Approved Document G: Water Supply, Hot Water Safety & Sanitation

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 Legionella control in hot water cylinders to unvented hot water systems (G3.64 notifiable work) to the minimum requirements for WCs and sanitation fittings.

The 2015 edition (with subsequent amendments) of ADG is the current version. Most of the contentious compliance questions arise from Part G3 — unvented hot water storage systems and thermostatic mixing 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:

• Installing or replacing a hot water system (cylinder, combination boiler, thermal store)
• Installing an unvented hot water cylinder (G3 notifiable work — requires Building Regulations sign-off)
• Installing a thermostatic mixing valve on a bath or shower in a care setting
• Specifying bathroom layouts in new dwellings (WC, wash basin, bath/shower requirements)
• Meeting water efficiency requirements in new dwellings

Key Facts

• G3 notifiable work — Unvented hot water storage systems are notifiable under Regulation 20 (Schedule 1 Paragraph G3). Must be installed by a competent person registered with an approved scheme (e.g., BPEC Unvented Hot Water, JIB CSCS unvented card holder, or BST SNIPEF) or notified to Building Control
• Hot water storage temperature — Part G3: hot water stored at a minimum 60°C to prevent Legionella. Systems must have a pasteurisation cycle: heated to 60°C+ periodically (typically daily for cylinders with electric immersion backup)
• Maximum bath temperature — domestic dwellings — 48°C (amended Part G 2016). Achieved by fitting a thermostatic mixing valve (TMV) on the bath fill. Required in new dwellings and specified care homes/sheltered housing. Not mandatory for individual dwellings unless occupied by vulnerable people (HSE guidance)
• TMV standard — TMV3 (BS EN 15092) for healthcare/high-risk settings. TMV2 (BS EN 15092 + WRAS approval) for domestic dwellings. The TMV must fail safe — if cold water fails, the valve must shut off to prevent scalding
• Legionella and sentinel taps — Where water is recirculated (hotels, apartment blocks, care homes), sentinel temperature checks of hot water at remote outlets are required. Not typically required for single-family dwellings
• Unvented cylinder safety devices: All unvented hot water cylinders must have:
  1. Temperature/pressure relief valve (T&P valve) — opens if temperature exceeds 90°C or pressure exceeds rated pressure. Discharge pipe must terminate safely (outside, within 100mm of floor, drain)
  2. Expansion vessel and pressure reducing valve (PRV) — limit incoming pressure to cylinder rated pressure (typically 3.0 bar)
  3. Thermostat and overheat thermostat (thermal cut-out) — stops heat source before T&P valve activates
• Water efficiency — new dwellings — Maximum 125 litres per person per day (l/p/d) for new dwellings (ADG1). Calculated using SAP methodology. Lower limit of 110 l/p/d may be required in water-stressed areas (developer's planning condition)
• Minimum sanitation provision — A dwelling must have: at least one WC (with wash basin adjacent), at least one bath or shower, and means of washing and food preparation

Quick Reference Table

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 requirement for all four safety devices to be correctly installed and maintained.

Competence requirement: Only engineers holding a specific unvented hot water qualification may certify G3 work to Building Regulations. Common qualifications:

• BPEC Unvented Hot Water Level 3
• ACS Unvented certificate (ACS AW6 category)
• Similar NVQ or SNIPEF certification

An engineer without this certification must notify Building Control who will inspect the installation. For a competent person scheme registration, the engineer notifies their scheme (e.g., Gas Safe for gas-fired indirect cylinders, or an electrical scheme for immersion-heated unvented cylinders).

T&P valve discharge pipe: The discharge from the T&P valve must terminate safely. Requirements:

• Must not discharge directly onto a person — terminate at low level (within 100mm of floor or external ground)
• Must terminate where the discharge is clearly visible and clearly not a hazard (ideally over a gully or drain)
• Discharge pipe must fall continuously to the outlet — no low points that would trap water
• Minimum 22mm diameter (same as or larger than T&P valve outlet)
• Must not be made of material that would soften at discharge temperature — copper or stainless steel preferred. CPVC and some engineering plastics acceptable — confirm with manufacturer. Standard PVC conduit is NOT acceptable

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:

• Test T&P valve (lift lever briefly and check discharge, then verify it reseats and does not drip)
• Check expansion vessel pre-charge pressure (should match manufacturer's specification, typically 3.0 bar) — re-pressurise if below
• Check PRV and verify it is not passing (continuous trickle from T&P discharge pipe indicates PRV failure or vessel pressure too low)
• Check thermostat setting (60°C minimum)
• Verify overheat thermostat is functional (should not have tripped unless system was overheated)

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:

