifiers inside air handlers all create or contact water that can aerosolize, and Legionella grows in warm, stagnant water in the range a cooling tower or a neglected humidifier basin can sit at.
The control framework is ASHRAE Standard 188, which requires a formal water management plan with a team, a system schematic, identified control points, and ongoing monitoring of temperature and disinfectant levels. For HVAC, that means keeping humidifier water clean and the equipment maintained, treating and inspecting cooling towers, and not letting condensate or basins go stagnant. This is a brief note inside an HVAC guide, but it sits next to the air work because the same building serves both, and the water plan is a separate document the facility maintains.
The energy cost of getting it right
Healthcare HVAC uses a lot of energy by design, and that is the price of the protection, not a flaw to engineer away. High air change rates move large volumes of air around the clock, all-outdoor-air spaces give up the energy savings of recirculation, and the dehumidify-then-reheat sequence that holds a tight humidity band burns energy on both ends. A hospital runs continuously, so none of this gets a nighttime setback the way an office does.
There are honest savings to take without touching the protection. Energy recovery on the exhaust and outdoor air, careful coil and fan selection, and demand-based control in spaces that allow it all cut the bill. What you do not do is set back the air changes or relax the pressure in a critical space to save energy while it is in use, because the saving comes straight out of the patient's protection. The setback rules for critical spaces are limited for that reason; the engineer and the adopted code define where reduced operation is permitted.
Maintenance that keeps the clinical performance
The clinical performance a room had at commissioning decays without maintenance, so the upkeep is part of the infection control, not a separate facilities chore. Filters load and need changing on condition from the differential pressure, not on a wishful schedule. Coils and humidifiers need cleaning so they do not become a microbial source feeding the supply air. Controls drift, and the sequences that hold the pressure offsets need to keep working through every load.
The pressure monitors and sensors are instruments that need calibration on a schedule, because a drifted monitor lies about the one thing it exists to report. The rooms themselves are recertified periodically, and after any HVAC change, to confirm they still hold pressure, air changes, and filtration. An operating room that passed two years ago and was opened up for a renovation is not certified anymore until someone proves it again.
The practical risk is the slow drift nobody logged: a filter left past its load, a monitor reading the wrong direction, a humidifier growing biofilm. Each is invisible until it is an outbreak or a failed recertification. Tie the maintenance, the calibration dates, and the recertification schedule to a record the facility actually keeps. The intervals come from the manufacturer, ASHRAE 170, and the facility's infection control and maintenance program.
What to document
A healthcare room's compliance is only as good as the record that proves it, and on these spaces the record can be subpoenaed, surveyed, and audited. The Joint Commission and the authority having jurisdiction want to see that the pressure relationships, air changes, filtration, and humidity were verified and stay verified, with dates and names attached.
Capture the room and space type, the required pressure direction and differential, the required and measured total and outdoor air changes, the filtration level and filter change and leak-test dates, the temperature and humidity range and readings, the continuous monitor calibration dates and alarm events, and the commissioning and recertification certificates. Keeping the per-room pressure checks, the alarm log, the filter and calibration dates, and the recertification schedule on a field platform like FieldOS puts the proof in one place instead of scattered across binders and a control screen nobody exports.
| Space | Requirement to record | Note |
|---|---|---|
| Operating room | Positive, ~20 ACH, HEPA, RH band | Particle count certification on schedule |
| AII room | Negative, ~12 ACH, exhaust path | Continuous monitor and alarm log |
| PE room | Positive, ~12 ACH, HEPA | Confirm not confused with AII |
| Hazardous pharmacy | Negative, USP 800 | Reconcile with ASHRAE 170 and the engineer |
| All critical spaces | Monitor calibration, recert dates | Re-validate after any HVAC change |
Common mistakes
- Reversing or losing a room pressure relationship, so a positive room goes negative or a negative room goes positive.
- Running air change rates below the ASHRAE 170 requirement for the space, or counting recirculated air where the space must run outdoor air.
- Using the wrong filtration for the space, such as skipping HEPA on a protective environment or operating room, or letting a loose filter bypass.
- Operating a critical space with no continuous pressure monitoring, or trusting a monitor that was never calibrated.
- Confusing an AII room with a protective environment room and setting the pressure the wrong direction for the patient.
- Ignoring ICRA during construction, so dust and spores from the work area reach occupied patient spaces.
- Balancing a room and calling it done without commissioning and validating the clinical performance.
Field checklist
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Standards and references
ANSI/ASHRAE/ASHE Standard 170, Ventilation of Health Care Facilities, is the governing technical standard, and the FGI Guidelines for Design and Construction adopt and reference it as the healthcare ventilation requirement. The space-by-space table in ASHRAE 170 controls the pressure relationship, total and outdoor air changes, recirculation, filtration, and temperature and humidity for each room type. Hedge the pressure, air change, filtration, and humidity values to ASHRAE 170 and the FGI Guidelines for the edition the jurisdiction has adopted, to the engineer of record, and to infection control, because the editions revise and states amend.
The CDC environmental infection control guidance and the ASHE ICRA process govern construction and renovation in occupied facilities. USP General Chapters 797 and 800 govern sterile and hazardous drug compounding in the pharmacy, with their own ISO classifications and pressure directions. ASHRAE Standard 188 governs the building water and Legionella management that the HVAC humidifiers and cooling towers touch. The companion Anvilfield guides on ASHRAE 62.1 ventilation rate and on DOAS cover the outdoor-air floor and the equipment that dries it.
