CO2 Fire Suppression Design to NFPA 12: Surface vs Deep-Seated Fires

A CO2 flooding system calculation under NFPA 12 starts with the type of fire. For surface fires (flammable liquids and gases), the design concentration is never below 34%, and you multiply the room volume by a flooding factor from Table 5.3.3(b). For example, a room between 127.36 and 1,415 m³ needs 0.80 kg of CO2 per m³. For deep-seated fires (electrical equipment, paper, dust), use Table 5.4.2.1, with design concentrations of 50% to 75% and higher factors.

After the base quantity, you add CO2 for openings, ventilation and temperature extremes. Then you design the life safety measures, because CO2 is lethal at extinguishing concentrations. This guide covers each step with a worked example. You can check your numbers in the free FM-200, Novec 1230 and CO2 calculator.

Surface Fires: Volume Flooding Factors

CO2 puts out fire mainly by reducing oxygen in the room (smothering). For surface fires, NFPA 12 gives flooding factors that already account for normal leakage in a closed room. Small rooms have more leakage per cubic metre, so their factor is higher.

Multiply the room volume by the factor. If the result is less than the minimum quantity for that band, use the minimum. These factors are based on the 34% design concentration.

Material Conversion Factor (MCF)

Some fuels need more than 34% CO2. In that case you multiply the base quantity by a material conversion factor read from Fig. 5.3.4.

Where a room has more than one fuel, use the fuel with the highest design concentration. Site tip: ask the client for the real list of stored materials. A generator room with a hydrogen battery area is not the same hazard as a plain diesel room.

Openings, Ventilation and Temperature

The flooding factors do not cover large openings, running fans or extreme temperatures. Add CO2 for each one.

• Unclosable openings: add enough CO2 to replace the loss through the opening for 1 minute. A common preliminary manufacturer practice is 5 kg per m² of opening × MCF. Confirm the final value with the listed manufacturer.
• Forced ventilation that is not shut down: add the volume of air moved in 1 minute × flooding factor × MCF. It is better to shut the fans and dampers down on discharge.
• High temperature: add 1% of CO2 for every 2.8 °C above 93 °C.
• Low temperature: add 1% of CO2 for every 0.56 °C below −18 °C.

On site, walk the room with the HVAC drawings. Look for transfer grilles, louvres and cable openings that stay open. Every opening you close is CO2 you do not need to buy.

Worked Example: 192 m³ Surface Fire Room

The room is 8 × 6 × 4 m. It is a surface fire hazard at 34%. All openings close on discharge and the ventilation shuts down.

For surface fires, the design concentration must be reached within 1 minute. Pipe sizes and nozzle selection follow the listed manufacturer's flow calculation.

Deep-Seated Fires: Table 5.4.2.1

Deep-seated fires burn inside the material, like smouldering cables or packed paper. They need higher concentrations and a long hold so the fire does not restart.

For deep-seated fires:

• Reach the design concentration within 7 minutes.
• Reach 30% within the first 2 minutes.
• Hold the concentration for at least 20 minutes.
• Add CO2 for leakage and openings separately. The table factors include no leakage allowance.

The method is different from halocarbon agents. If you also design FM-200 systems, watch How to Calculate FM200 System Gas Quantity to compare the two approaches.

Life Safety: CO2 Can Kill

CO2 is lethal at extinguishing concentrations. This is the main difference from clean agents like HFC-227ea and FK-5-1-12, which work mainly by absorbing heat. Treat every CO2 system as a life safety system first.

• Avoid CO2 total flooding in normally occupied spaces.
• Fit supervised lockout valves, and lock out the system whenever people enter for work.
• Provide pneumatic pre-discharge alarms and time delays, so people can leave before discharge.
• Post warning signs at every entrance and inside the room.
• Train staff and contractors. Many CO2 deaths happen during maintenance, not during fires.

Low areas next to the protected room, such as pits and basements, can also collect CO2 after a discharge. Check them in your risk assessment.

CO2 Suppression FAQ

What is the minimum CO2 design concentration for surface fires?

NFPA 12 never allows less than 34% CO2 for surface fires. Fuels that need more, such as ethylene at 49% or hydrogen at 75%, use a material conversion factor.

How do I calculate CO2 quantity for a room?

Multiply the room volume by the flooding factor for its volume band in Table 5.3.3(b). Check the minimum quantity, apply the MCF if needed, then add CO2 for openings, ventilation and temperature.

What is the difference between surface and deep-seated fires in CO2 design?

Surface fires need 34% or more within 1 minute. Deep-seated fires need 50% to 75%, reached within 7 minutes, with 30% within 2 minutes and a hold of at least 20 minutes.

How much CO2 do I need for a 192 m³ room?

For a surface fire at 34%, the flooding factor is 0.80 kg/m³, so the quantity is 153.6 kg. This can be supplied with 4 × 45 kg cylinders.

Can CO2 systems be used in occupied rooms?

CO2 is lethal at extinguishing concentrations, so avoid it in normally occupied spaces. Where it is used, provide supervised lockout valves, pneumatic pre-discharge alarms, time delays and warning signs.

Related reading: Anti-Vortex Plate in Fire Pump Suction Tanks: What It Does

Sources: NFPA 2001, Standard on Clean Agent Fire Extinguishing Systems and NFPA 12, Standard on Carbon Dioxide Extinguishing Systems. Values are from NFPA 2001 (2018 edition) and NFPA 12. Always check the edition adopted by your Authority Having Jurisdiction and the listed manufacturer's design manual.