Does a combi boiler need a legionella risk assessment?
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Key points
- Yes, the duty applies. The need for a risk assessment comes from the property and its use, not from the type of boiler.
- A combi boiler usually makes the assessment short and reassuring: no stored hot water means the biggest domestic risk factor is gone.
- A combi removes the storage problem, not every problem: dead legs, little-used outlets, showers, and warming cold pipes remain.
- The assessment should still be written down. "It's a combi, it's fine" said aloud is not a record; noted in an assessment, it is.
- Void periods still matter. Flush every outlet through to temperature before a new tenancy starts.
Yes, you still need a legionella risk assessment, because the legal duty attaches to the property and its water system, not to the boiler. What a combi boiler changes is the content of that assessment, usually dramatically for the better. A combi heats water instantaneously as it flows, so there is no cylinder of stored warm water, and stored warm water is legionella's favourite habitat in a home. The assessment for a combi property is typically short. But "short" is not "skipped", and the duty to have assessed, and to have a record of it, does not evaporate with the cylinder.
Why storage, not the boiler, drives the risk
Legionella multiplies in water that sits still at roughly 20°C to 45°C with nutrients to feed on. A traditional vented or unvented cylinder is a reservoir: if the thermostat is set low, if the immersion fails, or if draw-off is low, water can spend days in exactly that range, which is why the guidance requires stored hot water to be kept at 60°C. A combi has no reservoir. Water enters cold from the mains, passes through a heat exchanger, and leaves hot seconds later. There is nowhere for the bacteria to establish inside the appliance, and the water is never held warm. That is why assessors and the HSE's landlord guidance treat combi systems as inherently lower risk.
The temperature control regime, storage at 60°C and outlets at 50°C, exists to tame stored systems. With a combi, most of it falls away, and what remains is a shorter list of things to check.
What the assessment still has to cover
Removing the cylinder does not remove the plumbing. A combi property still has a cold side, outlets, and occupants, and the assessment walks those:
- The cold supply. Mains-fed cold is normally cool and low risk, but cold pipework that runs through a warm loft or beside heating pipes can pick up heat. If a cold tap runs above 20°C even after two minutes of flushing, note it and find out why.
- Little-used outlets. An en suite nobody uses or an outside tap drawn twice a year stagnates in its branch whatever boiler feeds it. List them, flush them weekly or before use, and record it. Our guide to flushing little-used outlets covers the method.
- Leftovers from the old system. Many combi conversions leave pipe stubs behind where the cylinder and tanks came out. A capped stub is a dead leg: a pocket of stagnant water plumbed into the live system. Finding and removing these is often the single most useful thing the assessment turns up.
- The shower. Showers aerosolise water, which is the route into the lungs. Whatever the boiler, shower heads and hoses need descaling and disinfecting on a schedule, as covered in cleaning shower heads.
- Void periods. An empty property stagnates end to end. Before re-letting, run every outlet through to temperature, starting nearest the stop tap and working outward.
A combi boiler removes the storage cylinder, which removes the biggest risk. It does not remove the duty to assess, the dead legs from the old system, or the need to write the assessment down.
"It's a combi, it's fine" is not a record
The most common failure with combi properties is not bacterial, it is evidential. A landlord concludes, often correctly, that the risk is low, and records nothing. Then a tenant is ill, an insurer asks questions, or the property is sold with a compliance pack requested, and there is no paper trail showing the risk was ever considered. The low-risk conclusion is exactly the thing worth writing down: "combi boiler, no stored hot water, cold mains-fed throughout, no dead legs found, one little-used outlet flushed weekly" is a five-line assessment that takes minutes and closes the question properly.
The free risk assessment builder is set up for exactly this: it walks a simple domestic system, prompts the combi-specific points above, and produces the written record. It structures your judgement; it does not replace it, and it certifies nothing. Our landlord guide explains where the duty comes from if you want the legal background.
When a combi property is not simple
A few combi properties deserve a second look rather than a five-liner. Conversions where old tanks were abandoned but left connected, houses with long pipe runs to distant bathrooms where hot water arrives slowly, and properties with rainwater reuse or private water supplies all add features the simple picture does not cover. If the hot tap furthest from the boiler will not reach 50°C within a minute, or you find pipework you cannot account for, treat the assessment as a real investigation rather than a formality, and get competent help if it outruns your confidence.
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.
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
Is a combi boiler a legionella risk?
A combi boiler is a lower-risk way to make hot water because it heats water on demand as it flows through, so there is no stored volume of warm water for legionella to multiply in. That removes the single biggest domestic risk factor, the storage cylinder. It does not remove risk entirely: cold water can still warm in pipework, outlets can still stagnate, and showers still produce aerosol, so the rest of the system still needs a look.
Do landlords need a legionella risk assessment with a combi boiler?
Yes. The legal duty to assess the water system applies to the property, whatever heats the water. A combi boiler usually makes that assessment short, because there is no stored hot water to manage, but the duty itself does not disappear with the cylinder. The assessment should still cover the cold supply, little-used outlets, showers, and what happens during void periods, and it should be recorded.
Does a combi boiler kill legionella?
A combi boiler is better described as avoiding the problem than killing the bacteria. It heats water to well above the growth range as the water passes through the heat exchanger, and because there is no storage vessel the water is never held warm for days at a time. Legionella's foothold in a home is stored or stagnant water in the growth range, and a combi largely removes that foothold from the hot side.
Can legionella grow in a combi boiler system?
Yes, just not usually in the boiler itself. The realistic growth points are elsewhere in the same system: a dead leg of pipework left from an old cylinder or removed appliance, a little-used outlet, a scaled shower head and hose, or cold pipework running through a warm loft. A combi removes stored hot water, not these, which is why the assessment still walks the whole system rather than stopping at the boiler.
Do I still need to flush taps with a combi boiler?
Yes, where outlets are little-used. Stagnation does not care how the hot water is made: any outlet drawn off less than about once a week can stagnate in the branch feeding it, whatever boiler is on the wall. After a void period, flush every outlet through to temperature before the new tenancy starts, and in a long let with a rarely used bathroom, put that outlet on a weekly flush and record it.
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