Do I need a Legionella risk assessment with a combi boiler?

Legionella bacteria multiply most readily in water held between roughly 20°C and 45°C. A stored hot-water cylinder, especially one running below its intended temperature or with a cool base and sediment, provides exactly that. A combi boiler heats mains water on demand, so there is no reservoir of warm water sitting between uses.

Combi properties also tend to have shorter, simpler pipe runs and no loft tank, which removes two more common problem areas.

• Little-used outlets — a guest bathroom, an en-suite, a utility or outside tap that goes weeks without flowing
• Dead legs and capped pipework left behind after a bathroom or kitchen refit
• Shower heads and hoses, which hold water between uses, scale up and produce a breathable spray
• Cold water that does not stay cold, for example a run of pipe passing through a warm airing cupboard or a poorly insulated run alongside heating pipes
• Any cold-water storage tank still in place, including a redundant one that was never removed
• Void periods, where the whole system stagnates regardless of how the water is heated
• Occupants who are more susceptible to infection

1. 1

   Confirm there really is no stored water

   Check the loft, airing cupboard and under stairs. Redundant tanks and abandoned cylinders are common in properties converted to a combi.

2. 2

   List every outlet and how often it is used

   Usage frequency matters more than anything else in a combi property, because stagnation is the main remaining risk.

3. 3

   Look for dead legs

   Refits often leave capped pipe behind a panel or under a unit. Water in a dead leg never moves.

4. 4

   Inspect shower heads and hoses

   Scale and biofilm are visible and easy to deal with. Descaling or replacing a head is a cheap, effective action.

5. 5

   Record hot and cold observations

   Note whether the cold runs genuinely cold after a short run, and what the hot outlet reaches, remembering that a blended outlet reads differently.

6. 6

   Give the tenant simple advice

   Run all outlets, including the shower, after a period away, and report any persistent problem with water temperature or appearance.

Stored hot water

A cylinder holding warm water between uses. Storage temperature, sediment, cylinder condition and the temperature reaching outlets all matter.

Combi / instantaneous

No hot-water reservoir. Attention shifts to stagnation in little-used outlets and dead legs, shower head condition and any cold-water storage.

Electric instantaneous shower

Heats on demand at the point of use. The head and hose still hold water between uses and still produce a spray.

Describe the system as instantaneous with no storage and the workflow drops the prompts that do not apply, leaving the outlet register, the usage pattern, the shower checks and the observations that do matter.

You end up with a structured, versioned report specific to the property, rather than a generic form with the cylinder questions crossed out.