Legionella Risk in HMOs and Shared Houses

The Health and Safety Executive is clear that the Legionella risk in most rented homes is low, and that most landlords can assess it themselves. An HMO is where that stops being the whole story. Not because different law applies — the duty is the same — but because the things that keep risk low in an ordinary house are precisely the things a shared house tends not to have.

This article sets out what actually differs, and what a workable control regime looks like in shared housing.

Why an HMO is a different problem

Risk in a domestic water system is a function of two things: how warm the water sits, and how long it sits there. A family home scores well on both because the same few people use the same few outlets every day. Break either condition and the picture changes.

Stored water. Larger shared houses frequently have a cold water storage tank and a hot water cylinder rather than a combi boiler, because the demand needs it. Stored water is water that waits.
Longer, more complicated pipework. A house converted from three bedrooms to six, with en-suites added, has runs the original plumber never drew.
Dead legs left by conversion. Every bathroom moved, every basin removed, every capped-off run from an old layout is a length of pipe holding water that never moves.
Outlets nobody uses. The shower in the room that is currently empty. The basin in the box room. The outside tap.
Voids that empty the whole building at once. This is the one that matters most, and it is specific to shared housing.

The summer void is the real issue

A student HMO in a university city can stand substantially empty from June to September. During that period the water in the tank, the cylinder and every pipe run sits undisturbed at ambient summer temperature — which in a loft tank is comfortably inside the range where Legionella multiplies.

Then in late September six people arrive on the same afternoon and all take a shower.

That is the scenario the assessment exists for, and it is entirely predictable, which means it is entirely manageable. What it needs is a flushing regime during the void and a proper recommissioning before re-occupation — not a certificate, and not a water sample in March when the house was full.

What control actually looks like

The HSE's guidance on hot and cold water systems gives the practical measures. In a shared house they come down to five things.

1. Temperature

Hot water stored at 60°C or above, reaching at least 50°C at the outlet within a minute of running. Cold water held below 20°C in storage and at the tap. The zone between those numbers is where the bacteria are comfortable, so the control is simply to keep the hot genuinely hot and the cold genuinely cold. Turning a cylinder thermostat down to save money is the single most counterproductive thing a landlord can do here.

2. Flushing what nobody is using

The HSE recommends a programme of regular — weekly — flushing for outlets that are used infrequently. In an HMO that means the empty room's shower and basin, and every outlet in the building during a void. Run each one long enough for the water to reach temperature, and run showers gently or with the head removed to avoid generating spray.

3. Removing dead legs

The guidance is to remove dead ends and blind ends from the system where you can, and flush what you cannot remove. In a converted property this is usually the highest-value physical work available, and it is often a plumber's morning rather than a project.

4. Tanks and cylinders

Cold water storage should be no larger than the building actually needs — roughly a day's usage — because oversized tanks mean slower turnover. Tanks need a close-fitting lid and insulation to keep them below 20°C and to keep contamination out. Where there is more than one tank they should be linked so water flows through each rather than stagnating in one.

5. Showers

Showers matter disproportionately because they produce the fine aerosol that makes infection possible. The guidance is to dismantle, clean and descale shower heads quarterly, or at a frequency your assessment sets. In a six-person house with six showers that is a real task, and it belongs in a schedule rather than in someone's memory.

Monitoring, and what to write down

The practical monitoring regime for hot and cold water systems is straightforward: check sentinel outlet temperatures monthly, check cold water storage temperatures every six months, and descale shower heads quarterly. Keep a simple log — date, what was checked, the reading, who did it.

That log is the evidence. If anyone ever asks whether you were managing the risk, a year of dated readings answers the question in a way no certificate does.

Where this meets HMO licensing

Legionella control is not itself a licensing condition in the way a fire risk assessment is. But licensing brings the council into the property, and the Housing Health and Safety Rating System covers hazards from water supply and personal hygiene — so a system with obvious stagnation, an unlidded tank or hot water running cold is not a matter the council will simply pass over.

Practically, if you are already having an HMO assessed for fire safety, having the water system looked at during the same visit costs very little extra and closes a gap that most landlords have not thought about.

A workable regime for a shared house

Assess the system once, properly, and write it down — including where the tanks are, what temperature things run at, and where the dead legs are.
Set the cylinder to 60°C and leave it there.
Flush unused outlets weekly; flush the whole building weekly during a void.
Recommission properly before a new cohort moves in.
Descale shower heads quarterly.
Check sentinel outlet temperatures monthly and tank temperatures twice a year.
Tell incoming tenants to run anything that has been standing, and to report hot water that is not hot.
Keep the log.

None of that is expensive. All of it is defensible. And unlike a certificate, it is what actually keeps the risk down.

Frequently Asked Questions

Do HMOs need a Legionella risk assessment?

The legal duty is the same as for any let — assess and control the risk under section 3 of the Health and Safety at Work etc. Act 1974. What differs is that HMOs commonly have the features that raise risk: stored water in tanks and cylinders, longer and more complex pipework, dead legs left by conversion, unused outlets, and void periods when the whole building empties. Those make a written, considered assessment genuinely worthwhile rather than a formality.

Why are student HMOs a particular Legionella risk?

Because they empty for the summer. Water sits undisturbed in tanks, cylinders and pipe runs for two or three months at ambient summer temperature, which is inside the range in which Legionella multiplies. Then the building is fully occupied again within a day. The risk is predictable, which means it is manageable — a flushing regime during the void and a proper recommissioning before re-occupation.

What temperature should the hot water cylinder be set to in an HMO?

60°C or above for stored hot water, with water reaching at least 50°C at the outlet within a minute of running. Cold water should stay below 20°C. Turning the cylinder thermostat down to reduce energy costs removes the main control and is the most common self-inflicted problem we see in shared housing.

What is a dead leg and why does it matter?

A dead leg is a length of pipework that no longer serves an outlet in use — a capped run from a removed bathroom, a disconnected basin, an isolated outside tap. Water in it never moves, sits at room temperature and provides ideal conditions for bacterial growth while remaining connected to the rest of the system. HSE guidance is to remove dead ends and blind ends where possible and flush what cannot be removed.

How often should shower heads be cleaned in a shared house?

Quarterly is the standard frequency, or as your risk assessment determines. Shower heads matter more than other outlets because showers generate the fine aerosol that allows the bacteria to be inhaled, which is how infection occurs. Dismantle, clean and descale rather than simply wiping.

What records should a landlord keep?

A simple log is enough: the date, what was checked, the temperature reading, and who did it. Monthly sentinel outlet temperatures, six-monthly cold water storage temperatures, quarterly shower descaling, and any flushing carried out during voids. A year of dated readings is far better evidence of control than any certificate.

Does Legionella come up in HMO licensing?

It is not a licensing condition in the way a current fire risk assessment is. But licensing brings the council into the property, and the Housing Health and Safety Rating System covers hazards relating to water supply and personal hygiene. Visible stagnation, an uncovered storage tank or hot water that does not reach temperature are not things an inspecting officer will overlook.

Can I do the HMO assessment myself?

You can for a simple property, and the HSE says most landlords can assess domestic risk themselves. The judgement call is whether your building is still simple: stored water serving multiple units, pipework altered over several conversions, and seasonal voids move a property out of that category. If you cannot draw your own water system from memory, that is usually the answer.