Hot water: scalds, legionella and the tank
70 min
Two hosts talk the lesson through. The voices are synthetic; the script was written from this lesson and checked against it, and asserts nothing the lesson does not.
- Compare the scald and legionella temperature advice, and explain what each body's advice protects against and what its page leaves out
- Apply CPSC's tap-thermometer check and read the result against CPSC's exposure times
- Identify a storage water heater's T&P valve or an unvented cylinder's tundish, and state what the makers read here say about checking each and who deals with a fault
The same tap can hurt two ways. Set the heater at 140°F (60°C) and, on US figures, the water that comes out of it can give most adults a third-degree burn in six seconds; most people hurt that way are very young or very old. And, on British guidance, stored water that isn't kept hot and moving can grow the bacteria behind Legionnaires' disease, a potentially fatal pneumonia. The official advice on each is clear, and the two pull in opposite directions. The US Consumer Product Safety Commission's scald sheet says nothing about legionella, and the legionella page from Great Britain's Health and Safety Executive says nothing about scalds, so a household that reads only one of them has heard about one hazard. This lesson gives you both, shows you how to measure what your own tap delivers, and then looks at the valve on a hot water tank whose whole job is to stop it bursting.
This course is education, not a trade qualification, and not legal advice about your own home. It teaches no gas work and no wiring. If you smell gas in Great Britain, open doors and windows, turn the gas off at the meter if you can do it safely (not if the meter is in a cellar or basement), and call 0800 111 999; in Northern Ireland the number is 0800 002 001. In the US, leave straight away and call 911 or your gas utility once you're outside. In either country, don't light a flame or touch an electrical switch. Isolate the water or power before you open anything up, and if a breaker, RCD or GFCI keeps tripping, leave it off and call an electrician. Which jobs the law leaves to a registered or licensed trade differs between England, Wales, Scotland, Northern Ireland and each US state, and it changes.
Nothing in this lesson opens a water heater or a cylinder. The measuring is done at a tap, and the valves are for looking at. Where a source describes changing a setting, the lesson says whose instructions to follow, and any thermostat that sits behind a cover on an electric heater, a US electric water heater or a UK cylinder's immersion heater, is a job this course labels Call someone. Parts of this lesson describe what British guidance asks of landlords. This is general information, not legal advice. The rules differ between England, Wales, Scotland and Northern Ireland, and between US states and cities, and they change.
Two warnings that pull opposite ways
One household keeps its hot water at 60°C and another at 49°C. Before reading on: roughly how long would it take water from each tap to cause a third-degree burn in most adults? Guess in seconds.
Show the answer
On the US Consumer Product Safety Commission's figures (CPSC, a US federal agency, on a sheet dated 2012), about six seconds at 60°C (140°F) and about five minutes at 49°C (120°F).1 The chart below has the four temperatures CPSC gives.
The scald side
CPSC says most tap water scald injuries and deaths are to "the elderly and children under the age of five", and "urges all users to lower their water heaters to 120 degrees Fahrenheit."1 Its exposure times:
CPSC Publication 5098, "Avoiding Tap Water Scalds", dated 2012: "Most adults will suffer third-degree burns if exposed to 150 degree water for two seconds", six seconds at 140, thirty seconds at 130, and "Even if the temperature is 120 degrees, a five minute exposure could result in third-degree burns."1 A US figure for adults. CPSC says most tap water scald injuries and deaths are to the elderly and children under five. The time axis is logarithmic, so equal steps are tenfold.
Look at the second bar. 140°F is 60°C, the temperature HSE names for stored hot water, which you're about to meet. At that temperature CPSC's figure for most adults is six seconds. Every ten Fahrenheit degrees cooler (about five and a half Celsius) multiplies the time by three, then five, then ten, on this course's arithmetic from CPSC's four figures.
CPSC's way to check yours is safe and needs no tools: hold a candy (sugar) or meat thermometer under the hot tap first thing in the morning, or at least two hours after water was last used.1 That test is Yours. Changing the setting is not always. For an electric water heater, CPSC describes turning off the power at the breaker, removing access panels and setting two thermostats, and suggests calling the electricity company; for a gas one it says to call the gas company for instructions.1 This course labels any thermostat behind a cover on an electric water heater or immersion heater Call someone, because it means opening panels on a wired appliance, and this course never treats a switched-off breaker as proof that what's behind a panel is dead (lesson 5, "Electricity: what trips, and why"). One maker, A. O. Smith, calls 120°F the "preferred starting point" on the dial of its own gas water heaters and warns that turning past it "will increase the risk of scald injury".3 If you change a setting you can reach without opening anything, do it by the heater's manual.
