Chlorination or thermal disinfection: which should you use?
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Key points
- Chlorination treats hot and cold alike; thermal disinfection can only treat the hot side, because you cannot heat the cold system.
- Thermal disinfection avoids chemicals entirely but creates a genuine scald hazard and depends on the calorifier reaching and holding temperature.
- Chlorination involves COSHH-controlled chemicals, material-damage risk, and trade-effluent rules on the flush water.
- Both struggle with established biofilm, and neither fixes the underlying fault that let legionella establish.
- Both are remediation, not ongoing control. The temperature regime and the written scheme are what keep the system controlled afterwards.
Chlorination disinfects a water system chemically, by holding a high free-chlorine dose in the pipework for a set contact time. Thermal disinfection does it with heat, by raising the stored hot water to at least 60°C and drawing it through every outlet. They are the two standard ways of disinfecting a system, and the right choice depends on which side of the system is affected, what the plant can achieve, and what the building can tolerate.
This page sets them side by side as a decision aid. The methods themselves are covered in more depth in chlorination of water systems and the wider options in legionella treatment. The first thing to hold onto is that both are remediation: they clean a system on a particular day, usually after a positive sample, after plumbing work, or when bringing a shut-down system back into use. Neither replaces the day-to-day control regime.
The two methods compared
| Chlorination (chemical) | Thermal disinfection (heat) | |
|---|---|---|
| How it works | System dosed to a high free-chlorine concentration, around 50 ppm, held for at least an hour, then flushed clear | Stored hot water raised to at least 60°C and drawn through every outlet until each reaches temperature |
| Reaches the cold side? | Yes: the disinfectant travels with the water to hot and cold pipework alike | No: only the hot-water system can be heated |
| Scald risk during the work | Low; the water is not unusually hot | High: outlets run at 60°C or above, a real burns hazard in occupied buildings |
| Chemicals and COSHH | Yes: a hazardous oxidiser, dosed and measured under COSHH controls | No chemicals involved |
| Effect on materials | Can pit some stainless steels and degrade rubber seals and washers, especially if repeated | None beyond normal thermal stress, though very old plant may struggle to reach temperature |
| Disposal | Flush water must be neutralised; discharge may need trade-effluent consent | Hot water goes to drain as normal |
| Against established biofilm | Weak: chlorine kills planktonic bacteria fast but penetrates biofilm poorly | Also limited: heat reaches the water, less reliably the sheltered film on the pipe wall |
| Depends on | Accurate dosing and residual measurement | The calorifier and circulation actually achieving temperature at every branch |
When each method wins
Choose by the fault, not by habit. If the problem is in the cold-water system, for example a storage tank that has warmed and fouled, thermal disinfection is not an option at all: chlorination is the only one of the two that can reach it. If the building is occupied by people at scald risk, care settings above all, running every outlet at 60°C or more is a serious undertaking, and chlorination avoids it. If the calorifier is old or undersized and cannot reliably hold disinfection temperature, a heat treatment is undermined before it starts, and again the chemical route remains.
Thermal disinfection comes into its own where the hot side is the problem and the plant is sound. It needs no chemical handling, no COSHH assessment for a dosing operation, no neutralisation, and no trade-effluent question. For a straightforward hot-water system with a healthy calorifier, it is often the simpler, cheaper, and gentler operation, and it can be repeated without the cumulative material damage that repeated chlorination can cause.
The cold side settles the argument. If the problem is in cold-water pipework or storage, only chlorination can reach it. If it is confined to a healthy hot system, heat is usually the simpler job.
What neither method does
Both methods share the same two blind spots, and honest guidance has to name them. The first is biofilm. Legionella living inside an established slime layer on the pipe wall is sheltered from both chlorine and heat, so a disinfection can clear the running water while leaving the reservoir in place, ready to reseed the system. That is why disinfection is paired with physical cleaning, descaling, and removal of dead legs rather than relied on alone, and why persistent positives point to a design fault rather than another dose.
The second is the future. A disinfection is a snapshot intervention. If the system returns to water held in the 20 to 45°C growth range, or to branches that stagnate between uses, the bacteria return with the conditions. The ongoing controls are unglamorous: hot stored at 60°C and delivered at 50°C or more within a minute, cold below 20°C within two, weekly flushing of little-used outlets, all recorded. Check your own figures against those targets with the temperature checker.
Free legionella risk assessment template
A structured Word document following the five-step approach in ACOP L8. Covers risk identification, written scheme, monitoring, and records. If it isn't written down, you can't evidence it.
Follows ACoP L8 and HSG274 Part 2. Free. No spam.
Whichever you choose: confirmation and records
A disinfection certificate records that a procedure was carried out to a specified dose or temperature for a specified time. It does not evidence that the water is clear. Only post-disinfection sampling does that, which is why sampling follows a proper disinfection as surely as the flush follows the dose. The certificate, the sample results, and the record of any underlying fault you fixed all belong in your records, kept for at least five years, and the whole event should be visible in your written scheme of control as an action taken, not hidden as an awkward exception.
Free legionella risk assessment template
A structured Word document following the five-step approach in ACOP L8. Covers risk identification, written scheme, monitoring, and records. If it isn't written down, you can't evidence it.
Follows ACoP L8 and HSG274 Part 2. Free. No spam.
Frequently asked questions
Which is better, chlorination or thermal disinfection?
Neither is universally better; the system decides. Chlorination reaches hot and cold pipework equally and avoids the scald risk of running outlets at very high temperature, so it suits cold-water problems and occupied buildings. Thermal disinfection needs no chemicals, no trade-effluent handling, and no risk to materials, so it suits hot-water systems whose calorifier can reliably reach and hold disinfection temperature. Both are weaker against established biofilm than people expect, and both need the underlying fault fixed or the problem returns.
What temperature is needed for thermal disinfection?
Thermal disinfection works by raising the stored hot water to at least 60°C and drawing that water through every part of the hot system, holding each outlet at temperature long enough to kill legionella throughout the pipework. It is a demanding operation: every branch and outlet has to reach temperature, and water above 60°C at outlets is a serious scald hazard, so the building's occupants need protecting while it is carried out. It only treats the hot side; cold-water pipework cannot be disinfected this way.
Does disinfection remove the need for temperature control?
No. Disinfection, whether chemical or thermal, cleans the system on the day it is done. It is a remediation step, not an ongoing control. The everyday defence remains the temperature regime: hot water stored at 60°C and delivered at at least 50°C at outlets, cold below 20°C, with little-used outlets flushed weekly. A disinfection followed by a return to tepid, stagnant conditions is a temporary result, which is why the written scheme of control, not the disinfection certificate, is the backbone of legionella management.
Can I disinfect my own water system?
Thermal disinfection of a simple hot-water system is sometimes within the reach of a competent maintenance person, but it carries a real scald risk and every outlet must reach temperature. Chlorination is firmly a contractor job: it involves dosing a hazardous oxidising chemical under COSHH, measuring free chlorine residual through the contact period, and neutralising the flush water before lawful discharge. Either way, the work should end with confirmation sampling, because the certificate records the procedure, not the microbiological result.
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