TL;DR
CO2 extinguishers are a well-established choice for electrical fires and server environments, leaving no residue and posing no damage risk to sensitive equipment. Water mist extinguishers are a more versatile, modern alternative that can tackle multiple fire classes, including Class A, B, C, and even some electrical risks, making them increasingly popular in mixed-use and residential settings. The right choice depends on your building type, the fire risks present, and your compliance obligations under UK fire safety legislation.
Key Takeaways
- CO2 extinguishers work by displacing oxygen and are rated for Class B and electrical fires, while water mist extinguishers use fine droplets to cool and suppress fires across a broader range of classes.
- Water mist extinguishers are tested and rated under BS EN 3 and can carry multiple fire class ratings, making them suitable for a wider variety of premises.
- CO2 extinguishers pose an asphyxiation risk in confined spaces and must be used with caution, particularly in small rooms or poorly ventilated areas.
- Water mist extinguishers typically cost more to purchase than CO2 models, but their versatility can reduce the number of different extinguisher types required on a premises.
- Both types require annual servicing and maintenance by a competent person, in line with BS 5306-3 and the Regulatory Reform (Fire Safety) Order 2005.
Contents
What is a CO2 Extinguisher?
A CO2 (carbon dioxide) extinguisher works by flooding the area around a fire with carbon dioxide gas, which displaces the oxygen that flames need to sustain combustion. Because the agent is a gas, it evaporates completely and leaves no residue, making it particularly suitable for use around electrical equipment, servers, and data centres where water or powder would cause significant secondary damage. CO2 extinguishers are rated for Class B fires (flammable liquids) and are widely used for electrical hazards, though they do not carry a formal electrical fire class rating under BS EN 3.
In London’s commercial sector, CO2 extinguishers are a common sight in offices, control rooms, and retail environments with electrical infrastructure. They are available in a range of cylinder sizes, typically from 2 kg to 5 kg, and must be installed and maintained in accordance with BS 5306-3. One important limitation is the asphyxiation risk they present in enclosed or poorly ventilated spaces, so placement and user training are both critical considerations. If you are weighing up CO2 against other agent types, it is worth reviewing resources such as the CO2 Extinguisher vs Foam Extinguisher guide for additional context.
What is a Water Mist Extinguisher?
Water mist extinguishers are a newer technology that discharges a fine spray of microscopic water droplets at high pressure. These droplets cool the burning material, displace oxygen, and reduce radiant heat, achieving suppression without saturating the area with water. Because the droplets are so small, they are safe to use on electrical equipment up to a tested voltage threshold and can tackle Class A fires (solid combustibles), Class B fires (flammable liquids), Class C fires (gases in some models), and Class F cooking oil fires depending on the specific product rating.
Their versatility has made water mist extinguishers increasingly popular in environments such as care homes, hotels, hospitals, and residential blocks, where a single extinguisher type may cover a broader risk profile. They are also considered an environmentally friendly option, as they use only distilled water and contain no chemical agents. For premises managers considering water mist alongside other modern agents, the Dry Powder Extinguisher vs Water Mist Extinguisher comparison provides a useful additional reference. Always confirm the specific class ratings of any water mist model with a competent fire safety professional before purchasing.
Head-to-Head Comparison
| Feature | CO2 Extinguisher | Water Mist Extinguisher |
|---|---|---|
| Mechanism | Displaces oxygen with CO2 gas | Cools and suppresses with fine water droplets |
| Best suited for | Class B fires, electrical equipment, server rooms | Class A, B, C, and electrical risks (model dependent) |
| Cost range | Typically lower purchase cost | Typically higher purchase cost |
| Installation complexity | Straightforward; standard bracket mounting | Straightforward; some models are heavier |
| Maintenance | Annual inspection; periodic extended service required | Annual inspection; refill with distilled water |
| Compliance standards | BS EN 3, BS 5306-3, RRO 2005 | BS EN 3, BS 5306-3, RRO 2005 |
| Typical lifespan | Typically 10 years (with correct servicing) | Typically 10 years (with correct servicing) |
Costs and specifications vary. Contact London City Fire for a site-specific assessment.
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Which is Right for You?
If your premises contains significant electrical infrastructure, such as a server room, data centre, or office with a high density of computers and networking equipment, a CO2 extinguisher is often the go-to recommendation. It acts quickly, leaves no residue, and avoids the damage that water or powder agents would cause to sensitive electronics. However, its effective range is relatively short, and it should not be the only extinguisher type on a mixed-use floor where Class A fire risks are also present.
