Summary

Approved Document G sets out the requirements for water supply, sanitation, and hot water in dwellings and some non-domestic buildings. It is the plumber's most directly relevant building regulation, covering everything from unvented hot water system safety devices to bath outlet temperature limits to the minimum requirements for WCs and sanitation fittings.

The current version is the 2015 edition incorporating 2016 and 2024 amendments. The 2024 amendments took effect on 1 October 2024 and updated the G4 sanitary convenience guidance to cross-refer to the new Approved Document T (Toilet accommodation) for buildings other than dwellings, and to refresh the BS 6465 references. Most of the contentious compliance questions arise from Part G3 — unvented hot water storage systems and blending valves — because these are safety-critical systems where incorrect installation can cause scalding injuries or Legionella growth.

For plumbers, the key triggers for Part G compliance are:

Key Facts

Quick Reference Table

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Component Role Standard
Thermostat (cylinder stat) Primary control — maintains stored temperature Integral to cylinder or immersion
Non-self-resetting energy cut-out Safety device 1 — cuts the heat supply on overheat; manual reset only BS EN 60335-2-73 / BS EN 60730-2-9 / BS EN 257
Temperature relief valve, or combined temperature & pressure relief valve Safety device 2 — discharges to keep stored water ≤100°C BS 6283-2 (temperature relief) / BS EN 1490 (combined T&P)
Expansion vessel or expansion valve Functional — absorbs expansion of heated water Sized per cylinder manufacturer
Pressure reducing valve (PRV) Functional — limits incoming supply pressure to the cylinder rating Typically set to 3.0 bar
Thermostatic mixing valve (TMV) Blends hot and cold to a safe outlet temperature BS EN 1111 / BS EN 1287; TMV2 or TMV3 scheme approval
Outlet Maximum Temperature Source
Bath in a new dwelling (or one formed by material change of use) 48°C Building Regulations requirement G3(4) — mandatory
Shower in a dwelling No Building Regulations limit; thermostatic shower valves are standard practice —
Bath in a care home / healthcare setting 44°C HSE / NHS HTM 04-01 (TMV3)
Shower or washbasin in a care home / healthcare setting 41°C HSE / NHS HTM 04-01 (TMV3)
Bidet in a care home / healthcare setting 38°C HSE / NHS HTM 04-01 (TMV3)
Stored hot water ≥60°C, reaching 50°C at outlets within 1 minute (55°C in healthcare) HSE ACOP L8 / HSG274 Part 2

Detailed Guidance

Part G3 — Unvented Hot Water Cylinders

Unvented hot water cylinders (Megaflo, Gledhill Torrid, Telford, etc.) store hot water at mains pressure — unlike vented cylinders which operate at low pressure from a header tank. The advantages are high-pressure hot water, no header tank, and more flexible installation. The disadvantages are: higher installation cost, mandatory notifiable status, and the requirement for the control thermostat plus two independent safety devices — and the discharge arrangement — to be correctly installed and maintained.

Competence requirement: ADG 3.40 says building owners should choose installers with the skills to carry out the work, demonstrated for example by registration with a competent person scheme for unvented hot water systems or by holding a current registered operative skills certification card. Common qualifications:

An engineer not registered with a relevant competent person scheme must notify the building control body before work commences (ADG 3.41), and the BCB may then inspect. A registered installer self-certifies through their scheme and the owner receives a building regulations compliance certificate (ADG 3.42).

Discharge pipe from the safety device (ADG 3.50–3.62): The discharge arrangement is the part most often got wrong on retrofits. Requirements:

Expansion vessel sizing: The expansion vessel must be sized to accommodate the expansion of water when heated from cold (typically 10°C) to stored temperature (60°C). Water expands by approximately 1.7% per 10°C rise. For a 150L cylinder heated from 10°C to 60°C (50°C rise): expansion = 1.7% × 5 × 150L = 12.75L. Expansion vessel must accommodate this volume — a 12L expansion vessel at the correct pre-charge pressure is typically adequate. Always calculate per the cylinder manufacturer's specification.

Annual service: Unvented hot water cylinders require annual servicing:

Thermostatic Mixing Valves (TMVs)

A TMV blends hot water from the cylinder/boiler with cold water to deliver mixed water at a preset temperature. The key safety characteristic is fail-safe operation: if cold water supply fails, the valve must close to prevent scalding from unmixed hot water.

