Water Temperature Monitoring for Legionella Control

Temperature control is the single most important weapon against legionella bacteria in water systems. Get the temperatures right, and you dramatically reduce the risk. Get them wrong, and you create ideal breeding conditions for dangerous bacteria.

This guide explains exactly what temperatures you need to maintain, how to monitor them, where to take readings, how to record them, and what to do when things go wrong.

Why temperature monitoring matters

Legionella bacteria multiply rapidly in water between 20°C and 45°C. At around 35-37°C (human body temperature), they thrive. Temperature control works by keeping water either:

• Too cold for bacteria to multiply effectively (below 20°C)
• Too hot for bacteria to survive (above 60°C storage, above 50°C distribution)

Temperature monitoring tells you whether your control measures are working. It's your early warning system. If temperatures drift into the danger zone, you know you have a problem before bacteria populations explode.

What the regulations say

The Control of Substances Hazardous to Health Regulations 2002 (COSHH) require you to:

• Prevent or control the risk of exposure to legionella
• Ensure control measures are used and maintained
• Monitor to ensure controls remain effective

The HSE's Approved Code of Practice L8 and technical guidance HSG274 specify that temperature control is a primary control measure, and that temperatures should be monitored and recorded regularly.

Monitoring frequency:

L8 and HSG274 recommend monthly temperature monitoring for most premises as a minimum. Higher-risk premises (healthcare, care homes, large buildings) may require weekly or even daily checks.

Critical temperature thresholds

These are the temperatures you're aiming for:

Hot water system temperatures

Storage (calorifiers, cylinders, tanks):

• Target: 60°C or higher throughout the stored water
• At this temperature, legionella bacteria are killed within minutes
• Most UK guidance specifies 60°C as the minimum

Distribution (pipes carrying hot water):

• Target: 50°C or higher throughout the system
• Hot water should reach outlets at 50°C or above within one minute of running the tap

Outlets (taps, showers):

• Target: At least 50°C after one minute of running
• This confirms that hot water distribution is effective
• Sentinel outlets (furthest or most vulnerable) are particularly important

Cold water system temperatures

Storage (tanks, cisterns):

• Target: Below 20°C
• Cold water stored above 20°C allows legionella growth
• Insulation should keep heat out, not in

Distribution (pipes carrying cold water):

• Target: Below 20°C throughout the system
• Pipes should be insulated to prevent warming from surrounding air or proximity to hot pipes

Outlets (taps):

• Target: Below 20°C after running for 2 minutes
• If cold water reaches outlets above 20°C, you have a problem with your cold water system

The danger zone: 20-45°C

Water in this temperature range is where legionella thrives. If your monitoring reveals temperatures in this zone, you need immediate action. This is particularly problematic in:

• Poorly insulated cold water tanks in warm loft spaces
• Cold water pipes running alongside hot pipes
• Blended water systems with TMVs
• Little-used outlets where water sits stagnant
• Systems during summer months when ambient temperatures rise

What to monitor

You can't check every tap and pipe in your building every month. Instead, you monitor representative points that give you confidence in the overall system.

Hot water system monitoring points

1. Calorifiers and hot water cylinders

• Flow temperature: Hot water leaving the calorifier (should be 60°C+)
• Return temperature: Hot water returning to the calorifier after circulation (should be at least 50°C, ideally 55°C+)
• If the return temperature is too low, it suggests heat is being lost in distribution or circulation isn't adequate

2. Sentinel outlets

These are the outlets most vulnerable to legionella risk:

• Furthest outlet: The tap or shower furthest from the hot water source (longest pipe run, most heat loss)
• Least-used outlet: Outlets that are used infrequently (guest bathrooms, vacant rooms)
• Most vulnerable: Outlets serving vulnerable users (care home resident rooms, immunocompromised patient areas)

At sentinel outlets, measure:

• Hot water temperature after running for one minute
• Cold water temperature after running for two minutes

3. TMV outlets (if fitted)

• Measure the blended water temperature
• Separately measure the hot and cold feed temperatures before the valve
• TMVs can create lukewarm water, which may sit in pipework at ideal legionella growth temperatures

Cold water system monitoring points

1. Storage tanks (cisterns)

• Measure the water temperature inside the tank (ideally using a pocket thermometer or probe thermometer, not just surface temperature)
• Take readings in different parts of large tanks (top, middle, bottom) as temperature can stratify

2. Sentinel outlets

Same principle as hot water:

• Furthest outlet from the storage tank
• Least-used outlet
• Outlets in the warmest parts of the building

Measure cold water temperature after running for two minutes to get water that has traveled from the tank, not just water sitting in the outlet.

How many sentinel outlets?

This depends on the complexity of your system:

• Simple small building: 2-4 sentinel outlets may be sufficient
• Large building with multiple floors/wings: At least one sentinel outlet per zone or floor
• Complex system: Your risk assessment should identify specific monitoring points

Monthly monitoring requirements

For most premises, monthly temperature monitoring is the baseline. Here's what a monthly monitoring cycle should include:

Hot water checks (monthly)

1. Calorifier flow temperature: 60°C or higher
2. Calorifier return temperature: 50°C minimum (55°C+ preferred)
3. Sentinel outlet temperatures: 50°C or higher after one minute

Cold water checks (monthly)

1. Storage tank temperature: Below 20°C
2. Sentinel outlet temperatures: Below 20°C after two minutes

TMV checks (monthly, if fitted)

1. Blended water temperature: Check it's at the designed temperature (usually 41-44°C)
2. Hot and cold feeds to TMV: Confirm hot is 50°C+ and cold is below 20°C

What else should be monitored?

In addition to temperatures, monthly checks often include:

• Visual inspection of tanks (lids secure, no contamination, no light ingress)
• Checking insulation is intact on pipes and tanks
• Flushing little-used outlets
• Checking water flow rates (poor flow can indicate problems)
• Checking for leaks, corrosion, or scale buildup

How to take readings correctly

Temperature monitoring is only useful if you do it properly. Here's how to get accurate, meaningful readings:

Equipment you'll need

1. Thermometer

• Digital probe thermometer: Most versatile, suitable for tanks and outlets (recommended)
• Infrared thermometer: Quick surface readings, but less accurate for water temperature
• Pocket dial thermometer: Simple and robust, good for tank readings

Calibration: Your thermometer should be calibrated regularly (at least annually) to ensure accuracy. Keep records of calibration.

2. Personal protective equipment (PPE)

• Heat-resistant gloves: Water at 60°C can cause severe burns
• Safety glasses: Protect against splashes when checking hot systems
• Consider face protection when working near hot calorifiers

3. Recording equipment

• Pre-printed logbook or temperature monitoring sheets
• Clipboard
• Pen (not pencil - records should be permanent)

Taking readings at outlets (taps and showers)

For hot water:

1. Run the tap or shower at full hot (no cold water blended)
2. Let it run for one minute by the clock
3. Fill a container or hold the probe in the water stream
4. Take the temperature reading
5. Record the temperature, date, time, location, and your initials

For cold water:

1. Run the tap at full cold
2. Let it run for two minutes by the clock (to ensure you're getting water from the tank, not just pipe water)
3. Take the temperature reading
4. Record the result

Why these timings?

• One minute for hot water ensures you're measuring distributed water, not just the small amount sitting in the pipework near the outlet
• Two minutes for cold water ensures you're drawing water that has traveled from the storage tank

Taking readings at calorifiers

Flow temperature:

• Locate the flow thermometer or temperature gauge on the calorifier outlet pipe
• If using a surface probe, ensure good contact with the pipe (dry the pipe first)
• Wait 10-15 seconds for the reading to stabilize
• Record the temperature

Return temperature:

• Locate the return thermometer on the return pipe to the calorifier
• Same process as flow temperature
• The return should be at least 50°C, ideally 55°C+

Many modern calorifiers have built-in thermometers or digital displays. Use these, but verify them periodically with your own calibrated thermometer.

Taking readings at storage tanks

For cold water tanks:

1. Remove the lid (check it's secure and fitted correctly first)
2. Use a probe thermometer to measure water temperature
3. Take readings in different parts of large tanks (temperatures can vary)
4. Don't just measure surface temperature - measure at depth
5. Replace the lid securely

Safety note: Cold water tanks can be in awkward locations (lofts, basements). Ensure safe access, adequate lighting, and don't work alone if access is difficult.

