📊 Total Air Changes
📈 Pressure Relationship
Design Breakdown
Full Calculation Table
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How to Use the Hospital Ventilation Calculator
Choose the hospital space, from operating room to general patient room, to load its ASHRAE 170 air change and pressure targets.
Enter length, width and ceiling height. The room volume is what the air change rate scales against.
Click Calculate Airflow to see total supply airflow, the outdoor air portion, the recirculated balance and the pressure direction against the corridor.
Hospital Room Ventilation: Air Changes, Outdoor Air and Pressure
Hospital air systems have two jobs that an office building does not: dilute and carry away airborne contaminants fast, and steer air so it moves from clean spaces toward dirty ones. Both come down to a room volume, an air change rate and a pressure direction, which is what this calculator works from.
Standards Behind the Numbers
The air change figures here come from Table 7.1 of ASHRAE Standard 170, Ventilation of Health Care Facilities. Canada's counterpart, CSA Z317.2, covers the planning, design, construction, commissioning, operation and maintenance of HVAC systems in health care facilities. Its current edition, CSA Z317.2:24, was published on 31 October 2024 and supersedes the 2019 edition. CSA keeps its table of design parameters behind a paywall, so this page does not quote it. Treat the results as a first check, confirm each room against the standard and edition your province or health authority has adopted, and have the infection prevention team confirm how each room is classified.
Total ACH and Outdoor Air ACH
Every space carries two air change numbers. Total ACH counts all the air moving through the room, and outdoor ACH is the part that must come from outside. Take an operating room 6 m by 6 m with a 3 m ceiling, which holds 108 m³. At 20 total ACH the supply is 108 × 20 ÷ 3.6 = 600 L/s (1,271 CFM). Four of those air changes are outdoor air, 120 L/s (254 CFM), and the other 480 L/s is filtered air recirculated within the system. A 54 m³ airborne infection isolation room at 12 ACH moves 180 L/s, with 30 L/s outdoors, while a patient room of the same size at 4 ACH takes only 60 L/s. The table values are minimums. Heat gains, humidity control and supply air temperature limits can push the design airflow higher, and the calculator does not model them.
Pressure Direction and the 2.5 Pa Differential
An airborne infection isolation room runs negative, so air flows in from the corridor and not out. A protective environment room and an operating room run positive, so air flows out toward the corridor. ASHRAE 170 sets the minimum differential at 0.01 in. w.g. (2.5 Pa), negative for isolation rooms and positive for protective environment rooms, as summarised in an NIH technical bulletin on converting patient rooms, which also lists patient rooms at 4 total and 2 outdoor air changes. Older editions used 6 total air changes for a patient room, so a facility built to an earlier edition may show a higher figure. A differential that small is easy to lose when a door opens, so the supply and exhaust offset, door seals and continuous monitoring matter as much as the design number. Where an anteroom is provided, it needs its own supply and exhaust terminals and a pressure relationship to both the room and the corridor, and exhaust from isolation rooms, their toilet rooms and anterooms goes directly outdoors. The space pressurization calculator helps check a cascade of spaces.
Filtration and Recirculation
Because total ACH is several times the outdoor air portion, most of the air in a hospital room is recirculated, and the filters decide how clean it is. ASHRAE 170 allows HEPA-filtered recirculated air to make up total air changes that the central system cannot deliver, and the CDC environmental infection control guideline places protective environment rooms on HEPA-filtered supply at 12 or more air changes. The HEPA filter calculator covers filter sizing for a space like this.
What High Air Change Rates Cost the Building
Twenty air changes an hour is a heavy load on fans and coils, and the 4 ACH of outdoor air is conditioned in every season. The hospital HVAC calculator looks at the air handling side of a facility, the clean room calculator compares the same dilution logic in a manufacturing setting, and the makeup air calculator checks the replacement air for exhaust-driven spaces.
Frequently Asked Questions
ASHRAE 170 Table 7.1 lists an operating room at 20 total air changes per hour, 4 of them outdoor air, held at positive pressure to adjacent spaces. In a 6 m by 6 m room with a 3 m ceiling (108 m³) that is 600 L/s (1,271 CFM) of supply, of which 120 L/s (254 CFM) is outdoor air. Unidirectional flow over the surgical field and any extra requirements for the room class come from the full standard and the edition your province has adopted.
Both are listed at 12 total air changes per hour with 2 outdoor, but the pressure runs in opposite directions. An airborne infection isolation room is held negative to adjacent spaces so air from an infectious patient cannot leave, while a protective environment room is held positive so contaminated air cannot reach an immunocompromised patient. ASHRAE 170 sets the minimum differential at 0.01 in. w.g. (2.5 Pa) in each case, negative for the isolation room and positive for the protective room.
In the current ASHRAE 170 table a patient room needs 4 total air changes per hour with 2 of them outdoor air and no required pressure relationship. A critical care patient station is listed higher, at 6 total and 2 outdoor. Older editions of the standard used 6 total air changes for a patient room, so a facility built to an earlier edition may show a higher figure than this calculator does. A 5 m by 4 m patient room with a 2.7 m ceiling needs 60 L/s at 4 air changes.
CSA Z317.2 is the Canadian standard for HVAC systems in health care facilities, and its 2024 edition replaced the 2019 one. It applies where a province, health authority or facility owner adopts it, and many projects also cite ASHRAE 170. This calculator uses the ASHRAE 170 table values because CSA's design parameter table is behind a paywall, so confirm each room against the standard and edition your project is bound to.