Water mist extinguishers offer a broader coverage of fire classes, which can be a practical advantage in settings such as care homes, hotels, or residential blocks where multiple risk types exist and simplicity of use for untrained occupants matters. Their environmental credentials and the absence of chemical agents also make them an attractive option for premises with sustainability policies. That said, their higher upfront cost and the need to verify specific class ratings for the model in question mean they are not automatically the right fit for every building. For premises that also involve cooking hazards, it is worth reviewing the Wet Chemical Extinguisher vs Water Mist Extinguisher comparison to understand where each type excels.
Ultimately, neither option is universally superior. The correct selection depends on a proper assessment of the fire risks in your building, the layout of the premises, the occupants present, and the requirements of your fire risk assessment under the Regulatory Reform (Fire Safety) Order 2005. A competent fire safety professional can help you determine the right combination of extinguisher types, placement, and quantities for your specific site.
Why Choose London City Fire?
London City Fire has been protecting commercial and residential properties across the capital for over 20 years. Our team includes IFSM-registered assessors who understand the full spectrum of fire safety compliance requirements, from fire risk assessments conducted under the Regulatory Reform (Fire Safety) Order 2005 and the Fire Safety Act 2021, through to the supply, installation, and annual servicing of fire extinguishers in line with BS 5306-3. We work with landlords, managing agents, property developers, and facilities managers across London, providing tailored advice rather than a one-size-fits-all approach.
When it comes to choosing between CO2 and water mist extinguishers, or any other combination of fire safety equipment, our team can assess your premises and provide clear, compliant recommendations backed by documentation suitable for audit and inspection. Whether you manage a single commercial office or a portfolio of mixed-use buildings, London City Fire offers the expertise and local knowledge to keep your property safe, secure, and audit-ready. Contact us today on 0204 572 0661 or email info@londoncityfire.co.uk to arrange a site assessment, or visit our Fire Extinguisher Quote page to get started.
Frequently Asked Questions
Q: “Which is cheaper, a CO2 extinguisher or a water mist extinguisher?”
A: CO2 extinguishers are typically less expensive to purchase than water mist models. However, water mist extinguishers may reduce the overall number of different extinguisher types needed on a premises, which can offset the higher unit cost. For an accurate comparison based on your specific requirements, it is best to request a site-specific quote from a competent fire safety supplier.
Q: “Do both CO2 and water mist extinguishers meet UK fire regulations?”
A: Yes, both types, when correctly specified and maintained, can meet the requirements of the Regulatory Reform (Fire Safety) Order 2005 and relevant British Standards including BS EN 3 and BS 5306-3. However, compliance depends on selecting the right type for the fire risks present, placing extinguishers correctly, and ensuring they are serviced annually by a competent person. Your fire risk assessment should determine which types are appropriate for your premises.
Q: “Can I use a water mist extinguisher on electrical fires?”
A: Many water mist extinguishers are independently tested and approved for use on electrical equipment up to a specified voltage, often 35 kV at one metre, though this varies by model. Always check the label and manufacturer data for the specific extinguisher you are considering. A CO2 extinguisher is the more traditional choice for electrical risks, but certain water mist models offer a viable alternative where versatility is a priority.
Q: “Which is better for a large commercial property in London?”
A: Large commercial properties in London often require a combination of extinguisher types rather than a single solution. CO2 extinguishers are well suited to server rooms and areas with concentrated electrical equipment, while water mist or other agent types may be more appropriate in common areas, kitchens, or storage spaces. A professional fire risk assessment is the most reliable way to determine the correct provision for your building.
Q: “Do I need a professional to decide between a CO2 and water mist extinguisher?”
A: While building owners and responsible persons can research extinguisher types independently, the final selection should be informed by a fire risk assessment carried out by a competent person, as required under the Regulatory Reform (Fire Safety) Order 2005. Getting professional advice ensures that the type, size, quantity, and placement of extinguishers are all appropriate for your specific fire risk profile and compliant with current UK standards.
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Disclaimer: The comparisons on this page are for general informational purposes only. The suitability of any fire safety equipment or system depends on the specific risks, layout, and occupancy of your property. Always consult a qualified fire safety engineer before selecting or replacing equipment. London City Fire recommends a site-specific fire risk assessment as the starting point for any fire safety decision.