Common mistakes to avoid

Mistake 1: Not running water long enough

• Taking hot water readings immediately means you're measuring water sitting in the pipe, not system temperature
• Solution: Always wait the full minute for hot, two minutes for cold

Mistake 2: Using an uncalibrated thermometer

• Inaccurate readings give false confidence or trigger unnecessary alarms
• Solution: Calibrate annually, check against known temperature points periodically

Mistake 3: Taking readings at inconsistent locations

• One month you check the staff toilet, next month a different outlet
• Solution: Use the same sentinel outlets each month so you can spot trends

Mistake 4: Measuring blended water when you should measure hot

• If the cold tap is slightly open or a TMV is active, you're not measuring true hot water temperature
• Solution: Ensure only hot water is running during hot checks

Mistake 5: Not recording immediately

• Relying on memory leads to errors and lost data
• Solution: Record the reading on the spot before moving to the next location

Record keeping requirements

Temperature monitoring is only useful if you keep proper records. These records:

• Demonstrate ongoing compliance with L8 and COSHH
• Help you spot deteriorating performance or seasonal variations
• Provide evidence in case of an investigation or prosecution
• Show trends over time that might indicate developing problems

What to record

Every temperature reading should be documented with:

1. Basic information:

• Date and time of reading
• Location (specific tap, shower, or equipment)
• Temperature reading (in °C)
• Name or initials of person taking reading

2. Additional useful information:

• Weather/ambient temperature (can affect cold water tanks)
• Any observations (water discolored, low flow, unusual odor)
• Any remedial actions taken
• Follow-up required

Record formats

Paper logbooks:

• Pre-printed sheets with monitoring locations listed
• Simple to use, no technology required
• Should be kept in a dedicated folder or binder
• Use permanent ink, not pencil

Digital records:

• Spreadsheets, databases, or specialist software
• Easier to analyze trends and generate reports
• Should be backed up regularly
• Some systems allow thermometers to upload readings directly

Photographs:

• Some premises photograph calorifier gauges as supporting evidence
• Not a replacement for written records, but useful supplementary documentation

How long to keep records

Minimum: 5 years

The HSE recommends keeping legionella control records for at least five years. This aligns with general health and safety record-keeping requirements.

Better: Keep indefinitely

Many organizations keep water safety records indefinitely because:

• Legionnaires' disease has an incubation period of 2-10 days, but cases can be linked to exposures months or years earlier during investigations
• Long-term trend data is valuable for understanding system performance
• In the event of a prosecution, older records may be relevant

Storage:

• Keep records secure, dry, and organized
• Ensure records are accessible to those who need them (responsible person, auditors, inspectors)
• Back up digital records
• Consider keeping copies off-site in case of fire or flood

What to do if temperatures are out of range

Finding an out-of-range temperature is not a failure - it's the monitoring system working. Now you need to respond appropriately.

Hot water too low

If hot water at outlets is below 50°C:

Immediate actions:

1. Recheck the reading to confirm it's accurate
2. Check other hot water outlets - is it localized or system-wide?
3. Check calorifier temperature - is it at 60°C?
4. Record the findings

Investigation:

• Is the calorifier thermostat set correctly?
• Is the calorifier heating element working?
• Are circulation pumps operating?
• Is there an issue with the specific outlet (TMV fault, pipe problem)?
• Has there been a change in demand or usage pattern?

Short-term actions:

• Increase calorifier temperature if it's too low
• Adjust boiler/heating controls
• Isolate and label the affected outlet if it serves vulnerable users
• Increase flushing frequency if it's a little-used outlet

Long-term solutions:

• Install trace heating on long pipe runs
• Improve insulation
• Upgrade undersized calorifier
• Remove dead-legs or rarely-used outlets
• Install a more powerful heating system

Cold water too warm

If cold water at outlets is above 20°C:

Immediate actions:

1. Recheck the reading to confirm
2. Check if it's localized (one outlet) or system-wide
3. Check the cold water storage tank temperature
4. Record findings

Investigation:

• Is the tank insulated properly? (It shouldn't be - insulation keeps heat in)
• Is the tank exposed to heat sources (hot pipes nearby, warm loft space)?
• Are cold pipes running alongside hot pipes?
• Is it summer and the ambient temperature is particularly high?
• Is the cold water being stored for too long (oversized tank)?

Short-term actions:

• Increase flushing frequency to bring fresh, cooler water through
• Investigate and rectify any immediate causes (leaking hot pipe warming the area)

Long-term solutions:

• Relocate cold water tank to a cooler location
• Install cold water tank in an air-conditioned plant room
• Insulate cold pipes to keep heat out (different from insulating to keep heat in)
• Separate hot and cold pipe runs
• Reduce tank size to increase turnover
• Install a chilled water system (rare, used in some specialist premises)

Calorifier return temperature too low

If return temperature is below 50°C:

This suggests heat is being lost in the distribution system, or circulation isn't adequate.

