Legionella control and safe hot water storage

What the HSE guidance asks of stored hot water, why, and how a well-designed cylinder or calorifier helps you meet it, including with a low temperature heat pump. For specifiers, installers and facilities managers.

Heat pumps and Legionella Specifier’s checklist

World Heat stainless steel storage calorifier for commercial hot water

Guidance, not a risk assessment

This page points you to the current HSE guidance and explains how cylinder and calorifier design fits with it. It isn’t a risk assessment and it doesn’t replace a competent person. Duty holders are responsible for their own Legionella risk assessment and control scheme, and should refer to the current HSE guidance.

HSE: Legionnaires’ disease guidance

The guidance

L8 and HSG274: what they are

In Great Britain, the control of legionella in water systems is set out by the Health and Safety Executive in two documents. The Approved Code of Practice, L8, covers the legal duties: assessing the risk, having a written scheme to control it, putting that scheme into practice and keeping records. It applies to employers and people in control of premises, including landlords.^{1, paras 2, 58}

The technical detail is in HSG274. Part 2 covers hot and cold water systems: the water supplied for drinking, cooking, washing and bathing.^{2, para 2.1} L8 also puts duties on anyone who designs, makes, supplies or installs water systems, to make them safe to use, clean and maintain, and to provide the information needed to run them safely.^{1, paras 78, 80, 81} That’s where a cylinder manufacturer comes in.

Why it matters

Where legionella grows in a hot water system

Legionnaires’ disease is normally caught by breathing in tiny droplets of contaminated water. The bacteria multiply where water sits at the wrong temperature, stands still, or has deposits to feed on.^{1, para 13}

Lukewarm water
L8 identifies water at 20–45 °C as suitable for legionella growth, and asks for those temperatures to be avoided.^{1, paras 13, 59}
The base of the vessel
Cold water usually enters at the base of an indirect calorifier, leaving a cooler zone below the coil where sediment also collects.^{2, paras 2.4, 2.25}
Water that doesn’t move
Dead legs, capped pipes, little-used outlets and poorly circulating parts of the system, where water stands and cools.^{2, para 2.4}
Deposits and spray
Sludge, scale, rust and biofilm feed the bacteria; showers and taps can create the fine droplets people breathe in.^{1, paras 13, 41}

That’s why temperature is the traditional control: keep hot water hot enough and cold water cold enough that neither sits in the range where the bacteria grow, and keep it all moving.^{2, paras 2.6, 2.81}

Storage and distribution temperatures

The temperatures at a glance

Where temperature is the control method. Each one is summarised from the HSE guidance, with the paragraph it comes from: check the document itself before you rely on it.

20–45 °CThe temperature range L8 describes as suitable for legionella growth, to be avoidedL8, paragraphs 2(a), 13(a), 41(b) and 59(a)
at least 60 °CHot water storage temperature (and the supply temperature from the heat source or calorifier)HSG274 Part 2, paragraphs 2.6, 2.37 and 2.82
50 °C within one minuteHot water at outlets, reached within one minute of running (55 °C in healthcare premises)HSG274 Part 2, paragraphs 2.6, 2.37 and 2.82
at least 50 °CReturn temperature to the calorifier from each circulating loop (55 °C in healthcare premises)HSG274 Part 2, paragraphs 2.18 and 2.37; Table 2.1
a temperature of at least 60 °C throughout the vessel for at least one continuous hour a dayWhere a timer-controlled shunt pump is used in an indirect calorifier, with the heat source onHSG274 Part 2, paragraph 2.25
as close to 60 °C as practicable without going below 60 °CCalorifier thermostat setting, checked monthlyHSG274 Part 2, Table 2.1
below 20 °CCold water, where possibleHSG274 Part 2, paragraph 2.6

The healthcare figure is 55 °C in place of 50 °C at outlets and on the return. Where more than one calorifier is used, HSG274 says they should be connected in parallel and each deliver water at the storage temperature.^{2, para 2.37} The same storage and outlet temperatures are on HSE’s web guidance.³

The storage vessel

Getting the whole vessel hot, not just the top

A thermostat near the top can read the storage temperature while the water at the bottom is much cooler. Cold water comes in at the base, and in a large calorifier the water can settle into layers, leaving a cool zone where the bacteria can grow. HSG274 says that stratification in large calorifiers should be avoided.^{2, para 2.25}

The fix it describes is to heat the whole vessel through once a day: a timer-controlled shunt pump moves water from the top to the base while the heat source is on, at the time of least demand, to achieve a temperature of at least 60 °C throughout the vessel for at least one continuous hour a day. Ideally the calorifier has its own connection for the shunt pump return, as low down as possible. As an alternative, some calorifiers have coils extending to the base to promote mixing as they heat.^{2, paras 2.25, 2.26}

The vessel design should also make it easy to run and check:

Temperature pockets on the flow and return, and in the base, so you can see the bottom is getting hot.^{2, para 2.37}
A drain valve at the lowest point, accessible and close to the vessel, so sediment can be drained off.^{2, paras 2.26, 2.37}
A way to inspect inside: HSG274 lists an internal inspection through the hatch, and cleaning, as a routine check.^{2, para 2.37, Table 2.1}
Storage and recovery sized to the demand, so normal daily use doesn’t pull the supply temperature down, while avoiding excessive storage.^{2, paras 2.32, 2.37}
A thermostat set as close to 60 °C as practicable without going below 60 °C, checked monthly.^{2, Table 2.1}

