Technical·12 min read
CO₂ Gas Suppression — Total Flooding (Volume Protection): System Types, Design Concentration and Quantity Calculation
The engineering of CO₂ total flooding (volume protection): the difference between HP and LP system types, the oxygen-smothering + cooling mechanism, CO₂'s lethal profile (unlike inert gases, not suitable for occupied spaces), material-dependent design concentration and KB factor, ISO 6183 and NFPA 12 quantity formulas, additional-agent calculation for unclosable openings via the surface term, discharge/hold times, reserve/supplementary quantities and life-safety measures.
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Carbon dioxide (CO₂) is one of the oldest and most powerful agents in gaseous fire suppression, yet it differs decisively from the other clean agents (FM200, NOVEC, inert gases) in two fundamental ways: (1) it extinguishes fire by smothering oxygen, and its design concentrations are lethal to humans — that is, unlike the inert gases it cannot be used in occupied spaces; and (2) besides volume protection (total flooding) it is the only agent that permits object-based protection on its own (local application). CO₂ is also governed by a separate standard — TS ISO 6183 / NFPA 12 (FM200/NOVEC/inert gases fall under ISO 14520/NFPA 2001). This article covers CO₂’s total flooding (volume protection) application from an engineering standpoint; for object/device protection see our CO₂ Local Application (Object Protection) article, and for general agent selection see Gas Suppression: Design and Agent Selection.
What is CO₂ and how does it extinguish?
Carbon dioxide is a colourless, odourless, electrically non-conductive gas; its molecular weight is 44 and it is heavier than air (density ≈1.8 kg/m³ at 15 °C). As a natural component it has zero ozone-depletion potential (ODP = 0) and is subject to no F-gas regulation; but it is an asphyxiant and is dangerous to life safety.
CO₂ extinguishes fire by two physical mechanisms:
- Oxygen smothering (primary): the CO₂ discharged into the space displaces and dilutes the oxygen in the air. Total flooding design concentrations are 34–75%; this reduces oxygen from a normal 21% to roughly 14% (—34% CO₂) down to 5% (—75% CO₂). Once below the oxygen threshold needed for combustion, the flame goes out.
- Cooling (secondary): as high-pressure liquid CO₂ discharges, part of it flashes to solid “snow” and gas; this phase change draws heat from the environment and cools the material — especially valuable in deep-seated fires to prevent re-ignition.
CO₂ leaves no residue, does not harm electronic or mechanical equipment, and offers an extinguishing capability in deep-seated fires that clean agents cannot match.
Why it cannot be used in occupied spaces (life safety)
This is the critical point that separates CO₂ entirely from the inert gases (IG-541/IG-01). Inert gases are designed to keep oxygen within a human-safe margin (≈12%); in IG-541 even the 8% CO₂ in the mixture stimulates respiration. In CO₂ total flooding, by contrast, oxygen is reduced to lethal levels (≈5–14%), and the high CO₂ concentration itself is directly toxic. Even the lowest design value of 34% is far above life-safety thresholds; CO₂ has no safe human-use margin.
Consequently, CO₂ total flooding is used only in normally unoccupied technical spaces — generator and turbine rooms, oil-filled/dry transformer cells, flammable-liquid stores and pump rooms, cable galleries, paint booths, technical spaces with deep-seated loads. In all spaces where people may be present the following are mandatory: an adequate time delay, a pre-discharge audible-visual alarm, automatic/manual switching, a door abort/lock-off, and purge/ventilation after egress. For continuously occupied spaces such as server or control rooms, choose not CO₂ but IG-541 Inergen or halocarbon agents.