Three things carry the whole guide. The air is infection control, so hold the pressure relationship in the direction that protects the right party. Meet the air changes and filtration the space requires, not a comfort norm. Monitor and validate the rooms to a clinical standard, and keep the proof. The authority having jurisdiction and infection control have the final say on every space.
Units and terms
Healthcare ventilation borrows its units from HVAC and adds a vocabulary tied to infection control, so the same room can be described in pressure, airflow, and clinical terms on the same drawing.
Pressure differential is given in inches of water column (in. wc), sometimes inches of water gauge (in. wg) or pascals (Pa) on metric or instrument readouts, where 0.01 in. wc is about 2.5 Pa. Air change rate is given in air changes per hour (ACH), the number of times the room volume is replaced each hour. Filtration is given as a MERV rating for the filter banks and as HEPA for the highest level, where HEPA removes at least 99.97 percent of particles at 0.3 micron.
- Healthcare HVAC
- Ventilation and air conditioning that performs infection control in a hospital, treating the room's air as a clinical device
- ASHRAE 170 / FGI
- ANSI/ASHRAE/ASHE Standard 170 and the FGI Guidelines, the governing requirement for healthcare ventilation, with a space-by-space table
- Positive pressure room
- A room held above its surroundings so clean air flows out, protecting the patient from the room (OR, PE)
- Negative pressure room
- A room held below its surroundings so air flows in, protecting everyone else from the patient (AII)
- AII vs PE
- Airborne infection isolation (negative, contains the patient's pathogens) versus protective environment (positive, protects the immunocompromised patient)
- Air change rate (ACH)
- How many times per hour the room's full air volume is replaced, the dilution rate the table sets by space
- Anteroom
- A small airlock between an isolation room and the corridor that holds an intermediate pressure for a staged cascade
- ICRA
- Infection control risk assessment, the required process for containing construction dust and protecting patients during work
- Room pressure monitor
- A permanently installed instrument that continuously shows the pressure direction at the door and alarms when it is lost
FAQ
What is healthcare HVAC?
Healthcare HVAC is the ventilation and air conditioning that performs infection control in a hospital, where a room's air is a clinical device rather than a comfort system. It holds room pressure relationships, delivers high air changes and the required filtration, and controls humidity, all set by ASHRAE 170 and the FGI Guidelines.
What is ASHRAE 170?
ASHRAE 170 is ANSI/ASHRAE/ASHE Standard 170, Ventilation of Health Care Facilities, the governing technical standard for hospital ventilation. Its space-by-space table sets the pressure relationship, total and outdoor air changes, recirculation, filtration, and temperature and humidity for each room type. The FGI Guidelines adopt it, and the jurisdiction controls the edition.
What is a negative pressure isolation room?
A negative pressure isolation room, or airborne infection isolation (AII) room, is held below corridor pressure so air flows in under the door and the patient's airborne pathogens cannot escape. It runs about 12 air changes per hour, exhausts outdoors or through HEPA, and is monitored continuously while an infectious patient is present.
Why are operating rooms positive pressure?
Operating rooms are positive because the patient and the open surgical site are what need protecting, so the room pushes clean air out and keeps dirtier corridor air from drifting in. Positive pressure pairs with roughly 20 air changes per hour, HEPA-level filtration, and tight humidity to protect the patient from the room.
How many air changes per hour does an isolation room need?
An airborne infection isolation room needs about 12 total air changes per hour under recent ASHRAE 170 editions, with a portion from outdoor air. A protective environment room runs about 12 as well, and an operating room runs about 20. The adopted edition table and the engineer set the exact values.
What is the difference between an AII room and a protective environment room?
An AII room is negative to contain an infectious patient and protect the corridor, while a protective environment room is positive to protect an immunocompromised patient from the room. They look similar and are opposites. A room set up for one cannot be flipped to the other without reworking the airflow, exhaust, and controls.
What filtration do hospital operating rooms require?
Operating rooms use two filter banks plus HEPA in most designs: a MERV 7 or higher pre-filter, a MERV 14 or higher second bank, and a HEPA final filter removing at least 99.97 percent of 0.3 micron particles. Protective environment rooms also use HEPA. The exact class comes from ASHRAE 170 and the engineer.
What is ICRA in healthcare construction?
ICRA is an infection control risk assessment, the required process for evaluating how construction or renovation affects patient air and water and what containment is needed. The work area is held negative with HEPA negative-air machines and sealed barriers so dust and spores like Aspergillus cannot reach patients. Infection control and the FGI Guidelines govern it.
Why does hospital HVAC need continuous pressure monitoring?
Because a reversed pressure relationship is a patient-safety event, operating rooms and isolation rooms get continuous monitors that show the direction at the door and alarm when it is lost. Staff confirm the room is safe before entering rather than trusting the last balance. The monitor itself must be calibrated, or it can read the wrong direction.
What happens if a hospital room loses its pressure relationship?
If a positive room goes negative, it pulls contaminated corridor air toward a vulnerable patient. If a negative room goes positive, it pushes infectious air into the corridor. Either reversal can spread infection, so the relationship is monitored, alarmed, and corrected immediately, and the room is re-validated against ASHRAE 170 before it is trusted again.
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Codes cited in this guide
This guide is written and reviewed against the published standards below. Always confirm the current adopted edition with the authority having jurisdiction.