• TMV2: Approved to BS EN 15092, tested and certificated for domestic dwellings. Less stringent performance requirement
• TMV3: Approved to BS EN 15092 with additional healthcare standard (NHS Estates HTM 04-01). Required in care homes, hospitals, nurseries, and high-risk settings. Tighter temperature stability tolerance (±1°C vs ±2°C for TMV2)

TMV installation:

• Install as close to the outlet as possible (minimise the length of un-mixed hot water pipe after the valve)
• Cold water supply must be from a dedicated uncontaminated source — do not tee off the cold supply after the TMV
• Annual service: check temperature setting (test at outlet with thermometer), check cold supply failure (briefly close cold isolation — outlet should shut off within a few seconds). Record service on a maintenance tag fixed to the TMV

Bath fill TMV — domestic dwellings: ADG 2016 requires that baths in new dwellings are fitted with a device to limit outlet temperature to 48°C. A TMV fitted to the bath tap supply (or to the bath tap itself if a thermostatic tap) meets this requirement. A thermostatically controlled bath/shower valve at 48°C setpoint is equivalent.

Sanitation Requirements (Part G2)

Every dwelling must have access to:

• A WC with adequate flushing mechanism — dual flush (6L/4L or 4L/2.6L) is the water efficiency standard for new build
• A washbasin adjacent to the WC
• Either a bath or shower with hot and cold water supply
• A means of food preparation (sink with hot and cold water)

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.

WC flushing volumes: Part G water efficiency requires dual flush WCs in new dwellings. Single flush is permitted only if it delivers ≤6L per flush. Dual flush must be: full flush ≤6L, reduced flush ≤4L (or reduced flush ≤2⁄3 of full flush volume if not 6/4L).

Part G1 — Water Efficiency (New Dwellings)

For new dwellings, the total water consumption of all fixtures (WC, basin, shower, bath, kitchen, washing machine connection points, dishwasher connection points, garden tap) must not exceed 125 l/p/d. This is calculated using the method in ADG Appendix A.

Typical contribution of each fixture:

• WC dual flush 6/4L: 32 l/p/d
• Shower (8 l/min at 8 minutes/day): 64 l/p/d
• Basin (6 l/min at 4 minutes/day): 8 l/p/d
• Kitchen sink: 12 l/p/d
• Dishwasher: 2 l/p/d
• Washing machine: 6 l/p/d

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 G3 notifiable work. You need to: (a) have the appropriate unvented hot water qualification; (b) notify the relevant competent person scheme (or Building Control); (c) install all four safety devices (T&P valve, expansion vessel, PRV, overheat thermostat); (d) issue a commissioning certificate. The discharge pipe from the T&P valve must be correctly terminated — usually the most complex part to retrofit.

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

In new dwellings, yes — ADG 2016 requires bath water temperature to be limited to 48°C. A TMV on the bath fill is the standard solution. This does not apply to showers (48°C is guidance for showers, not mandatory), though thermostatic shower valves are standard in new-build practice anyway.

What Legionella checks are required in a domestic dwelling?

For a single-family dwelling, HSE Approved Code of Practice L8 (Legionella in water systems) does not require formal risk assessment or monitoring. The practical requirement is: hot water stored at ≥60°C, cylinder heated to 60°C+ daily, pipework flushed regularly, no dead-legs (sections of pipework that don't circulate). For rented properties with multiple occupancies or complex water systems, a formal L8 risk assessment may be required by the landlord.

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

A dripping T&P discharge pipe usually means: (a) the expansion vessel pre-charge pressure has dropped, causing water to discharge through the T&P valve on heating; (b) the T&P valve is failing to re-seat; or (c) the PRV is set too high. Check and re-pressurise the expansion vessel first (most common cause). If the problem persists, replace the T&P valve. Always investigate — a T&P valve that routinely opens is a sign of system fault.

Regulations & Standards

• Approved Document G (2015 with 2016 amendments) — Water efficiency, sanitation, hot water supply

• BS EN 15092 — Thermostatic mixing valves: requirements and test methods (TMV2/TMV3)

• HSE L8 (ACOP) — Approved Code of Practice: Legionella in water systems

• NHS HTM 04-01 — Safe water in healthcare premises (TMV3 specification)

• Water Supply (Water Fittings) Regulations 1999 — Prohibition on unfit fittings and waste/misuse of water

• GOV.UK — Approved Document G — Full text of the 2015 edition with 2016 amendments

• TMV3 Scheme (NHS) — TMV3 approved products and scheme information

• WRAS — Water Regulations Advisory Scheme — Approved products and guidance for Water Fittings Regulations compliance

• HSE — