What temperature must hot water be stored at?

Hot water must be stored at a minimum of 60°C to prevent Legionella bacteria growth. The distribution temperature should be at least 55°C at the return to the cylinder (or at the furthest tap in systems without a return). These requirements come from AD-G Section G3 and HSE Approved Code of Practice L8.

What is the maximum bath outlet temperature?

AD-G requires that the hot water temperature at bath outlets in new dwellings does not exceed 48°C. This is achieved by fitting thermostatic mixing valves (TMVs) compliant with BS EN 1111 or BS EN 1287 at bath taps. This requirement was introduced to prevent scalding, particularly of children and vulnerable people. It applies to baths only — basins and showers are not covered by this specific limit.

Do I need to notify Building Control for an unvented cylinder?

Yes. Installation of an unvented hot water storage system over 15 litres capacity is notifiable building work under Regulation G3. The installer must either notify Building Control directly or be registered with a competent person scheme (e.g. OFTEC, Gas Safe where applicable). The cylinder must comply with BS EN 12897 and have appropriate safety devices including temperature/pressure relief valve, expansion vessel, and tundish.

What safety devices does an unvented cylinder need?

An unvented hot water cylinder must have: (1) a thermostat to control normal operating temperature, (2) a high-limit thermal cut-out (non-self-resetting) set at approximately 90°C, (3) a temperature and pressure relief valve (T&P valve) set at 90-95°C and 6-10 bar, (4) an expansion vessel or internal air space, and (5) a tundish and discharge pipe to safely dispose of any relief valve discharge. These are specified in AD-G Diagram 1.

Does Part G apply across the UK?

AD-G applies to England. Wales has its own edition of Approved Document G. Scotland covers hot water safety under Technical Handbook Section 4 (Safety) clause 4.9. Northern Ireland uses Technical Booklet P. Requirements are broadly similar across jurisdictions but specific edition dates and details may differ.

Part G Hot Water Safety Standards

Hot Water Temperature Requirements (AD-G Section G3)

Parameter	Requirement	Reason
Storage temperature	≥ 60°C	Legionella prevention
Distribution temperature	≥ 55°C at return / furthest point	Prevent bacterial growth in pipework
Bath outlet (new dwellings)	≤ 48°C (via TMV)	Scald prevention
High-limit cut-out	~90°C (non-self-resetting)	Over-temperature safety
T&P relief valve	90-95°C / 6-10 bar	Pressure and temperature safety

Legionella Temperature Risk Zones

Temperature Range	Risk Level	Notes
Below 20°C	Low — bacteria dormant	Cold water should be kept below 20°C
20°C to 45°C	High — rapid multiplication	Danger zone: avoid water sitting in this range
45°C to 50°C	Moderate — some bacteria survive	Not hot enough to reliably kill bacteria
50°C to 60°C	Low — bacteria die slowly	Takes hours to kill; not sufficient for storage
Above 60°C	Very low — bacteria killed quickly	Storage temperature target for safety

Unvented Cylinder Safety Devices (AD-G Diagram 1)

Device	Function	Standard
Thermostat	Controls operating temperature (60-65°C)	Integral to cylinder
High-limit thermal cut-out	Shuts off heat at ~90°C, requires manual reset	Non-self-resetting
Temperature & pressure relief valve	Opens at 90-95°C or 6-10 bar	BS 6283-2
Expansion vessel / air gap	Accommodates water expansion when heated	Sized for cylinder volume
Tundish	Visible air break in T&P valve discharge	Within 500 mm of valve
Discharge pipe	Safely routes T&P discharge to safe point	Min. 1 pipe size larger than valve outlet

G3 Notification Requirements

Work Type	Notifiable?	Route
New unvented cylinder > 15 litres	Yes	Building Control or competent person scheme
Replacement unvented cylinder (like-for-like)	Yes	Building Control or competent person scheme
Vented cylinder installation	No (unless other notifiable work)	N/A
Unvented cylinder ≤ 15 litres	No	N/A (e.g. under-sink heaters)

Related Calculators

Use the Hot Water Cylinder Calculator for sizing, or the Pipe Sizing Calculator for distribution pipework.

Always check the latest edition of Approved Documents on the Planning Portal (planningportal.co.uk) as regulations are updated periodically.

Part G Hot Water Safety Standards

Hot Water Temperature Requirements (AD-G Section G3)

Legionella Temperature Risk Zones

Unvented Cylinder Safety Devices (AD-G Diagram 1)

G3 Notification Requirements

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