Legionella and hot water temperatures in commercial buildings
Commercial engineers covering Suffolk, Essex and Cambridgeshire
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In most commercial buildings, the same plant that heats the building also heats the water. Boilers feed calorifiers, or gas-fired water heaters store hot water for kitchens, washrooms and showers. That means the condition of your heating plant has a direct effect on how well you control legionella. This guide sets out the hot water temperatures HSE expects in commercial buildings, the routine checks behind them, how heating plant problems can undermine them, and what duty holders need to have in place. It is written for facilities managers, building owners and site teams, and it is based on HSE's legionella guidance, the L8 Approved Code of Practice and HSG274 technical guidance.
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Why temperature is the main control
HSE says there is a reasonably foreseeable legionella risk if your water system has a water temperature between 20 and 45°C, creates or spreads breathable droplets (for example from taps and showers), stores or recirculates water, or is likely to contain nutrients such as rust, sludge, scale, organic matter and biofilms. Hot and cold water systems are among the most common sources.
HSE explains that legionella can grow in water systems between 20°C and 45°C, especially where there is a risk of stagnation, and that avoiding that range "for as long as possible is a key control measure." It describes temperature control as "the traditional strategy for reducing the risk of legionella in hot and cold water systems." Other control methods exist, but where temperature is not used, HSE says you still need to show you are achieving effective control.
The temperatures HSE expects
HSE's guidance on hot and cold water systems gives these figures:
| Where | Temperature |
|---|---|
| Hot water storage (calorifier or water heater) | Stored at 60°C or above. HSE says outgoing water from calorifiers should be at least 60°C. |
| Hot water outlets | Distributed so it reaches 50°C within 1 minute at the outlets (55°C in healthcare premises). |
| Hot supply to thermostatic mixing valves (TMVs) | At least 50°C within 1 minute of running the water. |
| Cold water | Maintained, where possible, below 20°C. HSE says cold sentinel taps should be below 20°C after running for 2 minutes. |
HSE also says pipe runs downstream of TMVs should not exceed 2m. Hot water is a scald risk as well as a control measure: HSE notes there is a risk of scalding where water comes out of taps above 44°C, and where vulnerable people can get into baths or showers and the risk is significant, a Type 3 TMV must be fitted to stop water being discharged above 44°C. Your risk assessment has to balance both.
How heating plant affects legionella control
The temperatures above depend on the plant producing and circulating enough heat. A few common ways a heating plant fault turns into a water hygiene problem:
- The calorifier or water heater is not reaching 60°C. If a boiler serving an indirect calorifier is off, locked out, short of output or on the wrong time schedule, or a gas-fired water heater has a fault, the stored water can sit below the temperature HSE expects. HSE suggests checking flow and return temperatures at calorifiers, and says the thermometer pocket at the top of the cylinder, and on the return leg if fitted, is a useful point for accurate measurement.
- The circulation loop is not working. HSE says the calorifier flow and return check "makes sure the supply and return temperatures on each hot water loop are unchanged (the loop is functioning as required)." A return temperature that has dropped can point to a circulation problem on the secondary hot water system.
- Heat is getting into cold water pipework. HSE advises avoiding transfer of heat from other sources and insulating water pipes in work areas where heat may be transferred from hot pipes. Cold water pipework that runs through a warm plant room or alongside heating pipework can warm up into the growth range.
- Scale and sludge in the vessel. HSE lists sludge and scale as nutrients for legionella and suggests a visual check of the internal surfaces of cylinders for scale and sludge when possible, and checking the vessel remains clean at least annually.
- Plant switched off or barely used. Closed floors, empty units and holiday shutdowns leave water standing. HSE says to remove dead ends, flush dead legs regularly, and run a weekly flushing programme for infrequently used outlets.
When your monitoring shows the stored or return temperature is low, the cause is often on the heating side. Our engineers repair commercial water heaters and calorifiers and the boilers that serve them, and we cover the wider plant room in our commercial plant room safety checklist.
Routine checks HSE suggests for hot water
HSE's "things to consider" page lists typical checks for hot and cold water systems. The right frequency for your site comes from your risk assessment, but HSE's examples include:
| Check | HSE's suggested frequency |
|---|---|
| Calorifier flow and return temperatures (outgoing at least 60°C) | Monthly, with findings recorded |
| Hot water sentinel outlets (at least 50°C within 1 minute) | Monthly, plus representative outlets on a rotational basis |
| Hot water inlet to TMVs, where fitted | Monthly |
| Cold water sentinel outlets (below 20°C after 2 minutes) | Monthly |
| Cold water storage tank temperature at the ball valve outlet | Every 6 months |
| Visual check of calorifier internal surfaces for scale and sludge | When possible, and cleanliness checked at least annually |
| Flushing infrequently used outlets | Weekly |
| Cleaning and descaling showerheads and hoses | Quarterly, or as the risk assessment decides |
HSE notes that a building management system can take the calorifier measurements if one is installed, and that remote temperature monitoring is increasingly used and allows early detection of control failures. It recommends that remote monitoring is based on a specific legionella risk assessment and agreed by a competent person.
For enclosed hot and cold water systems, HSE says microbiological monitoring is not usually required. Testing for legionella may be necessary where there is doubt about how well the control regime is working, or where recommended temperatures are not being consistently achieved. Where sampling is needed, HSE says it should follow BS 7592 and be analysed by a UKAS-accredited laboratory.
Duty holder checklist
HSE says employers and people in control of premises, including landlords, must identify and assess sources of risk, manage, prevent or control those risks, and keep the correct records. The legal framework is the Health and Safety at Work etc Act, the Management of Health and Safety at Work Regulations and COSHH, with practical guidance in the L8 Approved Code of Practice and HSG274.
- Have a current risk assessment. Review it when anything significant changes, such as an extension of the water system or a change in who uses the building.
- Appoint a responsible person. HSE says you must appoint someone competent, often known as the responsible person, with sufficient authority, competence, skills, knowledge of the system and experience. If you use contractors for water treatment, the responsible person still has to make sure the work is done to the required standard.
- Keep a written control scheme. Where a risk cannot be prevented, HSE describes a written control scheme setting out your system (for example a schematic), who is responsible, safe operation, the control methods and what checks are carried out and how often.
- Keep records. If you have 5 or more employees you must record the significant findings. HSE says records should be kept for as long as they remain current and for at least 2 years after, and records of monitoring inspections for at least 5 years.
- Keep the heating plant maintained. Regulation 35 of the Gas Safety (Installation and Use) Regulations 1998 requires employers to ensure gas appliances, installation pipework and flues at a place of work under their control are maintained in a safe condition. Boilers and gas-fired water heaters that hold your hot water at temperature fall under that duty, and the work must be done by a Gas Safe registered engineer.
Taking calorifier flow and return readings
The monthly calorifier check HSE suggests is simple, but readings are only useful if they are taken the same way each time. A practical routine for site teams:
- Use the same points every month. HSE says the thermometer pocket at the top of the cylinder, and on the return leg if fitted, is a useful point for accurate measurement. If you use a contact thermometer on the pipe instead, mark the spot so the next person uses it too.
- Read the outgoing (flow) temperature. HSE says outgoing water from calorifiers should be at least 60°C.
- Read the return temperature where the system has a circulation loop. HSE describes the flow and return check as making sure the supply and return temperatures on each hot water loop are unchanged, showing the loop is working as required.
- Record the date, time and who took the reading, alongside the sentinel outlet checks, so trends show up.
- Do not change any settings. If a reading is low, report it to your responsible person.
Reading at a similar time each month helps, because a calorifier just after a heavy draw-off, such as straight after a kitchen's morning wash-down, can read lower than it will an hour later.
What a low reading can point to on the heating side
| What the log shows | Possible plant-side causes an engineer will check |
|---|---|
| Flow below 60°C at every check | Calorifier or water heater thermostat set too low or faulty, boiler not prioritising hot water, primary pump or valve fault, boiler short of output or locking out |
| Flow fine in the afternoon, low first thing | Time schedule not giving the calorifier enough heat-up time before demand starts, or boiler off overnight with no hot water priority |
| Flow fine but return well down | Secondary circulation pump stopped or air-locked, a balancing valve out of position, or a long poorly insulated run |
| Readings falling over several months | Scale on the heat exchanger or coil reducing heat transfer, or a boiler whose combustion is drifting and needs a service |
| Gas water heater keeps going out | Ignition, flame sensing, flue or air supply fault on the appliance itself |
These are the faults a Gas Safe registered engineer can find and fix on the plant. The water hygiene side, such as reviewing the risk assessment, flushing and any sampling, stays with your responsible person and water hygiene contractor. HSE also notes that testing for legionella may be needed where recommended temperatures are not being consistently achieved.
