A combi boiler heats water the moment you open the tap, so there is no hot cylinder sitting warm between uses. That single fact removes the largest reservoir Legionella usually has to grow in. So the honest answer is this: a combi genuinely lowers your risk. It does not take it to zero.
The belief that “no stored hot water means no Legionella” is half right, which is exactly what makes it worth correcting. It fixes one big problem and quietly leaves several smaller ones in place.
The part that is actually true
Legionella multiplies in standing water that sits in the wrong temperature band — roughly 20°C to 45°C — and it needs time plus a surface to colonise [1]. A traditional system hands it all three: a cylinder or calorifier holding tens or hundreds of litres, often drifting through the warm band, feeding the whole property.
Hot water storage: preventing Legionella in tanks and cylinders sets out why that stored volume is the feature a risk assessment worries about most. Take the store away and the biggest single hazard goes with it. An instantaneous heater — a combi, or a point-of-use electric unit over a basin — warms only the water passing through it, only when you draw it. There is no tank to keep at 60°C, and no large body of tepid water waiting downstream for the next person.
That benefit is real, and it is why combis are lower-risk than stored-water systems in the same building. The error is reading “lower” as “none”.
Myth versus reality
| The myth | The reality |
|---|---|
| ”No stored hot water, so there’s nothing for Legionella to grow in.” | The hot store is gone, but the incoming cold water, the pipework and the outlets are unchanged — and any of them can still harbour bacteria. |
| ”On-demand heating sterilises the water as it passes through.” | The unit warms water for the seconds it flows. It does nothing for pipework upstream, nothing for water sitting in pipes after you stop, and nothing for an outlet left unused. |
| ”A combi means I don’t need a Legionella risk assessment.” | A landlord still has a duty to assess and control the risk. The combi makes that assessment simpler, not unnecessary [2]. |
| ”The cold tap has nothing to do with Legionella.” | Cold water that warms past 20°C in a stagnant run is a textbook growth condition, whatever heats the hot side [3]. |
| ”If the shower runs properly hot, it’s safe.” | The showerhead atomises water into a breathable mist and collects scale and biofilm regardless of how that water was heated [4]. |
The rest of this piece is just those five rows, explained.
The cold side does not care what heats your hot water
Swapping a cylinder for a combi changes nothing about your cold-water supply. Cold should stay genuinely cold — below 20°C at the tap where practicable [3]. In a real house that is not guaranteed. A cold pipe run through a warm airing cupboard, a loft tank baking under a summer roof, or a long buried run on a hot day can all drift up into the growth band.
If the cold water arrives lukewarm and then sits, you have the same conditions that worry assessors on any system. The danger zone: Legionella risks in lukewarm water covers why tepid is the worst place for water to sit. The combi has no bearing on it.
Dead legs and the outlet nobody uses
A combi does not flush your pipes. It heats water for the outlets you actually open.
Think about the parts of a property that barely run: a guest en-suite between visits, an outside tap over winter, a utility sink used twice a year, the basin in a room that became storage. Water sits in those branches and the pipe feeding them, cooling or warming into the growth band and going nowhere. That is a dead leg, and on-demand heating does not touch it. If anything, a combi can lull people into ignoring stagnation precisely because they have stopped thinking about water sitting still.
The showerhead is its own small ecosystem
Of all the outlets in a home, the shower is the one that matters most, because it does the one thing Legionella needs to reach your lungs: it breaks water into a fine, breathable spray [4]. That is the main route of exposure, and it is identical whether the water came from a cylinder or a combi.
A showerhead and hose also collect scale and biofilm over time, especially in hard-water areas, and that scale gives bacteria a surface to live on. The heating method is irrelevant to what is growing in the head you spray over yourself each morning. Cleaning and descaling the head and hose periodically does more for shower safety than the choice of boiler ever will.
