Combi Boilers and Legionella: What Dutyholders Need to Know
A combi boiler lowers the Legionella risk but does not remove it, and it does not remove the duty to assess it. Because it heats water on demand, there is no large hot water cylinder in which water can sit warm for days. The risk moves to the pipework, the cold water side and the outlets, and that is where a competent assessment looks.
This guide is for landlords, facilities managers, estate managers and compliance managers who have combi boilers in flats, schools, surgeries, offices or small commercial units. It explains what the combi changes, what it leaves in place, and what evidence you should be able to show.
TL;DR
A combi boiler is an instantaneous heater with no stored hot water, so it does not create the same growth conditions as a storage cylinder or calorifier.
Legionella multiplies between 20°C and 45°C, so long pipe runs, dead legs and little-used outlets can still be a problem.
A combi does not exempt you from a risk assessment under UK health and safety law. The duty applies whatever heats the water.
Keep records: temperature checks, flushing of unused outlets, and maintenance of any thermostatic mixing valves (TMVs).
Do not assume a setpoint on the boiler equals a safe temperature at the tap. Measure at the outlet.
Table of Contents
Why a combi boiler changes the risk
Legionella bacteria are found commonly in water. They multiply where temperatures are between 20°C and 45°C and nutrients are available, stay dormant below 20°C and do not survive above 60°C. A traditional system with a hot water cylinder holds a large volume of water for hours or days. If the bottom of that cylinder stratifies and cools, you have a warm, nutrient-rich environment.
A combi works differently. The HSG274 Part 2 guidance describes combination boilers and instantaneous water heaters as providing hot water directly from the cold water supply as it passes through the heater. Nothing is stored, so the main storage-related risk factor is largely absent.
That is why combi systems are often described as lower risk. It is a fair description, as long as you read it as “lower” and not “none”.
Where the risk remains
The HSE’s guidance for landlords is clear that all man-made hot and cold water systems can provide an environment where Legionella bacteria grow. A combi still sits in the middle of such a system. Four weak points deserve attention.
Long pipe runs and dead legs
A combi heats water at the boiler. If the nearest tap is a few metres away, the water arrives hot. If a bathroom at the far end of a school wing, or a kitchenette on another floor, is fed by a long run, the water can cool in the pipe between draws. The research behind the HSE guidance points to long pipework runs, dead legs and infrequent use as the conditions that let water drift into the 20°C to 45°C range before it reaches the outlet.
A dead leg is a section of pipe that no longer serves an outlet, or one that is rarely used. Water in it stops moving and warms to room temperature. Alterations over the years, such as removed basins or capped spurs, are a common cause.
The cold water side
A combi takes its water from the mains, but many commercial buildings still have cold water storage tanks feeding other outlets. Cold water should be stored and distributed below 20°C, according to HSE guidance on hot and cold water systems. A warm plant room or a tank near a heat source can undo that. Where a tank is old, contaminated or poorly covered, cleaning, disinfection or replacement may be the right remedial action.
Infrequently used outlets
Showers in a closed changing room, a sink in a spare consulting room, a hand basin in a vacant flat: if no one uses them, the water in the branch stagnates. The risk is not the boiler, it is the standing water. HSE guidance for health services notes that little-used outlets should be flushed on a weekly basis to prevent stagnation.
Showers and aerosols
Legionnaires’ disease is a potentially fatal form of pneumonia caused by inhaling small droplets of contaminated water. Showers and spray taps generate those droplets. Even with a combi, a shower head with scale or biofilm, fed by water that has sat in the pipe, is a plausible route.
Temperatures: what the numbers mean for a combi
The traditional control strategy is temperature. The HSE sets the benchmark that hot water should be stored at 60°C or above, reach outlets at 50°C or above within one minute, and cold water should stay below 20°C.
For a combi there is no storage temperature to log, so the figures that matter are at the outlet.
Point in the system | Target | What to check on a combi |
Cold water at outlets | Below 20°C | Run the tap for up to two minutes, then measure. Warm cold water suggests heat gain from pipework or plant. |
Hot water at outlets | 50°C or above within one minute | Time and measure at the nearest and furthest outlets. |
Mixed water at TMVs | Set for scald protection (see below) | Check the valve is serviced and that supply temperatures are correct. |
Little-used outlets | Flushed weekly | Keep a flushing log with dates and initials. |
A boiler set to a high temperature does not guarantee the outlet temperature. A recent industry article on killing temperatures is not needed to see the logic: poor insulation and low use let sections of pipework sit in the growth range, whatever the boiler says. Measure at the tap.
If you want the numbers set out for dutyholders, our own note on HSG274 temperature limits covers 60°C storage and 50°C outlets.
Do you need a risk assessment if you only have a combi?
Yes. The HSE states that landlords and employers must assess and control the risk whatever the type of system, and a combi does not exempt anyone. What changes is the scale. For a single flat with a combi, regular use and short pipes, the assessment can be brief and the control measures simple. For a school, surgery, care setting or multi-storey block with a combi in each unit but shared cold water tanks, the assessment will be broader.
A competent assessment should record:
the type and age of the heater and where it sits;
the pipework layout, including long runs and dead legs;
any cold water storage and its condition;
outlets that are rarely used, and how they are flushed;
showers and other aerosol sources;
TMVs and who services them;
the monitoring and record-keeping in place.
An assessment is not a one-off. The HSE and others stress that risk management is an ongoing process. If the building changes, usage changes or a test result comes back poorly, review it. We explain how in reviewing a Legionella risk assessment.
Monitoring and flushing in practice
For commercial and public buildings, the HSE’s health services guidance points to sentinel outlets, the closest and furthest from the heat source, for routine temperature monitoring. On a combi system, the furthest outlet matters most, because it shows how far the water cools on its way.
A workable routine:
Identify the nearest and furthest hot outlets from each combi, and the coldest-risk cold outlets.
Measure hot and cold temperatures at those points on a schedule set in your risk assessment.
Flush unused outlets weekly, and record it.
Escalate any reading outside the target to the responsible person, and note the corrective action.
Keep the logs where an auditor can find them.
Regulators expect evidence, not intentions. The Approved Code of Practice L8 context is that temperature logs, maintenance records and flushing documentation show the duty of care is met continuously. A policy without records is weak.
Manual checks work for a handful of outlets. For larger estates, automated temperature monitoring removes gaps between visits. Our guides on why temperature monitoring matters and how to complete the checks set out the routine.
Combi boilers, TMVs and scalding
Controlling Legionella wants hot water above 50°C. Protecting vulnerable people from scalding wants it much lower at the point of use. The HSE’s scalding guidance for health and social care describes how higher water temperatures are used partly to control legionella, which is why the conflict arises.
The usual answer in schools, surgeries, care homes and healthcare premises is a thermostatic mixing valve fitted as close to the outlet as possible. The NHS’s Health Technical Memorandum 04-01 covers this, including that TMVs should sit close to the outlet and not be installed in series with other mixing taps.
TMVs bring their own risk. Mixed water at around 38°C to 41°C is in the growth range, and a valve that is neglected or a spur that is too long can become a stagnation zone. So a TMV needs servicing, a record and a place in the risk assessment. If you have a combi feeding TMV-protected outlets, treat the valves as part of the Legionella control scheme, not a separate safety fitting. We offer TMV servicing and remedial works for sites that need it.
Switching off and saving energy
A common belief is that hot water must run 24 hours a day for Legionella control. GOV.UK energy efficiency guidance for schools and colleges says you do not need to run hot water around the clock for legionella protection, and that systems can be switched off overnight, at weekends and in holidays, provided they are reheated to a safe temperature before use.
That suits a combi well, since it heats on demand anyway. The catch is the pipework: after a holiday, flush every outlet before reopening, and check temperatures at the sentinel points.
When temperature control is not enough
Sometimes holding 50°C at every outlet is impractical, or creates a scalding risk you cannot manage. Then HSG274 Part 2 allows alternative biocidal treatment such as chlorine dioxide as part of a formal water safety programme, managed by competent persons and monitored. Total oxidant levels at drinking outlets should stay below 0.5 mg/l. It is not a shortcut. Dosing needs monitoring and records. See our note on chlorine dioxide dosing for UK facilities.
Where a system has already shown contamination, cleaning and disinfection of tanks and pipework comes first. Our system disinfection and flushing service covers tank cleaning, replacement and repair, and ongoing maintenance contracts.
Our perspective: the combi is a starting point, not a conclusion
We see the same pattern on many sites. A building has combi boilers, the responsible person hears that combis are low risk, and the assessment is skipped or left to age. Then a refurbishment strands a dead leg, a wing closes for the summer, or a shared cold tank warms in a plant room, and nobody has a record of any of it.
Our view is simple. A combi lowers one risk and leaves the others. The useful question is not “is this a combi?” but “where can water sit between 20°C and 45°C in this building, and how would we know?” The answer comes from walking the system, measuring at outlets, and writing down what you find.
We build assessments, logbooks and monitoring around the site, not around a template. For a small combi-only unit that can mean a short, proportionate report. For a school or a surgery with TMVs and shared tanks, it means a fuller control programme, training for the people who do the checks, and sampling where the risk assessment calls for it. The goal is compliance that is simple to run and easy to evidence.
— Sammi
Common mistakes with combi systems
Treating the boiler setpoint as the outlet temperature. Measure at the tap.
Ignoring cold water. Warm cold water is a growth risk.
Leaving vacant units unflushed. Empty flats and spare rooms stagnate quickly.
Forgetting TMVs. Valves need servicing and records.
No review. After works or a change of use, the assessment is out of date.
No paperwork. If it is not recorded, it cannot be shown.
How we can help
If you manage sites with combi boilers and are not sure your assessment reflects the pipework, start with a Legionella risk assessment, which starts from £185. Where the assessment calls for sampling, Legionella water testing and analysis starts from £55 per sample. We also provide Legionella awareness training, on site or online, for the staff who carry out the checks.
FAQ
Do combi boilers get Legionella?
The boiler itself is unlikely to be the source because it stores no hot water. But the pipework, cold water tanks, showers and little-used outlets connected to it can still harbour Legionella if water sits between 20°C and 45°C. The system, not the boiler, is what you assess.
Is a combi boiler safer than a hot water cylinder for Legionella?
Generally it presents lower risk, because there is no large volume of stored water. It still needs a risk assessment, and the benefit depends on short pipe runs, regular use and good cold water control.
Do landlords need a Legionella risk assessment for a combi?
Yes. HSE guidance says landlords must assess and control the risk, whatever heats the water. For a simple property the assessment may be short and the controls few, such as flushing unused outlets and descaling shower heads.
Can I switch off a combi boiler to save energy?
Yes, in most cases. GOV.UK guidance says hot water does not need to run 24/7 for Legionella protection, as long as the system reheats to a safe temperature before use. After a long closure, flush all outlets before people use them.
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