CPSC's sheet doesn't rest on the thermostat alone. It says "Always hand-test before using, especially when bathing infants and young children", and that leaving a child unsupervised in the bathroom, even for a second, could cause serious injuries.1
The legionella side
This is general information, not legal advice.
HSE's guidance for landlords, from Great Britain's workplace health and safety regulator, calls Legionnaires' disease "a potentially fatal form of pneumonia" caught by breathing in droplets of contaminated water, and says: "All man-made hot and cold water systems are likely to provide an environment where legionella can grow."2 For most homes, it says, "temperature is the most reliable way of ensuring the risk of exposure to legionella bacteria is minimised", and it puts the rule in one line: "keep the hot water hot, cold water cold and keep it moving." One of the controls it names is setting the cylinder "to ensure water is stored at 60°C".2 That 60°C is a figure for the stored water in a cylinder. The same page's example of a typical low-risk home has hot water from instantaneous or low-volume heaters "supplying outlets at 50°C".2 HSE also gives a thermometer a place on its side: it calls temperature monitoring "a reliable method for confirming the water system is under control".2
Two homes: one has a combi boiler and an electric shower, the other a system boiler with a cylinder in the airing cupboard. Before reading on: on HSE's account, which carries the lower legionella risk, and why?
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The combi home. HSE says the risk is lower where instantaneous water heaters, "for example combi boilers and electric showers", are fitted, "because there is no water storage".2 A combi heats water as it's drawn, as the Energy Saving Trust describes it, so there's no store to sit and cool.[7]
HSE's page is about a landlord's duties in Great Britain. The duty it describes is to assess the risk from legionella to tenants, and it says that doesn't require "an in-depth, detailed assessment"; if a tenant catches Legionnaires' disease from the water system in their home, it says, the landlord "may be liable to prosecution under HSWA", the Health and Safety at Work Act.2 It says the risks in most homes are "generally considered to be low owing to regular water usage and turnover", and, as the answer above showed, lower again with no stored water; and that testing for legionella "is not usually required for domestic hot and cold water systems".2 Where a property stands empty, it says outlets should be used at least once a week to stop water standing.2 HSE says nothing on this page about owner-occupiers, so for them it's guidance by analogy, not a duty.
Holding both
One thing to know before you weigh them: the scald figures here are American (CPSC, a US federal agency) and the legionella guidance is British (HSE, for landlords in Great Britain). This course read no UK source on scald temperatures in private homes and no US source on legionella in homes, so it can't tell you how either country's authorities weigh the other hazard in a home. The one UK source it read that weighs both is written for care settings, and comes at the end of this section.
So "turn it down" answers CPSC's hazard and "turn it up" answers HSE's, and neither of those two pages speaks to the other hazard. Turning a cylinder down lowers the scald risk and moves the stored water away from the 60°C that HSE names as a control; that's this course's reading of HSE's page, which gives no figure for how much the risk changes. Both hazards are established. Which way a household leans is a judgement about who lives there: a toddler or a very old person in the bath, or a store of hot water that sits unused for weeks. This course doesn't pick a number for you.
One maker's manual mentions a device between the two. A. O. Smith, about its own heaters, says: "Valves for reducing the point-of-use temperature by mixing cold and hot water are available", and sends readers to "a licensed plumber or the local plumbing authority".3 It offers them as a scald measure, alongside the lowest setting that meets the household's needs, and says nothing about legionella.
HSE does weigh the two together, but not for homes. Its page on scalding and burning is written for health and social care settings in Great Britain, where the people using the water may be vulnerable, and it adds hostels and staffed or sheltered housing. It says scald precautions "should not introduce other risks, for example from legionella bacteria". Where stored water above 60°C and distributed water above 50°C are being used to control legionella, it says baths and showers should be fitted with "thermostatic mixer valves (TMVs)" so the water doesn't go above 44°C; at basins, where there's no full body immersion, it calls the risk much lower, though for someone assessed as vulnerable at basins it says controls on the outlet temperature "should be in place".[8] That's HSE's guidance for care settings, and it says nothing about private homes. This course read no source, in either country, on whether a mixing valve lets a household keep stored water hot for legionella and still deliver cooler water at the tap, so it can't tell you that. In a home, it's a question to raise with a plumber or installer.
A combi, and a man who lives alone
This is the simple case. A man in his eighties lives alone in a bungalow in Carmarthenshire. His hot water comes from a combi boiler, and he has an electric shower. His daughter worries about him scalding himself at the basin.
On the legionella side, HSE's account puts his home at the low end: no stored water, with a combi and an electric shower, and water used every day.2
Before reading on: which side leans in this home, and what's the first thing the family should do?
If the reading is high, whether and how a combi's hot water temperature can be set is in its user instructions, which this course didn't read for any model, so the family starts there or asks at the boiler's next service. Nothing in this case needs the boiler opened.