Investigation:

• Is the circulation pump working?
• Are all circuits circulating properly?
• Is insulation damaged or missing on return pipes?
• Are there dead-legs causing heat loss?

Actions:

• Check and repair/replace circulation pump
• Check insulation throughout system
• Balance the system to ensure even circulation
• Remove dead-legs where practical

Seasonal variations

Summer:

• Cold water temperatures may rise due to higher ambient temperatures
• Tanks in lofts can be particularly affected
• You may need more frequent flushing or temporary measures

Winter:

• Cold water temperatures naturally fall, often well below 20°C
• Hot water systems may struggle to maintain temperature in very cold weather
• Frozen pipes can cause system disruptions

Record seasonal trends:

Note seasonal variations in your records. This helps you:

• Anticipate problems each year
• Plan seasonal adjustments (increasing calorifier temperature in winter, more frequent flushing in summer)
• Distinguish between seasonal variations and genuine system failures

Remedial actions and escalation

When you find a temperature problem, your response should be proportionate to the risk:

Low-risk findings

Example: One sentinel outlet is slightly out of range on one occasion, but other readings are normal.

Response:

• Re-test the same outlet next day or week
• If problem persists, investigate that specific outlet
• Continue routine monitoring

Medium-risk findings

Example: Multiple outlets showing temperatures in the 20-45°C range, or calorifier return temperature consistently low.

Response:

• Immediate investigation by competent person
• More frequent monitoring until resolved
• Implement interim controls (increased flushing, usage restrictions)
• Fix within a defined timeframe (days to weeks depending on circumstances)

High-risk findings

Example: System-wide temperature failures, cold water consistently above 25°C, hot water storage below 55°C, presence of vulnerable users.

Response:

• Immediate action required
• Consider restricting use of affected outlets
• Call in specialist contractor if internal expertise is insufficient
• Increase monitoring frequency (daily or more often)
• Notify responsible person and senior management
• Consider water sampling to assess bacterial contamination
• Document all actions taken

When to call in external help

You should consult a water treatment specialist or legionella control expert if:

• Temperature problems persist despite your remedial efforts
• You don't understand the cause of the problem
• Remedial work requires specialist skills (calorifier repair, system redesign)
• You have a vulnerable user population
• Water sampling shows high levels of legionella
• You've had a suspected case of legionnaires' disease linked to your premises

Special considerations for different premises

Landlords and residential properties

Simple domestic properties:

• Monthly checks are often impractical for landlords of simple single-let houses
• Annual checks may be appropriate, combined with inspection during routine maintenance
• Provide tenants with guidance on running taps after holidays

HMOs and multi-occupancy:

• Monthly monitoring of communal hot and cold systems
• Check water temperatures in void rooms or rooms with vulnerable tenants
• Keep records accessible to managing agents and inspectors

Care homes and healthcare settings

Enhanced requirements:

• Weekly temperature monitoring (not just monthly)
• More sentinel outlets (at least one per floor or ward)
• Priority for monitoring near immunocompromised or elderly residents
• Immediate action on any out-of-range reading
• Regular audits by water safety group or responsible person

Hotels and guest accommodation

Focus on infrequently used rooms:

• Vacant rooms create stagnation risk
• Temperature check should follow vacancies
• Flushing regime before guest check-in
• Monitor after periods of low occupancy (off-season)

Offices and commercial premises

Lower intensity, but still essential:

• Monthly monitoring is usually sufficient
• Focus on bathrooms, kitchens, showers (if present)
• Monitoring after building closures (holidays, lockdowns)
• Consider seasonal variations in office temperature

Leisure facilities with spa pools

Spa pools require different approach:

• Spa water is typically 30-40°C (ideal for legionella)
• Temperature monitoring daily or more frequently
• Chemical disinfection (chlorine/bromine) is primary control, not temperature
• Regular water quality testing (chemical levels, pH, bacterial counts)

UK guidance: HSG274

The HSE's technical guidance HSG274 provides detailed advice on legionella control. It's published in three parts:

HSG274 Part 2: Hot and Cold Water Systems

Relevant sections on temperature monitoring:

• Section 2.53-2.62: Temperature control
• Section 2.82-2.88: Monitoring and inspection
• Appendix 2: Operational monitoring and inspection checklist

Key recommendations:

• Monthly monitoring of temperatures at sentinel outlets
• More frequent checks (weekly or continuous) for higher-risk premises
• Calorifier flow should be 60°C, return at least 50°C
• Hot water at outlets should reach 50°C within one minute
• Cold water should be below 20°C

HSG274 Part 3: Other Risk Systems

Covers cooling towers, spa pools, and other specialized systems. Temperature monitoring requirements vary by system type.

