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Part G Hot Water Safety Standards
Hot water storage temperatures, TMV requirements, Legionella prevention, and unvented cylinder regulations, referencing Approved Document G: Sanitation, Hot Water Safety and Water Efficiency (2015 edition incorporating 2016 and 2024 amendments).
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. See also the Part G Water Calculator.
Always check the latest edition of Approved Documents on the Planning Portal (planningportal.co.uk) as regulations are updated periodically.
Next up: related pages
Frequently Asked Questions
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.
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.
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.
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.
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.
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Last updated: April 2026
Verified against UK standards · estimates only, confirm with your supplier.