Commercial hot water temperature by type of system
The figures HSE sets out are the same whatever produces the hot water, but where you read the temperature, and what usually goes wrong, depends on the type of plant. Most commercial buildings we work on use one of the systems below, and some larger sites have more than one.
| System | Where to read the temperature | Plant faults we often find when it runs cool |
|---|---|---|
| Indirect calorifier heated by the boilers (offices, schools, care homes, leisure) | Outgoing flow at the top of the vessel and the return leg of the secondary loop | No hot water priority on the boiler controls, primary pump or diverting valve fault, scaled coil, boiler locking out or short of output |
| Direct gas-fired storage water heater (kitchens, washrooms, hotels) | Outlet from the heater and the return if the water is recirculated | Ignition or flame sensing fault, flue or air supply problem, thermostat drift, scale building up on the base of the vessel |
| Plate heat exchanger or instantaneous system (low storage) | Outlet temperature while water is being drawn, plus the primary temperature from the boilers | Scaled plates, a sticking control valve, primary flow too cool at peak times, sensor fault |
| Combination boiler in a small commercial unit (shops, small offices) | Hot water at the nearest and furthest taps | Low flow, diverter valve fault, scaled plate exchanger, boiler too small for the number of outlets |
If you are not sure what you have, a photo of the plant room and the data plate on each heater or calorifier is usually enough for us to tell you. Our commercial water heater repair page covers the appliances themselves, and plant room repair covers pumps, valves and controls.
Stratification: why part of a calorifier can run cooler
Hot water is lighter than cold, so a storage vessel naturally forms layers. Cold mains water comes in near the base, and the hottest water sits at the top where it is drawn off. In a large calorifier that sees heavy, intermittent use, such as a school at break time or a kitchen after a wash-down, the bottom of the vessel can sit a long way below the outgoing temperature for much of the day. HSE's technical guidance, HSG274 Part 2, covers calorifier design and maintenance, including stratification, and your responsible person and risk assessment decide how it is managed on your site.
On the plant side, the things an engineer will look at are:
- The destratification or shunt pump, where fitted: whether it runs, whether its timer is set as the control scheme says, and whether the pump or its valves have failed.
- Recovery time: whether the boilers, or the burner on a direct-fired heater, can bring the whole vessel back up to temperature between periods of heavy use.
- Controls and time schedules: whether the hot water gets enough heat-up time before demand starts, and whether the boiler controls give the calorifier priority when it needs it.
- Scale and sludge at the base: deposits insulate the water from the heat and give bacteria somewhere to settle. HSE suggests checking the vessel stays clean at least annually.
Please leave flushing, cleaning and any disinfection to your responsible person and water hygiene contractor. We fix the heating plant that keeps the vessel at temperature.
Where hot water temperatures most often slip
- Commercial kitchens: a big morning draw-off can pull a calorifier or water heater down quickly. If readings only fail at busy times, the plant may be undersized for how the kitchen now works. See our hotels, pubs and restaurants page.
- Schools and colleges after holidays: plant on reduced schedules and outlets unused for weeks. Our school heating winter readiness checklist covers getting plant back up before term.
- Care homes: hot water has to stay hot enough for legionella control while TMVs keep bathing water safe. Our guide to care home hot water, scald risk and TMVs goes into detail.
- Part-empty offices and multi-let buildings: floors and units standing empty leave long runs of pipe with little flow, and the return temperature often shows it first.
- Cold snaps: condensing boilers that lock out on frozen condensate pipes take the hot water down with them. See frost protection for commercial heating plant.
What to send us when a calorifier or water heater will not hold temperature
The quickest repairs start with good information. Before you call, gather what you can:
- The last few months of flow and return readings, so we can see whether the problem is constant, at certain times, or getting worse.
- A photo of the data plate on the water heater or calorifier, and on the boiler serving it.
- Any fault code or lockout on the boiler or heater, with the date and time. Our boiler lockout log sheet makes this easy to keep.
- Whether the shunt or circulation pump is running (you can often feel or hear it), and any recent works such as new outlets, a refit or a change in opening hours.
Send them to WhatsApp 07947 578332 or call 0800 0996 007. Keeping the primary side clean helps the boilers deliver their full output, so ask us about closed-system water treatment at the same visit.
