Fire and smoke exhaust ventilation must meet code by providing a dedicated, fire-rated fan path that removes smoke at 300 to 600°C for 30 to 120 minutes and replaces it with make-up air at 80% of exhaust flow. Typical design targets are 10 air changes per hour in the fire compartment, with fail-safe dampers released by the fire alarm.
The physics of a fire makes the code non-negotiable. Hot smoke is buoyant and forms a descending layer beneath the ceiling; once that layer drops to about 2 m above the floor, escape routes become untenable and occupants cannot see or breathe. A smoke-control system holds the layer high by extracting at the ceiling and feeding low-velocity make-up air, buying the 20 to 30 minutes that evacuation and firefighting need. Toxicity rises faster than heat: a small fire can fill a corridor with carbon monoxide above 1,000 ppm within minutes, and a blocked extract lets that layer fall well before flames arrive. Insurers and authorities therefore require a witnessed test, because a system that looks compliant on paper but fails under load is worse than none, it gives false confidence. The penalty for non-compliance is severe: failed occupancy permits, invalidated insurance after a loss, and criminal liability where the code is law. Designing to the numbers, 300 to 600°C fan rating, 10 air changes per hour, 80% make-up air, and fire-rated ducts, is the only defensible path, and the cost is a fraction of a single fire's aftermath.
Why Smoke Control Is a Life-Safety System
In a fire, toxic smoke kills long before flames reach occupants. A smoke-control system keeps escape routes clear, limits heat build-up, and helps firefighters see. It is not ordinary ventilation; it must keep working while the building burns. That demands fire-rated ducts, certified high-temperature fans, and automatic release on alarm. Codes treat it as a separate system from comfort HVAC, and mixing the two without proper isolation is a common and dangerous violation.
Key Code Thresholds
The table lists the typical values referenced across EN 12101, NFPA 92, and local equivalents so designers can see the envelope.
| Parameter | Typical code value | Standard | Purpose |
|---|---|---|---|
| Fan temperature | 300°C for 60 min | EN 12101-3 | Survive the fire |
| Large compartment | 400°C for 120 min | EN 12101-3 | Big-volume spaces |
| Clearance rate | 10 air changes / h | NFPA 92 | Clear escape routes |
| Make-up air | 80% of exhaust | Local codes | Prevent negative pressure |
| Duct rating | EI 90 to EI 120 | EN 13501 | Contain fire 90 to 120 min |
Tunnels and subways push ratings to 600°C for 60 minutes and require jet fans for longitudinal ventilation, a distinct discipline from building smoke extract.
When to Apply Each Strategy
- Choose mechanical smoke extract with a dedicated fan when the compartment is large and natural ventilation cannot clear it within 30 minutes.
- Choose pressurisation of stairs and lobbies to plus 50 Pa so smoke never enters the escape route even if the floor fills.
- Choose make-up air inlets at low velocity below 1 m/s to avoid pushing smoke across occupants.
- Choose fire-rated ducts EI 90 or EI 120 when the exhaust path passes through other fire zones.
- Choose fail-safe closed dampers that open only on alarm, so the system stays isolated during normal operation.
Commissioning and Acceptance
Acceptance tests prove the system under load: run the fan at design volume and measure temperature resistance with a calibrated heater or witness certificate, confirm dampers release on the fire signal, and verify make-up air reaches 80% of exhaust. A balanced compartment should reach tenable conditions within 20 to 30 minutes. Log every test because inspectors require a valid commissioning dossier, and an untested system is treated as non-compliant regardless of the paperwork supplied at install.
Worked Example: A 2,000 m² Warehouse
A single fire compartment of 2,000 m² and 6 m height holds 12,000 m³. Code asks for 10 air changes per hour during a fire, so the smoke-exhaust fan must move 120,000 m³/h, and it must survive 300°C for 60 minutes per EN 12101-3. Selecting a fan for 120,000 m³/h at 300 Pa static gives a 75 kW unit in a fire-rated EI 120 shaft. Make-up air is set at 80% of exhaust, 96,000 m³/h, delivered through low-velocity inlets below 1 m/s so it does not push smoke back toward the exit. Dampers are fail-safe closed and release only on the alarm signal. In a drill, the compartment reached a tenable layer within 25 minutes, inside the 30-minute target, and the manometer on the stair pressurisation showed plus 50 Pa, confirming escape routes stayed clear. The dossier recorded each damper release and the fan temperature witness, which the inspector accepted without re-test.
Tenability is the real yardstick behind the 10 air-changes rule. Fire engineers model the smoke layer descent and the gas concentration at breathing height, and they accept a design only if occupants can reach the exit within the available safe time, usually 20 to 30 minutes, before CO exceeds about 1,500 ppm or heat beats 60°C at 2 m. The exhaust rate is the lever that holds the layer high, so under-sizing by even 15% can erase several minutes of escape time in a large compartment. That is why the calculation, not a rule-of-thumb, sets the fan, and why the commissioning test must prove the rate under real resistance rather than nameplate optimism.
Common Mistakes to Avoid
- Using a comfort fan for smoke extract, which melts at 300°C and leaves occupants without protection.
- Running make-up air below 80% of exhaust, so the building goes negative and stalls the fire flow.
- Mixing smoke extract with normal HVAC without fire-rated isolation, breaching the compartment.
- Setting dampers to fail-open, which vents conditioned air daily and defeats the alarm release.
- Skipping the witnessed temperature test, so the certificate is an assumption, not evidence.
Keruite manufactures this equipment at an 80,000 m² facility in Dezhou, China, with an in-house AMCA-210-style test laboratory; every unit is balanced to G2.5 and performance-tested before shipment.
FAQ
What are the code requirements for smoke exhaust ventilation?
Codes require a dedicated smoke-exhaust fan sized for the fire compartment, capable of 300 to 600°C for 30 to 120 minutes, moving at least 10 air changes per hour or a calculated volumetric flow. Ducts must be fire-rated and dampers fail-safe closed until the fire alarm releases them.
How hot must a smoke exhaust fan withstand?
A smoke-exhaust fan must typically withstand 300°C for 60 minutes under EN 12101-3, or 400°C for 120 minutes for larger compartments; high-risk tunnels specify 600°C for 60 minutes. The rating is the continuous temperature the motor and impeller survive while still moving design volume.
What is a make-up air system for smoke control?
Make-up air replaces the air pulled out by exhaust fans so the building does not go negative and stall the fire. Codes require make-up air of at least 80% of exhaust flow, delivered at low velocity below 1 m/s so it does not push smoke back into evacuation paths.
How many air changes per hour for smoke clearance?
Smoke clearance usually needs 10 air changes per hour in the fire compartment, while car parks under EN 16798 use 6 to 10 air changes per hour for daily exhaust and a higher emergency rate. The exact figure comes from compartment volume divided by the required clearance time, often 20 to 30 minutes.