System types: high pressure (HP) and low pressure (LP)
A CO₂ feature not found in other agents is that it can be applied with two different storage architectures. Both can perform total flooding; the choice depends on total quantity, number of zones and space constraints.
| Criterion | High pressure (HP) | Low pressure (LP) |
|---|---|---|
| Storage | Steel cylinders at ambient temperature | Single insulated, refrigerated tank |
| Temperature / pressure | ≈58 bar at ≈20 °C | ≈20 bar at ≈−18…−20 °C |
| Typical scale | Small–medium quantity, single/few zones | Large quantity, multi-zone, central |
| Space / layout | Dedicated room for cylinder bank | Compact single tank, low footprint |
| Monitoring | Cylinder pressure/weight | Tank pressure + quantity + refrigeration continuously |
HP systems are stored at ambient temperature, so cylinder pressure varies significantly with temperature; they are practical for small and medium-scale, single- or few-zone applications. LP systems store large quantities economically in a single refrigerated tank, offering a central solution for multi-zone facilities and large volumes, but tank pressure, quantity and the refrigeration system must be continuously monitored. LP systems also have a pre-liquid vapour phase on first discharge, so discharge time and pipe hydraulics are calculated accordingly (below).
Design concentration and the KB (material) factor
Unlike clean agents, in CO₂ the design concentration is not an agent constant; it depends on the protected material/hazard and is expressed by a KB (material) factor. KB 1.0 is the baseline (34%) and rises with hazard. ISO 6183 and NFPA 12 tabulate these material-based values with minimum hold times; representative values:
| Hazard / material | Design concentration | KB factor |
|---|---|---|
| Acetone, butane, diesel, methane (surface/flammable liquid — baseline) | 34% | 1.0 |
| Electrical / switch rooms | ≈40% | 1.2 |
| Cable galleries, IT/EDP machine rooms | ≈47% | 1.5 |
| Generator, oil-filled transformer (deep-seated) | ≈57–58% | 2.0 |
| Data handling / EDP deep-seated load | ≈61–62% | 2.25 |
| Hydrogen, high-risk dust | 75% | 3.3 |
The distinction between a surface fire and a deep-seated fire is decisive in CO₂: surface fires need only a rapid discharge, whereas deep-seated fires that have penetrated the material mass require both a higher concentration and a long hold time — because extinguishment requires the material to cool below its re-ignition temperature.
Quantity calculation: ISO 6183 and NFPA 12 formulas
In Turkey CO₂ total flooding design is based on TS ISO 6183 / NFPA 12. There are two main approaches:
1) ISO 6183 / VdS 2093 (formula with surface term):
Q = KB × (0.7 × VR + 0.2 × AR)
- VR = corrected calculation volume = V + 4·VZ − VG (V = gross volume, VZ = ventilation volume that cannot be isolated, VG = deductible permanent structure volume)
- AR = surface term = A + 30·A0 (A = enclosure surface area m², A0 = area of unclosable openings m²)
- KB = material factor (table above)
- (VdS 2093 uses the same formula with 0.75 for the volume term: Q = KB × (0.75·VR + 0.2·AR).)
2) NFPA 12 (conversion and volume-factor approach):
Q = CF × VF × V
- VF (volume factor) varies with the size of the space (less gas per unit for large volumes).
- CF (material conversion factor) is applied for design concentrations above 34%.
- NFPA 12 also adds extra material mass for unclosable openings.
Additional agent for unclosable openings (surface term)
Total flooding essentially requires an enclosed volume; dampers are closed and doors kept shut before discharge. In practice, though, there may be unclosable openings (process ducts, permanently open passages). This is where CO₂’s strength comes in: the AR = A + 30·A0 term in the ISO 6183 formula counts each square metre of unclosable opening at 30 times the weight, directly producing the additional CO₂ quantity to compensate the escaping gas. Total flooding can thus be applied even with limited unclosable openings, via the additional-agent calculation. But if the openings are large or at floor level (heavy CO₂ flows out fast), total flooding may become invalid; in that case switch to a local application solution.
Opening compensation or local application? — decision logic in brief
The method is decided by whether a space can hold the design concentration for the required hold time. Because CO₂ is heavier than air, it sinks to the floor and drains fast through low openings; the decision follows from this physics:
- Can the openings be closed? → Yes: close them on discharge with dampers/automatic flaps and apply normal total flooding (A0 → 0).
- Unclosable but small and high up? → Yes: solve with total flooding + the 30·A0 surface-term compensation; since heavy CO₂ sinks, escape through a high opening is limited.
- Large opening, floor-level, or an open space? → concentration cannot be held → local application (ISO 6183 / NFPA 12 rate-by-area or rate-by-volume method).