HSG274 and L8 relationship

• L8 (ACOP): Approved Code of Practice, has special legal status
• HSG274: Technical guidance, provides practical detail on implementing L8
• Both are published by HSE and should be used together

Continuous monitoring systems

Some premises use automated continuous temperature monitoring:

How they work

• Temperature sensors fitted at key points (calorifier flow/return, sentinel outlets)
• Sensors connected to a data logging system or building management system (BMS)
• System records temperatures continuously (e.g., every 15 minutes)
• Alarms trigger if temperatures go out of range

Advantages

• Early detection of problems (hours, not weeks)
• Comprehensive data for trend analysis
• Reduces manual monitoring workload
• Provides evidence of continuous compliance

Disadvantages

• Higher initial cost
• Requires maintenance and calibration
• Can generate false alarms
• Doesn't replace the need for physical checks and system inspections

When continuous monitoring makes sense

• Large or complex buildings with extensive water systems
• High-risk premises (hospitals, care homes)
• Buildings where manual monitoring is difficult or dangerous
• Premises with history of temperature control problems

Continuous monitoring is a supplement to, not a replacement for, your regular inspection and maintenance program.

Training and competence

The person conducting temperature monitoring should be competent, meaning they:

• Understand why temperature control matters for legionella control
• Know what temperatures to look for
• Can take accurate readings using appropriate equipment
• Know how to record results properly
• Understand when to escalate concerns
• Can work safely around hot water and in tank spaces

Training options

In-house training:

• Suitable for simple systems
• Senior staff or responsible person can train operational staff
• Document the training (date, content, attendees)

External training:

• City & Guilds legionella awareness or control courses
• Legionella Control Association (LCA) training
• Water treatment company training sessions
• One-day legionella awareness courses widely available

Refresher training:

• Every 2-3 years, or when procedures change
• When new staff take on water safety duties
• After incidents or near-misses

Frequently asked questions

Temperature monitoring checklist

Use this checklist for your monthly monitoring:

Before you start:

• Calibrated thermometer available
• PPE ready (heat-resistant gloves, safety glasses)
• Logbook or recording sheets prepared
• Familiar with locations of calorifiers, tanks, and sentinel outlets

Hot water system:

• Calorifier flow temperature recorded (target: 60°C+)
• Calorifier return temperature recorded (target: 50°C+, ideally 55°C+)
• Sentinel outlet 1: Hot temperature after 1 minute (target: 50°C+)
• Sentinel outlet 2: Hot temperature after 1 minute (target: 50°C+)
• Sentinel outlet 3: Hot temperature after 1 minute (target: 50°C+)
• [Additional sentinels as identified in risk assessment]

Cold water system:

• Cold water storage tank temperature (target: below 20°C)
• Sentinel outlet 1: Cold temperature after 2 minutes (target: below 20°C)
• Sentinel outlet 2: Cold temperature after 2 minutes (target: below 20°C)
• Sentinel outlet 3: Cold temperature after 2 minutes (target: below 20°C)
• [Additional sentinels as identified in risk assessment]

TMVs (if fitted):

• TMV blended temperature at safe level (typically 41-44°C)
• Hot feed to TMV at 50°C+
• Cold feed to TMV below 20°C

Visual checks:

• Cold water tank lids secure and in place
• No visible contamination in tanks
• Insulation intact on hot pipes
• No leaks or corrosion visible
• No unusual odors or discoloration

Follow-up actions:

• Any out-of-range readings investigated
• Remedial actions documented
• Responsible person notified of any concerns
• Logbook completed and signed

Review:

• Trends analyzed (are any readings consistently borderline?)
• Seasonal patterns noted
• Equipment calibration up to date
• Next monitoring date scheduled

Next steps

Want to understand the bigger picture of legionella control?

What is legionella? →

Not sure if you need a formal risk assessment?

Do I need a legionella risk assessment? →

Concerned about your monitoring findings?

Related articles:

• What is legionella?
• Do I need a legionella risk assessment?
• Legionella testing and water sampling

Useful tools:

• Legionella Risk Self-Check
• Responsibility Checker