Heat pumps

Low temperature heat pumps and the storage temperature

Here’s the tension installers ask about. A heat pump is most efficient at a low flow temperature, but where temperature is the control method, HSG274 says stored hot water should be at a temperature of at least 60 °C. It is also clear that measures to save energy should be assessed at design stage so that legionella control isn’t compromised.^{2, paras 2.32, 2.82}

HSG274 doesn’t mention heat pumps by name. The closest match is its guidance on cylinders with a second, lower coil fed by solar or other microgeneration. It notes that the water around the lower coil can sit at a lower temperature, and that these systems should be designed so the hot water temperature isn’t compromised when the lower coil adds little heat.^{2, paras 2.27, 2.28} In practice, it’s resolved in one or more of these ways, chosen with the site’s risk assessment:

1.Let the heat pump do it, if it can
Some heat pumps can reach the storage temperature on their own; others can’t. Check the manufacturer’s data for the unit. Where it can, it does so at a lower efficiency than at its normal flow temperature: our stratification page explains why a heat pump pays for every degree.
2.A scheduled higher temperature cycle
For cylinders with a lower (solar or other microgeneration) coil, HSG274 says that if that coil can’t reach temperatures that bring about thermal inactivation of legionella, a further level of control is needed, for example programming the upper heat source to heat the entire contents once daily, preferably when there’s little demand for hot water.^{2, paras 2.27, 2.28} Heat pump controls often call it a legionella or pasteurisation cycle. It’s in addition to the storage temperature, not instead of it. Be careful what it actually heats: an upper coil or immersion heater mainly heats the water above it, and the cooler water below can stay cool. A cycle only does the job if the design gets the whole vessel, base included, up to temperature, for example with a timed shunt pump (below) or a heat source that reaches the base, and if a temperature reading at the base confirms it.
3.An immersion heater or second coil to finish the job
Where the heat pump can’t reach the storage temperature, a second heat source does: an electric immersion heater, or a boiler on an upper coil in a twin coil cylinder. In HSG274’s description of twin coil cylinders, the upper coil raises the water at the top of the vessel to 60 °C.^{2, para 2.27}
4.A timed shunt pump to reach the base
On a calorifier, a shunt (de-stratification) pump moves hot water from the top to the base while the heat source is on. HSG274 says it should run to a time clock, not continuously: about one hour daily on a solar or microgeneration calorifier, and on an indirect calorifier to achieve a temperature of at least 60 °C throughout the vessel for at least one continuous hour a day.^{2, paras 2.25, 2.28}

Why the coil size matters

A coil passes heat in proportion to its surface area and the temperature difference across it. A heat pump’s cooler flow gives a smaller difference, so it needs a much bigger coil to heat the vessel in good time. Too small a coil and the heat pump runs hotter and less efficiently, the reheat takes longer, and more of the work falls to the immersion heater. A coil that reaches low into the vessel also heats the water at the base rather than leaving it to sit cool.^{2, para 2.26} Our heat pump cylinder guide explains coil sizing in plain English.

Where stratification helps, and where it doesn’t

In a stored hot water vessel, you want the incoming cold water to enter gently, so it doesn’t jet up and cool the hot water being drawn off the top. You don’t want a cool layer at the base left unheated from one day to the next: that’s the part the daily whole vessel heat is for.^{2, para 2.25} Thermal stores and buffer vessels on the heating side are different: they hold primary water, and there a well-kept stratified store is what the heat pump wants. Our stratification page covers those.

Vessel design

Why a well-designed vessel matters

A vessel with poorly placed connections, fast jets of incoming water and no means of heating the base can leave pockets of lukewarm water where you don’t want them. How water enters and moves through the vessel is a design decision, made on the drawing.

We build commercial vessels to order with internal diffusers, sparge pipes and baffle plates to control how water enters and moves through the vessel, with the connections, pockets and drains the control strategy needs, and supply de-stratification pump sets for calorifiers that need them. What each internal does, and where it goes, is on our stratification page.

Storage Calorifiers/Cylinders Buffer vessels De-stratification pump sets Diffusers, sparge pipes & baffle plates

Thermal disinfection

The principle

Thermal disinfection uses heat to disinfect a hot water system that needs it, and is commonly used as a rapid response. In principle, the whole contents of the calorifier are raised to a high temperature and circulated through the system, and every hot outlet is then flushed so that the whole system, not just the vessel, is held hot for long enough.^{2, paras 2.130 to 2.134}

We don’t summarise the procedure here, because a shortened version could be mistaken for a complete one. The required temperatures, times and checks are set out in full in HSG274 Part 2 (opens in a new tab), and the procedure should be followed from there.

It only works if every part of the system gets hot enough for long enough: parts that don’t can make it ineffective, and it may not be practicable where the hot water supply can’t hold a high temperature throughout.^{2, paras 2.130, 2.134} A vessel that can heat all of its contents, and a system that circulates properly, make it far more likely to work.