So “opening compensation” is a fine adjustment that offsets small leaks; once the opening reaches a critical size or floor level, it is not the correction but the method that changes. The critical size is not a fixed number; it is a function of (opening ratio × location/height × required hold time), and the final call is made by a door-fan (retention) test — if the calculated retention time is shorter than the required hold time, total flooding is invalid regardless of the nominal opening area.
How to solve a small but floor-level opening
With a small but near-floor opening the problem is not size but the continuous drainage of heavy CO₂ — a rate-of-loss problem, not a volume leak; the one-time
30·A0 correction does not safely represent it. Solution order:- 1) Close it on discharge (best solution): fit a discharge-actuated damper / gravity self-closing flap so A0 → 0 and the opening drops out of the calculation. This is the standard preferred method for low-level openings.
- 2) If it can’t be closed, keep the gas “above the pool”: permanently seal penetrations (cable/pipe entries, drains) with fire-rated mastic/mortar; add a threshold/kerb (dam) in front of the opening so the CO₂ layer stays below it; raise the opening if possible.
- 3) If the leak is unavoidable and continuous, use makeup (extended) discharge: instead of the one-time
30·A0addition, design an additional/extended CO₂ flow that replaces the loss over the hold time (NFPA 12: continuous feed for openings below the concentration level). - 4) If it still can’t be held: if the door-fan/retention test does not deliver the required hold time, the space is no longer an enclosure → switch to a local application protecting the object directly.
Special note: for deep-seated fires (generator, transformer, cable) a long hold time is needed, so a low opening is critical; here the opening must be eliminated via option 1 or 2 — managing it with makeup is usually neither sufficient nor economical.
Worked example
A generator room with a 100 m² floor area and 3.0 m height (gross V = 300 m³); enclosure surface area A ≈ 2×(10×10) + 4×(10×3) ≈ 320 m²; an unclosable exhaust opening A0 = 0.5 m²; isolatable ventilation (VZ ≈ 0), deductible structure (VG ≈ 0). The generator is deep-seated, so KB = 2.0 (≈57%):
- VR = 300 + 0 − 0 = 300 m³
- AR = 320 + 30×0.5 = 335 m²
- Q = 2.0 × (0.7×300 + 0.2×335) = 2.0 × (210 + 67) = ≈554 kg CO₂
The additional contribution of the 0.5 m² unclosable opening: 2.0 × 0.2 × (30×0.5) = ≈6 kg — even a small opening shows up in the calculation. The quantity is distributed across the cylinder bank in HP or the tank capacity in LP, plus the reserve and supplementary quantities below.
Discharge and hold times
ISO 6183 Table 6 defines the discharge times for total flooding:
- HP total flooding (surface): the effective CO₂ quantity discharges within 60 seconds.
- LP total flooding: on first discharge the pre-liquid vapour flow is at most 60 seconds, and total discharge at most 120 seconds (to bring liquid from the LP tank to the nozzles).
- Deep-seated fire: after reaching design concentration the hold time is critical — the material must cool below its re-ignition temperature. Representative ISO 6183 hold times: cable rooms ≈10 min, data handling ≈20 min, generators “until stopped/cooled”. Hold time depends on enclosure integrity and is verified by a door-fan test.
Reserve and supplementary quantities
- Supplementary quantity (LP): in LP systems a supplementary quantity of typically ≈10% is provided for fill and drain tolerances.
- Reserve quantity: depends on the number of protected zones — few zones usually need no reserve, whereas many zones can require 100% (full) or even 200% (double) reserve.
- Multi-zone system: the main quantity is sized for the single largest zone; selective valves discharge only to the zone on fire.
System equipment, monitoring and damper automation
The basic logic of contact-output manometers/pressure switches, ventilation/damper automation and the correct discharge sequence is common to all gas systems; for details see the shared equipment and automation section of Gas Suppression: Design and Agent Selection. CO₂-specific points:
- LP tank monitoring: tank pressure, quantity and the refrigeration system are continuously monitored; low-level/high-temperature alarms are mandatory.
