[MEP Center Engineering]

Starter question — a frequently asked question posted by the MEP Center team to open this forum. Share your own experience below.

We are designing a pressurised escape stair for a 12-storey office building. With all doors closed the stair must hold the design pressure difference, but the door opening force must stay within the limit so that people can still open the doors towards the stair.

How do I calculate the door opening force for a given pressure difference, what limits apply under EN 12101-6 (and its successor) and NFPA 92 / IBC, and what design measures keep the pressure from rising too high when doors are closed?

[MEP Center Engineering]

The force to open a door against a pressure difference is the door closer force plus the pressure force acting about the hinge. EN 12101-6 limits the opening force to 100 N at the handle; NFPA 101 / IBC allow 30 lbf (133 N). Check the maximum pressure with all doors closed and control it with a pressure-relief damper or a variable speed fan on a pressure sensor.

1. Door opening force

F = F_dc + (W · A · Δp) / (2 · (W − d))

• F_dc: force to overcome the door closer and friction (N)
• W: door width (m), A: door area (m²), Δp: pressure difference (Pa)
• d: distance from the handle to the latch edge (m)

2. Worked example

Door 0.9 × 2.1 m (A = 1.89 m²), handle 0.07 m from the edge, closer force 40 N, Δp = 50 Pa.

• Pressure term: (0.9 × 1.89 × 50) / (2 × 0.83) = 85.05 / 1.66 ≈ 51.2 N.
• F = 40 + 51.2 ≈ 91 N (about 20.5 lbf), below the 100 N limit.
• Maximum allowed pressure with this closer: Δp = (100 − 40) × 1.66 / 1.701 ≈ 58.6 Pa.

So the margin between design pressure and maximum pressure is small. A heavier closer of 60 N would allow only about 39 Pa. Specify the closer force on the door schedule and measure it at commissioning.

3. Design criteria (typical)

• EN 12101-6 (being replaced by EN 12101-13): pressure difference with doors closed typically about 45 to 50 Pa depending on system class; air velocity through the open fire-floor door 0.75 m/s or 2 m/s depending on class; opening force ≤ 100 N.
• NFPA 92 / IBC: minimum pressure difference 12.5 Pa (0.05 in wg) in sprinklered buildings, higher for unsprinklered; maximum governed by the 30 lbf opening force.

4. Leakage and fan sizing

Leakage through a closed door: Q = 0.83 · A_L · Δp^1/2. With an assumed leakage area of 0.01 m² per door at 50 Pa: 0.83 × 0.01 × 7.07 ≈ 0.059 m³/s per door, about 0.7 m³/s for 12 doors, before lift, wall and roof leakage. The open-door velocity case usually governs the fan size; add a margin for leakage uncertainty.

5. Keeping pressure under control

• Relief damper from the stair to outside, or a variable speed fan controlled by a differential pressure sensor; fast response matters when doors close.
• For tall stairs, multiple injection points, commonly every two to three floors, to limit pressure variation and stack effect.
• Separate duty and standby fans with automatic changeover, on secondary power.
• Air intake away from smoke exhaust outlets.

Common mistakes

• Fan sized for the open-door case with no relief, so closed-door pressure exceeds 80 Pa.
• Ignoring the closer force in the calculation.
• Commissioning on a calm day only; wind and temperature change the result.

Related: Stair Pressurisation: Quick Airflow Design. For deeper study: HVAC Systems Design Course.

Check the edition of the code that applies to your project.

[Author]

Posts