TMV2 vs TMV3:

TMV installation:

Bath fill TMV — domestic dwellings: Requirement G3(4) requires the hot water supply to any fixed bath in a new dwelling (or a dwelling formed by a material change of use) to be designed and installed so the delivered temperature does not exceed 48°C. A TMV fitted to the bath tap supply, or a thermostatic bath tap or bath/shower valve set to 48°C, meets this. ADG's performance clause also requires that the device cannot be easily altered by building users, so a setpoint an occupant can wind up on the tap body is not compliant on its own. G3(4) does not apply to showers, and does not apply to a like-for-like bath replacement in an existing dwelling.

Sanitation Requirements (Part G2)

Every dwelling must have access to:

For shared houses and HMOs (Houses in Multiple Occupation), the HHSRS (Housing Health and Safety Rating System) supplements Part G with occupancy-based minimum provision. For buildings other than dwellings, the 2024 amendments redirect the detail on toilet accommodation to Approved Document T (Toilet accommodation), in force from 1 October 2024, alongside Part M for accessible provision.

WC flushing volumes: Under the ADG "fittings approach" (Table 2.1) a WC in a new dwelling must be either 6/4 litre dual flush or 4.5 litre single flush. Under the optional 110 l/p/d requirement (Table 2.2) the WC must be 4/2.6 litre dual flush. If a fitting exceeds these values, the full water efficiency calculator must be completed instead. Separately, the Water Supply (Water Fittings) Regulations 1999 cap any single-flush WC at 6 litres.

Part G1 — Water Efficiency (New Dwellings)

For new dwellings, the total water consumption of all fixtures (WC, basin, shower, bath, kitchen, washing machine, dishwasher) plus a fixed 5 l/p/d allowance for outdoor use must not exceed 125 l/p/d. This is calculated using the water efficiency calculator method in ADG Appendix A. An alternative "fittings approach" (ADG Table 2.1) lets you demonstrate compliance by specifying fittings that all meet stated maximums, without running the calculator. The notice given to the building control body under regulation 37 must state the calculated figure, or "Less than 125 litres/person/day using fittings approach".

Typical contribution of each fixture:

Specifying low-flow showers, 4/2.6L dual-flush WCs, and aerated taps is the main method of achieving the 125 l/p/d limit in new dwellings.

Frequently Asked Questions

My customer wants to replace their vented cylinder with an unvented one. What do I need to do?

This is notifiable building work under G3. You need to: (a) hold the appropriate unvented hot water qualification; (b) notify your competent person scheme, or notify Building Control before work commences if you are not scheme-registered; (c) fit the control thermostat plus the two independent safety devices required by ADG 3.18 — a non-self-resetting energy cut-out and a temperature relief (or combined temperature and pressure relief) valve — along with the expansion vessel or valve and the pressure reducing valve; (d) issue the compliance/commissioning paperwork. The discharge arrangement from the relief valve, through the tundish, to a safe termination is usually the most complex part to retrofit.

Do I need to fit a TMV to every bath in a new house?

Yes. Requirement G3(4) limits the temperature that can be delivered to any fixed bath to 48°C, and it applies whenever a dwelling is erected or formed by a material change of use — every bath in the house, regardless of who is going to live there. A TMV on the bath fill is the standard solution. It does not apply to showers, and it does not apply to a bath swapped out in an existing dwelling, though thermostatic shower valves are standard practice anyway.

What Legionella checks are required in a domestic dwelling?

For a single-family dwelling occupied by its owner, HSE Approved Code of Practice L8 does not require a formal risk assessment or monitoring regime. The practical requirements are: hot water stored at ≥60°C, hot water reaching 50°C at outlets within one minute, pipework flushed regularly, and no dead-legs (sections of pipework that don't circulate). For rented properties, HMOs and any complex or recirculating water system, the landlord or dutyholder does need a formal L8 risk assessment — current technical guidance is HSG274 Part 2.

My unvented cylinder is dripping from the discharge pipe. What's wrong?

A dripping discharge pipe usually means: (a) the expansion vessel pre-charge pressure has dropped, causing water to discharge through the relief valve on heating; (b) the relief valve is failing to re-seat; or (c) the pressure reducing valve is set too high or passing. Check and re-pressurise the expansion vessel first (most common cause). If the problem persists, replace the relief valve. Always investigate — a relief valve that routinely opens is a sign of system fault, and ADG's own user warning tells occupants to switch off and call the installer if water flows from the discharge pipe.

Regulations & Standards