- Life-safety chain: time delay → pre-discharge audible-visual alarm → HVAC shutdown and damper closure → discharge; door abort/lock-off and post-discharge purge.
- Discharge sequence: dampers are closed before discharge and kept closed throughout the hold.
- In IT/cable spaces galvanised pipework is preferred.
Design and application considerations
- Not an occupied space: CO₂ total flooding is only for normally unoccupied technical spaces; life-safety measures (delay, alarm, lock-off, purge) are mandatory.
- System-type choice: small/few zones → HP; large quantity/many zones → LP. Continuous monitoring is essential in LP.
- Correct KB: surface or deep-seated? The right design concentration and hold time are chosen by material.
- Unclosable openings: additional agent is calculated via the surface term (AR = A + 30·A0); large/floor openings invalidate total flooding → local application.
- Discharge time: HP ≤60 s; LP vapour ≤60 s / total ≤120 s.
- Hold and integrity: for deep-seated fires the hold time is verified by a door-fan test.
- Reserve/supplementary: ≈10% supplementary in LP; reserve by number of zones.
- Altitude correction: applied above ≈1,000 m (agent-independent); for the shared rules see our pillar article.
- Periodic maintenance: cylinder/tank weighing and pressure checks are done regularly; see our periodic maintenance of fire systems article.
Summary
CO₂ total flooding is a powerful volume-protection method, superior for deep-seated fires; but unlike the inert gases it is lethal, so it is used only in unoccupied technical spaces and with full life-safety measures. The backbone of the design is three decisions: choosing the right system type (HP/LP), determining the material-appropriate design concentration/KB factor, and correctly computing the quantity with the ISO 6183 / NFPA 12 formulas — including the additional agent in the surface term for unclosable openings. For applications requiring object/device protection, see our CO₂ Local Application (Object Protection) article; for agent selection, see Gas Suppression: Design and Agent Selection. For a CO₂ design and volume calculation specific to your facility, contact us for a free site survey within Ankara.
This content is for information only. Binding calculations and project design specific to your facility require project-based work with a fire engineer under TS ISO 6183 / NFPA 12 and the relevant nozzle approval listing.
© 2026 A-Pro Mühendislik. This is original technical content produced by A-Pro Engineering; all rights reserved. Copying, reproducing or republishing it in whole or in part without attribution and permission is prohibited.
Frequently asked questions
Why is CO₂ total flooding not used in occupied spaces?+
CO₂ extinguishes fire by smothering oxygen; total flooding design concentrations are 34–75%, reducing ambient oxygen from 21% to about 5–14%. These levels are lethal to humans — unlike the inert gases (IG-541/IG-01), CO₂ has no safe human margin. Even the lowest design concentration of 34% is far above life-safety thresholds. CO₂ is therefore suitable only for normally unoccupied technical spaces (generator, turbine, transformer, flammable-liquid store, cable gallery) and requires time delay, pre-discharge audible-visual alarm, lock-off and egress measures.
What is the difference between high-pressure (HP) and low-pressure (LP) CO₂ systems?+
In high pressure (HP), CO₂ is stored in steel cylinders at ambient temperature; at 20 °C the cylinder pressure is ≈58 bar and varies significantly with temperature. It suits compact installations and small-to-medium quantities. In low pressure (LP), CO₂ is held in a single insulated, refrigerated tank (≈−18 to −20 °C, ≈20 bar); it is economical for large quantities, multi-zone facilities and central solutions. Both types can perform total flooding; the choice depends on required quantity, number of zones and space constraints. LP systems must be continuously monitored for tank pressure, quantity and temperature.
How is the CO₂ total flooding design concentration determined?+
In CO₂ the design concentration is not an agent constant but depends on the protected material, expressed by a KB (material) factor. The baseline for most surface fires and flammable liquids is 34% (KB 1.0). Electrical/switch rooms are ≈40% (KB 1.2), cable/IT spaces ≈47% (KB 1.5), generators and oil-filled transformers ≈57–58% (KB 2.0), data handling / deep-seated loads ≈61–62% (KB 2.25), and high-risk environments such as hydrogen up to 75% (KB 3.3). ISO 6183 and NFPA 12 tabulate these material-based values and their minimum hold times.