Raising water above 60 °C creates a scalding hazard. HSG274 says a risk assessment and a safe system of work are needed throughout, with warning signs at outlets.^{2, Info box 2.6} This is work for a competent person.

Dead legs and stagnation

Keep the water moving

A dead leg is a length of pipe that water only passes through now and then: a branch to a little-used outlet, a capped spur, a loop whose return has failed. The water in it cools and stands, which is what the bacteria need.^{2, paras 2.4, 2.37}

Design out dead legs where you can, or keep them short; remove redundant pipework and standby plant.^{1, para 80}
Balance the circulation, so every loop returns hot and there are no long, stagnant runs.^{2, para 2.37}
Remove outlets that are rarely used, and cut the pipe back to a common supply. If an outlet stays, flush it weekly for several minutes when it isn’t in regular use, and keep it up.^{2, paras 2.77, 2.78}
Put expansion vessels on the cold side, close to the incoming supply, and consider flow-through types so water doesn’t stand in them.^{2, para 2.39, Info box 2.1}

Specifying a cylinder or calorifier

A practical checklist for Legionella control

1Can the vessel reach and hold the storage temperature, every day, with the heat sources it will actually have?With a heat pump, decide how the higher temperature will be reached: the heat pump itself, an immersion heater, a second coil, or a combination.²
2How will the bottom of the vessel get hot?The base, where cold water comes in, is the cool part. Plan a daily whole-vessel heat: a timed shunt (de-stratification) pump, a coil reaching down to the base, or a scheduled cycle whose heat source or circulation genuinely reaches the base. Check it with a temperature reading at the base.²
3Is the coil big enough for the heat source?A low temperature heat source needs a large coil surface to heat the vessel and recover in good time. Check the coil area against the heat pump output.
4Is the vessel sized to the demand, and no bigger?Enough storage and recovery to hold the supply temperature through normal daily use, while avoiding excessive storage that turns over slowly.²
5Are there the connections and pockets you’ll need to run and check it?A shunt pump return as low as possible, temperature pockets on the flow, the return and the base, a drain at the lowest point, and a way to inspect inside.²
6How does water enter the vessel?On larger vessels, internals such as a diffuser or baffle on the cold feed stop incoming water jetting up into the hot water being drawn off.
7Does the pipework avoid dead legs?Keep branches short, balance the circulation so every loop returns hot, and don’t leave capped or little-used spurs.¹²
8Has the risk assessment been involved?The temperatures, controls and monitoring should follow the site’s Legionella risk assessment, not the other way round.¹

Our cylinders and calorifiers

Built with the control strategy in mind

Specifying hot water storage?

Tell us the duty and the control strategy

The demand, the heat sources and the storage temperature your risk assessment calls for. Our engineers will size the vessel and design in the coils, connections, pockets and internals it needs.

Request a quote Ask us a question Call 0161 343 8610

Common questions

Can I just set the cylinder lower to save energy?+

Not as a rule. Where temperature is the control method, HSG274 says hot water should be stored at a temperature of at least 60 °C. It does allow lower hot water temperatures where they aren’t needed for other reasons and a biocide regime is used instead, but then the control system should be checked at least weekly, because a lapse leaves the system vulnerable. It also says energy and water saving measures should be assessed at design stage so that legionella control isn’t compromised. It’s a decision for the risk assessment, not the thermostat.^{2, paras 2.32, 2.82, 2.86}

Isn’t 60 °C a scalding risk?+

The storage temperature is for the vessel, not the tap. HSG274 notes that at 50 °C the risk of scalding is small for most people but rises rapidly with higher temperatures and longer exposure, and is greater for young children, older and disabled people and those with sensory loss. Where the risk assessment finds a significant scalding risk, thermostatic mixing valves on baths and showers, as close to the outlet as possible, should be considered.^{2, paras 2.74 to 2.76, 2.83}

Does the stratification page contradict this?+

No: they cover different water. The stratification page is mainly about thermal stores and buffer vessels, which hold heating (primary) water and work best with hot water kept on top of cold. HSG274 Part 2 covers water supplied for domestic purposes (drinking, cooking, washing). In a stored hot water calorifier, the cool layer at the base has to be heated through regularly, which is why HSG274 says stratification in large calorifiers should be avoided and suggests a timed shunt pump. Keeping incoming cold water from churning up the hot water at the top is still good design; leaving a cool layer at the base unheated is not.^{2, paras 2.1, 2.25}

What do you need from us to quote?+

The demand (or the number and type of outlets), the heat sources and their outputs, the storage temperature and control strategy from the risk assessment, the space available, and any connections, pockets or internals you need. A schedule or drawing is ideal.

Sources

The guidance is summarised, not reproduced, and paragraph numbers are given so you can check it. Refer to the current HSE documents for the requirements; start at hse.gov.uk/legionnaires (opens in a new tab).

Help choosing and specifying

Guides and tools

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General guidance for specifiers, installers and facilities managers. It isn’t a risk assessment: duty holders are responsible for their own, and the system designer remains responsible for the system.