A family in rural West Virginia have a gas storage water heater set, by the tap thermometer, at 140°F (60°C). They have a two-year-old. Using this lesson's sources, what are the two things they should weigh, and who does the one maker's manual this lesson read send them to about lowering the temperature at the tap?
Show the answer
The scald side: at 140°F CPSC gives six seconds to a third-degree burn in most adults, and CPSC says young children are among those most often hurt. CPSC urges 120°F.1 The legionella side: HSE, a British regulator writing for landlords, treats 60°C storage as a control against legionella, and says all man-made hot and cold water systems are likely to provide an environment where it can grow.2 Both are real. A. O. Smith, about its own heaters, describes mixing valves at the heater or at the taps as a way to cut scald risk, and sends people to a licensed plumber or the local plumbing authority.3 It doesn't say they answer legionella, and this course read nothing that says so for a home. The lesson picks no temperature; the family weighs who is in the house.
The tank's safety valve, and the tundish
A US storage water heater has a temperature and pressure relief (T&P) valve. A. O. Smith's manual for its own gas water heaters says what it is for: "The purpose of a temperature-pressure relief valve is to prevent excessive temperatures and pressures in the storage tank."3 On A. O. Smith's heaters a discharge pipe runs from the valve, and the manual requires it to end no more than 6 inches (about 15 cm, this course's conversion) above a floor drain, or outside the building; the manual's warning label says to keep clear of the discharge.3 The University of Georgia's home maintenance checklist lists testing the valve in spring and autumn, and A. O. Smith recommends checking it "at least once a year".43 The same manual warns that the water "may be extremely hot", and that if the valve won't reset after it is operated, the cold inlet has to be closed, the tank drained and the valve replaced.3 That's why this course teaches no test: the method is your heater's manual, and if you'd rather not, it's a job to have checked. Yours, on conditions (your own heater's manual), or Call someone.
A. O. Smith also says excessive water pressure is the most common cause of a leaking T&P valve, usually from thermal expansion in a closed system. In the manual's words, "As water is heated, it expands." Where devices such as pressure-reducing valves or check valves are fitted on the supply, the manual calls the system closed, and in a closed system, it says, the heated water's growing volume raises the pressure.3 The valve is not meant to relieve that constantly; for the expansion tank the manual calls for, it sends readers to a plumbing service agency or the retailer.3
Draining sediment from a tank follows the same rule. A. O. Smith recommends draining and flushing its tanks every six months, while the American Society of Home Inspectors suggests draining a gallon or two every month.3[5] They differ; your own heater's manual decides.
In a UK home with an unvented cylinder (fed straight from the mains, as lesson 7, "Heating: boilers, radiators and furnaces", described), the thing to watch is the tundish. Vaillant describes the tundish as a fitting near the pressure release valve that lets you see whether water has escaped, and says a dripping tundish is a sign of a fault with the valve or of pressure rising too high, for "a qualified boiler engineer".[6] Call someone.
What people get wrong
"Turn the water heater down to be safe." Safer from scalds, on CPSC's times; but HSE treats 60°C storage as a control against legionella. Half the picture.12
"Turn it up to be safe." Safer from legionella in a stored system, on HSE's account; but at 140°F (60°C) CPSC gives most adults six seconds to a third-degree burn. The other half.12
"Legionella means every home needs testing." HSE says testing "is not usually required for domestic hot and cold water systems", and that health and safety law doesn't require landlords to produce a "legionella test certificate".2 The general-information note at the top of "The legionella side" applies.
"A T&P valve that drips should be capped." A. O. Smith: "Do not plug, block, or cap the discharge line." A weeping valve most often means excessive water pressure, and the manual sends that to a plumbing service agency or the retailer.3
"A dripping tundish is normal." Vaillant sends it to a qualified engineer.[6]
Practice
Take 25 minutes over this. Nothing here opens or adjusts anything, so it suits a renter too.
Take the tap's temperature. First thing in the morning, before anyone has run hot water, run a hot tap you use for washing and hold a thermometer in the stream, not your hand. Write the reading in °C and °F, and put it against CPSC's four temperatures in the chart.
Find the safety valve and look at it. US storage heater: find the T&P valve and follow its discharge pipe to where it ends. Don't touch or operate the valve, and keep clear of the pipe's end. Note whether the pipe end is open and clear, and whether there's any sign of water. UK unvented cylinder: find the tundish, the funnel with an air gap in the pipe near the cylinder, and note whether it's dry. Vented cylinder, combi or tankless heater: write down what you have and skip this step.
