Set your hot water cylinder to store water at 60°C. HSE guidance on hot and cold water systems asks for stored water at 60°C or above, and for water distributed at 50°C or above, because legionella bacteria multiply between 20°C and 45°C. The temperature at your taps is a separate number. In England, the hot water supply to a bath in a new dwelling must not exceed 48°C under Approved Document G. Store hot, deliver cooler — that is the whole idea, and the rest of this page explains how the two numbers fit together.
What temperature should a hot water cylinder be set to?
Set a hot water cylinder to store water at 60°C. HSE guidance on hot and cold water systems asks for storage at 60°C or hotter, with water distributed around the property at 50°C or hotter. Legionella bacteria multiply between 20°C and 45°C and do not survive above 60°C. That single fact is where the number comes from.
Cylinder and boiler manufacturers give much the same figure. Vaillant states 60–65°C for a UK hot water tank, and notes that the legionella risk rises below 50°C. Viessmann quotes at least 65°C for systems with a cylinder. Read those as each manufacturer's figure for their own product, and follow the instructions supplied with your cylinder.
HSE also asks for stored temperatures to be checked monthly, measured at the cylinder outlet. On a domestic system that check normally happens when an engineer visits, not every month by the householder.
Store hot, deliver cooler. The cylinder holds water at 60°C so bacteria cannot establish. A thermostatic mixing valve on the outlet brings the temperature down before it reaches a bath or basin. Two different numbers, one system, and both are there for a reason.
Why is bath water cooler than 60°C? The 48°C rule in England
Bath water is limited on purpose, separately from the cylinder setting. In England, requirement G3(4) of Schedule 1 to the Building Regulations 2010, explained in Approved Document G, says the hot water supply to a bath must be designed and installed so the temperature does not exceed 48°C. It applies to new dwellings and to a material change of use. A thermostatic mixing valve (TMV) — a valve that blends hot and cold water to a set temperature — is the usual way that limit is met.
HSE sets a tighter figure where vulnerable people use a bath or shower and the scalding risk is significant: a Type 3 TMV delivering no more than 44°C. HSE also expects hot water to reach at least 50°C at the tap within one minute, and 55°C in healthcare premises. Outlet temperatures and the TMVs themselves need monitoring and maintenance to keep working.
So 60°C in the cylinder and 48°C at the bath do not contradict each other. One figure controls bacteria in stored water. The other controls scalding at the point where someone puts a hand in.
Can you turn the hot water cylinder temperature down to save energy?
Do not take stored water below 60°C to save energy. Advice about turning a boiler's flow temperature down is aimed at combi boilers, which heat water on demand and store none. Baxi states that with a system or heat-only boiler feeding a hot water cylinder the flow temperature should not be reduced, because of the legionella risk. Vaillant treats 60°C as the efficient setting for a tank rather than a compromise.
There is still room to cut waste without touching the stored temperature:
- Time the hot water so the cylinder reheats shortly before you need it, instead of all day
- Insulate the cylinder and the exposed pipe runs around it
- Set the room thermostat and programmer to your actual routine, not a default one
- Fix a dripping hot tap, because every drip is stored water you have already paid to heat
Programmer times and thermostat settings are yours to change. The stored temperature at the cylinder is not a dial for saving money.
What is already preset inside an unvented cylinder such as a Megaflo?
An unvented cylinder arrives with its controls and safety devices preset. An unvented cylinder is a sealed cylinder fed at mains pressure, so it needs more protection than an old open-vented tank. Heatrae Sadia's manuals describe a temperature and pressure relief valve set at 90°C and 10 bar, an indirect thermostat, and a separate thermal cut-out — a device that shuts the heat source off if the water gets too hot. Immersion thermostats are adjustable between 10°C and 60°C and leave the factory set at 60°C, as protection against waterborne bacteria including legionella.
The thermal cut-out must never be bypassed under any circumstances. Safety devices are preset and are not there to be altered. Working outside the manufacturer's instructions can invalidate the warranty. A boiler serving an unvented cylinder also has to be compatible and fitted with both a control thermostat and a thermal cut-out.
Stored temperature and the safety devices on an unvented cylinder are checked during the annual service. Between services, nothing inside the casing needs your attention.
Who may change the temperature setting on a hot water cylinder?
Opening a cylinder casing to reach an immersion thermostat or a safety cut-out is not a householder task. In England, requirement G3 of Schedule 1 to the Building Regulations 2010 covers hot water systems, vented and unvented, and unvented cylinder work calls for a person qualified for unvented systems — see who can install an unvented hot water cylinder in England. Manufacturer instructions treat preset safety devices as parts nobody adjusts on site.
Where the work involves the gas boiler itself, a Gas Safe registered engineer is needed. In England, regulation 3 of the Gas Safety (Installation and Use) Regulations 1998 as amended requires that no person carries out work in relation to a gas fitting unless competent to do so. Nothing inside a boiler casing is a do-it-yourself job.
What you can safely check yourself before calling anyone:
- The boiler pressure gauge, to see whether it sits in the normal band marked by the manufacturer
- The programmer, including whether hot water is set to come on at all
- The room thermostat setting and its batteries
- The reset button on the boiler, once
- Bleeding radiators that are cold at the top
Landlords in England: how the 60°C cylinder setting fits the legionella duty
Landlords in England control legionella risk mainly through temperature. HSE guidance for landlords names the control in plain words: setting the temperature of the hot water cylinder so water is stored at 60°C. HSE also says the risk in most residential premises is low, and that sampling for legionella in domestic water systems is not normally needed. The principle it gives is short — keep the hot water hot, keep the cold water cold, and keep it moving.
A risk assessment is expected, and it should be reviewed periodically, for example after work on the water system or a long void period. HSE notes there is no statutory duty on landlords to keep a record; that duty falls on employers with five or more employees. A short written note of what you assessed, what the cylinder is set to and when you checked it is still useful evidence.
Temperature control is the reliable one in a domestic system. A cylinder running at 45°C to save money moves the water into the range where bacteria multiply.
How long does a cylinder take to reheat, and how long does the water stay hot?
Reheat time on a hot water cylinder depends on the heat input in kilowatts. Heatrae Sadia explains that a higher kW input gives a faster recovery time for the same cylinder. Its Megaflo Eco SystemFit manual gives, for that model, roughly 30 minutes to reheat about 70% of the volume at around 12 kW heat input. Read that as the manufacturer's figure for one product, not a figure for your own cylinder.
How long the water then stays hot comes down to insulation and cylinder size, as Viessmann sets out. A cylinder too small for the household will run cold in the evening whatever the thermostat says, so sizing matters as much as temperature. Poor insulation shows up as a warm airing cupboard and a boiler that fires again and again.
If you are replacing or converting a cylinder in London or the surrounding counties, FM Power's work on Megaflo and unvented systems starts with a free home survey and a written fixed price.
Sources
- HSE — Hot and cold water systems
- HSE — Legionella and landlords' responsibilities
- HSE — Scalding and burning
- GOV.UK — Approved Document G: sanitation, hot water safety and water efficiency
- legislation.gov.uk — Building Regulations 2010, Schedule 1
- legislation.gov.uk — Gas Safety (Installation and Use) Regulations 1998
- Vaillant — Hot water tank temperature
- Baxi — What temperature should a boiler be set at
- Viessmann — What temperature should your boiler be set at
- Heatrae Sadia — RDT resettable thermostats
- Heatrae Sadia — Megaflo Eco SystemFit installation manual
- Heatrae Sadia — Recovery time vs size of a cylinder
- Viessmann — How long does water stay hot in a tank