Reading your hot water temperature log: patterns that point to the plant
Most commercial sites already record monthly calorifier and sentinel outlet temperatures. The numbers are only useful if someone looks at how they change over time. A single low reading can be a bad day; the same low reading three months running is a pattern, and patterns usually have a cause in the plant. These are the ones we are most often asked about:
| What the log shows | What it often points to | Who should look at it |
|---|---|---|
| Calorifier flow fine, return well below it | The hot water circulation loop is not moving enough water: a pump, valve or balancing problem on the secondary return | Responsible person, then a competent engineer for the pump and loop |
| Flow drops below 60°C at the same time each day | Demand peaks (kitchen service, shift changes, showers) outrunning the recovery of the boiler or water heater | Engineer to check the heat source and recovery rate against real demand |
| Slow fall over several months | Scale on heat exchange surfaces, or a boiler or water heater losing output and needing a service | Engineer, with the service history to hand |
| Readings fine in summer, low in winter | The boilers are shared between space heating and hot water and the controls give heating priority on cold days | Engineer to review controls and priorities |
| One sentinel outlet always slow to reach 50°C | A long dead leg, poor insulation or a branch that is out of balance with the rest | Responsible person; water hygiene contractor or engineer depending on the cause |
| Readings suddenly much higher | A control or thermostat fault, or a setting changed by someone on site, which raises the scald risk | Responsible person straight away, then an engineer |
When you call us about a hot water temperature problem, the log is the most helpful thing you can send. Three or four months of readings tells an engineer far more than one number on the day.
Quiet periods, closures and reopening
Commercial buildings rarely have steady hot water use all year. Schools close for the holidays, offices run half-empty, holiday sites shut for the winter and parts of a building can sit unused for weeks. When water stops moving, temperatures drift towards the range where legionella can grow, so low use needs planning as much as heavy use does.
- Little-used outlets: HSE guidance includes flushing outlets that are not used regularly, often weekly, and recording it. Your risk assessment should list which outlets fall into this group.
- Before a planned closure: decide with your responsible person whether hot water plant stays running at temperature or is shut down, and what happens before people return. Turning a calorifier off to save energy over a long closure is a decision for the risk assessment, not a quick fix.
- Reopening after a closure: make sure the boiler or water heater is back up to temperature well before people arrive, and that any flushing or checks your risk assessment requires are done and logged.
- Heating plant that was off over summer: where the same boilers serve heating and hot water, the first cold weeks of autumn are when we see hot water temperatures slip, because the plant is suddenly asked to do both. A service before the heating season helps.
If a closure is coming up, our planned maintenance visits can be timed so the plant is checked and running before you reopen. For records that insurers and auditors ask to see, our guide to heating maintenance records sets out what to keep.
Where Fast Commercial Boiler Help fits
Legionella risk assessment, water treatment and sampling are a specialist field, and HSE notes that testing can be carried out by a water treatment company or consultant, or by a trained and supervised operator. Our part is the heating and hot water plant itself. If your responsible person's records show a calorifier or water heater not holding 60°C, a weak return temperature or a unit that keeps locking out, our Gas Safe registered engineers can find and fix the fault on the boilers, water heaters and controls. We also service that plant on a planned preventive maintenance basis, and our guide to water heater pressure relief valve checks covers the safety devices on the same equipment.
Please don't adjust gas water heater or boiler controls to chase a temperature yourself. Talk to your responsible person, because a higher setting changes the scald risk, and get a competent engineer to find out why the plant is underperforming.
Safety note
Under the Gas Safety (Installation and Use) Regulations 1998, gas work must be carried out by a Gas Safe registered engineer, and we apply that rule on every site. Don't attempt DIY work on gas appliances, burners, flues or safety devices. On pressure systems, trained site staff should stick to visual and gauge checks, log the readings and report anything abnormal. Never adjust, cap, plug or test-lift a relief valve yourself.
Talk to a Gas Safe commercial engineer
Fast Commercial Boiler Help is a Gas Safe registered (985448) commercial heating business based at 4 Market Hill, Clare, Sudbury, Suffolk CO10 8NN. Our team of over 10 local engineers covers East Anglia and East London, working on commercial boilers, plant rooms, water heaters, and warm air, radiant and unit heaters. We work on commercial premises only.
Call or WhatsApp 07947 578332 (a photo of the appliance data plate or the fault display helps) · Email info@fastcommercialboiler.help
Frequently asked questions
What temperature should hot water be stored at to control legionella?
HSE says hot water should be stored at 60°C or above and distributed so that it reaches 50°C within 1 minute at the outlets, or 55°C in healthcare premises. Outgoing water from a calorifier should be at least 60°C.