Long runs still pass through the growth band
Even the hot side is not as clean a story as “on demand, therefore safe”. Water leaves a combi hot, but it loses heat as it travels. On a long run to a far bathroom it can arrive cooler than you would like, and the moment you turn the tap off, the water left standing in that pipe cools to room temperature and sits there until the next draw — squarely in the band where Legionella is happy [1].
This is why the temperatures still matter even without a tank. What temperature kills Legionella? explains the kill-versus-control distinction, and Hot water temperature guidelines to prevent Legionella covers the delivery targets. A combi removes the stored reservoir; it does not warm your pipework between uses.
What is actually proportionate in a domestic let
Here is the reassuring half, because the point of this piece is not to frighten a landlord with one combi-fed flat. HSE is clear that the duty to control Legionella applies to rented homes, but that for most residential lettings the risk is low and a simple assessment is enough — you do not need a consultant, a certificate, or routine sampling for a typical small property [2].
In my view, the proportionate package for a low-risk domestic let with a combi is short and unglamorous: keep cold water cold and hot water hot, run any tap or shower that has sat unused (a weekly flush of little-used outlets is sensible practice), clean and descale showerheads periodically, and flush through thoroughly before re-letting a property that has stood empty. That is a recommendation, not a legal line — your situation may call for more or less. But it is far closer to the real obligation than either “do nothing, I have a combi” or “pay for a full survey of a one-bedroom flat”.
The combi has genuinely earned you a simpler regime. It has not earned you no regime.
This is general guidance, not an assessment of your property. What counts as adequate control — the outlets you flush, how often, whether your building’s size or occupants make it higher-risk — is a judgement for a competent person working from a current, site-specific risk assessment under the ACoP L8 framework. We do not give legal, medical or design advice, and a combi does not exempt any building from that assessment.
What to do this week
Walk your property with one question in mind: which water sits still? List the outlets nobody used in the last fortnight, then run each one — hot and cold — until the temperature settles. Take the worst showerhead off and check it for scale while you are there. That ten-minute walk tells you more about your actual risk than the boiler on the wall does.
If you manage more than a handful of lets, the part that slips is not the flushing itself but proving it happened — which tap, which week, by whom. That recurring, easy-to-forget record is exactly what a digital logbook keeps visible, so an empty flat or a forgotten outside tap does not quietly become the one stagnant spot you never flushed.
FAQ
Does a combi boiler mean I don’t need a Legionella risk assessment as a landlord?
No. The duty to assess and control Legionella risk still applies to rented homes [2]. A combi removes the stored hot water that drives most of the risk, so the assessment is usually simpler and the controls lighter, but the assessment itself is not optional. For a typical small let it can be a straightforward, proportionate exercise rather than a paid survey.
Can Legionella still grow if I have an instantaneous water heater?
Yes, just with one major hazard removed. The heater itself holds little or no warm water, but the cold supply, long pipe runs that cool between uses, dead legs to unused outlets, and the showerhead can all still support bacteria [1][4]. On-demand heating lowers the risk; it does not eliminate the parts of the system it never touches.
What’s the single most useful thing to do on a combi system?
Run the outlets that sit unused and keep the showerhead clean. Flushing a little-used tap or shower displaces the water that has been standing in the pipe, and descaling the head removes the scale and biofilm that bacteria colonise [3][4]. Both are quick, cheap, and target the parts of a combi system that genuinely still carry risk.
Sources
[1] HSE, “Legionnaires’ disease. The control of legionella bacteria in water systems - ACoP and guidance (L8)”. https://www.hse.gov.uk/pubns/books/l8.htm [2] HSE, “Legionella and landlords’ responsibilities”. https://www.hse.gov.uk/legionnaires/legionella-landlords-responsibilities.htm [3] HSE, “Hot and cold water systems”. https://www.hse.gov.uk/legionnaires/hot-and-cold.htm [4] CDC, “How Legionella Spreads”. https://www.cdc.gov/legionella/causes/index.html