Find what your manual says. Look up your heater's or cylinder's manual (a drawer, a pocket on the heater, or the maker's website). Write down what it says about the temperature setting, the relief valve and draining, and who it says should do each.
Check your notes against this. What does each finding tell you, and whose job is the next step?
Show the answer
Tap reading at or above 60°C (140°F): on CPSC's figures, six seconds or less to a third-degree burn for most adults, and it's at or above the 60°C that HSE names for stored water against legionella. Weigh who lives there, and HSE's page on legionella; if you want it lower, follow the heater's own instructions, and for any thermostat behind a cover on an electric heater or immersion heater, this course's label is Call someone. If you rent in Great Britain, HSE's guidance is that tenants don't adjust the cylinder and tell the landlord about problems; in a US apartment whose hot water comes from the building's furnace, CPSC says to contact the building manager.12 Reading at or below 49°C (120°F): in CPSC's words, a setting of 120°F "may be necessary for residential water heaters to reduce or eliminate the risk of most tap water scald injuries".1 If the water comes from a stored cylinder or tank, it's also below the 50°C at the outlets in HSE's own low-risk example, and HSE's control for stored water is 60°C.2 Weigh who lives there, as above, and the heater's manual. Water at the T&P pipe, or a dripping tundish: Call someone, a plumber for a weeping T&P valve (A. O. Smith) or a qualified engineer for the tundish (Vaillant).3[6] The manual's own intervals for testing the valve or draining the tank: those go on your calendar, with the manual as their source.
For your plan
This lesson adds to the calendar: the tap temperature, taken again whenever the setting or the household changes (this course's suggestion; no source read gives an interval), and your heater's own intervals for checking the T&P valve and draining the tank, from its manual (A. O. Smith's are once a year and every six months for its own heaters). It adds to the map: the water heater or cylinder, its type, and where the T&P valve's pipe or the tundish is. It adds to the stop list: a T&P valve test without the heater's manual, a capped or blocked discharge pipe, a dripping tundish, and any thermostat behind a cover on an electric water heater or immersion heater.
Connections
Lesson 1, "How a house works, and where it switches off", listed what this course leaves out; this lesson adds testing a water heater's relief valve to that list, and says why. Lesson 7, "Heating: boilers, radiators and furnaces", sorted the boilers and cylinders this lesson measured. First Aid and CPR lesson 10, "Burns, cuts and small injuries", covers what to do for a scald; this lesson covered only how hot the water is. Lesson 5, "Electricity: what trips, and why", is where the breaker behind an electric water heater comes in. Next, lessons 9 and 10, "Gas: the smell and the valve" and "Gas: the law and the engineer", take the fuel side of a gas water heater or boiler.
Go deeper
- US CPSC, Publication 5098, "Avoiding Tap Water Scalds", free. Read here in full. One page: the exposure times and how to take your tap's temperature.
- HSE, "Legionella and landlords' responsibilities", free. Read here in full. Written for landlords in Great Britain: why HSE counts the legionella risk in most homes as low, what it names as controls, and what a landlord's assessment covers.
- A. O. Smith, Residential Gas Water Heaters: Installation and Operating Manual, 2014, free. Read here in its sections on temperature, the relief valve and its installation, thermal expansion, maintenance and draining. One maker's manual for its own direct-vent models, and an example of what a heater's manual may cover.
Sources
- US Consumer Product Safety Commission, Publication 5098, "Avoiding Tap Water Scalds", dated 032012 on the sheet. A US federal agency. Read: full text; re-read 25 September 2026.
- Health and Safety Executive, "Legionella and landlords' responsibilities". Read: full text; re-read on the live page 25 September 2026. Guidance on landlords' duties in Great Britain.
- A. O. Smith, Residential Gas Water Heaters: Installation and Operating Manual, direct vented models, 326940-001,
- Read: the sections on temperature regulation, the temperature-pressure relief valve and its test, the relief valve's installation requirements and Figure 8, closed systems and thermal expansion, maintenance, and draining and flushing, 25 September 2026. A maker's manual for its own heaters.
- P. R. Turner and L. Bailey, Home Maintenance Checklist, Circular 1082, University of Georgia Cooperative Extension, reviewed June 2021. Read: full text.
- American Society of Home Inspectors, "Plumbing System". Read: full text. A US trade body for home inspectors.
- Vaillant, "What is a tundish?". Read: full text, 25 September 2026. A maker's page about unvented systems; used only for what a tundish shows and who to call when it drips.
- Energy Saving Trust, "Boilers", read from Internet Archive capture 20260919232017 (the live site refuses automated readers). Read: full text.
- Health and Safety Executive, "Scalding and burning", updated 14 November 2024. Read: full text, 25 September 2026. Guidance for health and social care settings in Great Britain, not private homes.
Check your understanding
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