How is additional CO₂ calculated for unclosable openings?+
Total flooding needs an enclosed volume; where openings cannot be closed, additional CO₂ is calculated to compensate for the gas that escapes. The ISO 6183 / VdS 2093 formula does this with a surface term: in Q = KB × (0.7·VR + 0.2·AR), AR = A + 30·A0, where A is the enclosure surface area and A0 is the area of unclosable openings. Thus each square metre of unclosable opening is counted as additional agent. NFPA 12 adds extra material mass for unclosable openings; large or floor-level openings can invalidate total flooding altogether and require local application.
When is opening compensation used, and when is local application preferred?+
The decisive question is whether the space can hold the design concentration for the required hold time. (1) If the openings can be closed → close them on discharge with dampers/automatic flaps and apply normal total flooding (A0 → 0). (2) If unclosable but small and high up → solve with total flooding + the 30·A0 surface-term compensation; because CO₂ is heavier than air it sinks, so escape through a high opening is limited. (3) If the opening is large, at floor level, or the space is open → concentration cannot be held and you switch to local application. The critical size is not a fixed number; it is a function of (opening ratio × location/height × required hold time), and the final call is made by a door-fan (retention) test — if the calculated retention time is shorter than the required hold time, total flooding is invalid regardless of the nominal opening area.
How do you solve a small but floor-level opening?+
With a small but near-floor opening the problem is not size but the continuous drainage of heavy CO₂; the one-time 30·A0 correction does not safely represent it. Solution order: (1) The best solution is to close it on discharge — fit a discharge-actuated damper / gravity self-closing flap so A0 → 0. (2) If it cannot be closed, keep the gas above the pool: permanently seal penetrations (cable/pipe entries, drains) with fire-rated mastic/mortar, add a threshold/kerb (dam) in front of the opening, and raise the opening if possible. (3) If the leak is unavoidable and continuous, design an additional/extended (makeup) CO₂ flow that replaces the loss over the hold time. (4) If it still cannot be held, switch to local application. For deep-seated risks (generator, transformer, cable) a low opening is critical and the opening must usually be eliminated rather than managed with makeup.
What are the CO₂ total flooding discharge and hold times?+
Per ISO 6183 Table 6, for surface fires in total flooding the effective CO₂ quantity discharges within 60 seconds in HP systems; in LP systems the pre-liquid vapour flow is at most 60 seconds and total discharge at most 120 seconds. For deep-seated fires the hold time after reaching design concentration is critical, to let the material cool: ISO 6183 gives values such as ≈10 min for cable rooms, ≈20 min for data handling, and 'until stopped' for generators. Hold time is verified by a door-fan test and enclosure integrity.
Do CO₂ systems require reserve and supplementary quantities?+
Yes. In LP systems a supplementary quantity of typically ≈10% is provided for fill and drain tolerances. The reserve (backup) quantity depends on the number of protected zones: few zones usually need no reserve, whereas more than five zones can require a full reserve (100%) and many zones up to a double reserve (200%). In multi-zone systems the main quantity is sized for the single largest zone, and selective discharge valves direct gas only to the zone on fire.
What is the key difference between CO₂ total flooding and an inert gas (IG-541)?+
Both extinguish physically by diluting oxygen; but IG-541 is designed to keep oxygen within a human-safe margin (≈12%) and its 8% CO₂ even stimulates respiration — suitable for occupied spaces. CO₂ reduces oxygen to lethal levels (≈5–14%) and cannot be used in occupied spaces. In return, CO₂ is the only agent that can perform object-based local application on its own and is superior for deep-seated fires. See our Gas Suppression: Design and Agent Selection article for the agent comparison.
Does altitude above sea level affect the CO₂ quantity?+
Yes; the atmospheric/altitude correction applies to all gas systems including CO₂ and is agent-independent: above roughly 1,000 m the ambient pressure drops significantly, so the required agent quantity is scaled by a correction factor. Up to ≈1,000 m the factor is 1.00. For the shared calculation rules and the factor rationale, see the relevant section of our pillar article.
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