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Room pressurization for isolation rooms and clean rooms
Pressure control gives a room a deliberate leakage direction. Making supply either greater or less than exhaust pushes air through the room's leakage paths, mainly the gap under the door, in one direction. The offset is separate from the room's general ventilation rate, which the ACH calculator covers.
The physics is the orifice flow equation, Q = Cd × A × sqrt(2 × ΔP / ρ). Airflow grows with the square root of the pressure difference, so doubling the pressure raises the required offset by only about 41 percent. A small offset therefore buys a measurable pressure, and each extra pascal costs less airflow than the one before.
Negative pressure isolation rooms
An airborne infection isolation room is held negative to the corridor, so exhaust exceeds supply and air moves into the room when the door opens. A National Institutes of Health technical bulletin on converting patient rooms to isolation or protective environment rooms quotes Standard 170 for a minimum differential of 0.01 in. w.c. (2.5 Pa) and for 12 total air changes per hour, 2 of them outdoor air.
Positive pressure protective and clean rooms
Protective environment rooms for immunocompromised patients are held positive to adjacent spaces at the same 0.01 in. w.c. minimum, so supply exceeds exhaust and corridor air stays out. Cleanrooms are often held at a higher differential and add fine filtration on the supply. For the wider set of hospital space types and their air change rates, see the hospital HVAC calculator.
How big the leakage area really is
The door gap usually dominates. A 36 in wide door with a 1 in undercut leaves about 36 in², which is 0.25 ft² or 0.023 m². With Cd 0.65 and a 2.5 Pa target, the offset is about 31 L/s (65 CFM), so a room with 400 CFM of supply needs 465 CFM of exhaust to stay negative, an offset of about 16 percent. A tighter room needs less offset and a leakier one more. The calculation covers only the leakage area you enter, so add other paths and expect some margin to be needed. Confirm the real pressure with a smoke test at the door gap, and see the smoke test calculator for the field procedure. Because the airflow stations are only accurate to a few percent, an offset that is a small fraction of the room airflow can be lost in measurement error, so a pressure gauge or monitor is the final check.
Canadian standards and the final airflow check
Canadian healthcare facilities design HVAC to CSA Z317.2, the standard for HVAC systems in health care facilities, whose 2019 edition the CSA store lists as withdrawn with an update available. Check the pressure criteria, anteroom rules and monitoring requirements in the edition your province has adopted. Once the offset is set, make sure the exhaust fan can deliver the higher airflow with the exhaust fan sizing calculator and that the room still meets its required air changes.
Frequently Asked Questions
Treat the room's leakage paths as one orifice: offset = Cd × A × sqrt(2 × ΔP / ρ), with Cd about 0.65, A the total leakage area, ΔP the target pressure difference in pascals and ρ the air density, about 1.2 kg/m³. For a 0.25 ft² (36 in²) door gap and a 2.5 Pa (0.01 in. w.c.) target, the offset is about 31 L/s (65 CFM). A negative room exhausts that much more than it supplies, and a positive room supplies that much more than it exhausts.
ASHRAE Standard 170 sets a minimum of 0.01 in. w.c. (2.5 Pa), negative for airborne infection isolation rooms and positive for protective environment rooms, relative to adjacent spaces. Both room types also need 12 total air changes per hour with 2 of them outdoor air. Canadian hospitals design to CSA Z317.2 and the codes adopted by their province, so confirm the pressure values, anteroom rules and monitoring requirements against the edition your authority enforces. Pair this tool with the ACH calculator for the air change check.
Start with the door, which is usually the dominant path. A 36 in wide door with a 1 in undercut leaves about 36 in², or 0.25 ft² (0.023 m²). Add sealed or unsealed penetrations, ceiling and wall joints and any transfer grilles, then confirm by measurement. A leakier room needs a larger offset for the same pressure, and a well-sealed room can reach the target with very little offset, which makes the pressure unstable and harder to control.
Check the pressure with a calibrated differential pressure gauge or a smoke test at the door gap that shows air moving in the intended direction. Rooms that need continuous assurance also use a pressure monitor with an alarm, because the pressures involved are only a few pascals and wind, stack effect and door openings can disturb them. After setting the airflows, confirm that the exhaust fan capacity from the exhaust fan sizing calculator still covers the higher of the two flows.