What temperature range does legionella grow in?
HSE says legionella can grow in water systems between 20°C and 45°C, especially where water stagnates. That is why cold water should be kept below 20°C where possible and hot water stored at 60°C or above.
How often should hot water temperatures be checked in a commercial building?
It depends on your risk assessment. HSE's examples include monthly checks of calorifier flow and return temperatures and of sentinel outlets, with findings recorded, plus representative outlets on a rotational basis.
Who is responsible for legionella control in a commercial building?
The employer or person in control of the premises, including landlords. HSE says they must assess and control the risk, keep records and appoint a competent responsible person to take responsibility for controlling the risk.
Can a boiler fault cause a legionella risk?
It can. If the boiler serving a calorifier, or a gas-fired water heater, is faulty or not running as it should, stored hot water may not reach 60°C. A Gas Safe registered commercial engineer should find the fault, and your responsible person should review the monitoring records and the risk assessment.
How do you take a calorifier flow and return temperature?
Use the same measuring points each month, such as the thermometer pocket at the top of the cylinder and on the return leg if fitted. HSE says outgoing water should be at least 60°C, and the return should show the hot water loop is working. Record the date, time and who took it.
What is the right commercial hot water temperature at the taps?
HSE guidance is for hot water to reach at least 50°C within 1 minute at outlets, or 55°C in healthcare premises. Where vulnerable people bathe or shower, thermostatic mixing valves limit the temperature delivered, so a risk assessment must balance scald and legionella risk.
Our calorifier reads below 60°C. Should we turn the thermostat up?
Not yourself. Report it to your responsible person, because the setting affects scald risk too, and ask a competent engineer to find out why the plant is not reaching temperature. Common causes include controls, pumps, scale and boiler faults.
Why is the bottom of our calorifier colder than the top?
Cold mains water enters near the base and hot water rises, so a large vessel can layer, with the lower part sitting well below the outlet temperature, especially where draw-off is heavy and intermittent. A destratification or shunt pump, if fitted, mixes the contents. If the base stays cool, ask your responsible person to review it and have an engineer check the pump, its timer and whether the plant has enough output.
Do plate heat exchanger or instantaneous hot water systems still need temperature checks?
Yes. They store little or no hot water, but the outlets still need to reach the temperatures set in your risk assessment. Low readings on these systems often come from a scaled plate, a sticking control valve or primary water from the boilers that is not hot enough at times of high demand.
Can you repair a gas-fired water heater that will not hold temperature?
Yes. Our Gas Safe registered engineers (985448) repair and service commercial gas-fired storage water heaters, calorifiers and the boilers that serve them. Send a photo of the data plate and any fault display to WhatsApp 07947 578332 or call 0800 0996 007.
Is one low hot water reading something to worry about?
Report it to your responsible person and follow your risk assessment, but look at the trend too. A single low reading may follow a demand peak; the same low reading over several months usually points to a cause in the plant, such as scale, a weak circulation pump or a boiler losing output.
Should we turn the calorifier off while the building is closed?
Only if your legionella risk assessment allows it and sets out how the system will be brought back into use. Stored water that cools sits in the temperature range where legionella can grow, so a shutdown needs a plan for reheating and any flushing before people return.
Why does our hot water get cooler in winter?
On many commercial sites the same boilers serve both heating and hot water. On cold days the heating takes much of the output, and if the controls do not give hot water enough priority the calorifier struggles to recover. An engineer can check the controls and whether the plant is sized for both loads.
Sources and further reading
- HSE: Legionnaires' disease, hot and cold water systems
- HSE: Hot and cold water systems, things to consider
- HSE: Legionella risks in your workplace
- HSE: Legionnaires' disease, what you must do
- HSE: Legionnaires' disease, managing the risk (the responsible person)
- HSE: Legionnaires' disease, preventing or controlling the risk (written control scheme)
- HSE: Legionnaires' disease, monitoring and keeping records
- HSE: Testing and monitoring your water system for legionella
- HSE L8: Legionnaires' disease, the control of legionella bacteria in water systems (ACOP, fourth edition)
- HSE HSG274: Legionnaires' disease, technical guidance (Part 2 covers hot and cold water systems)
- Gas Safety (Installation and Use) Regulations 1998, regulation 35 (legislation.gov.uk)
- HSE: Gas safety for employers
Call or WhatsApp 07947 578332 · info@fastcommercialboiler.help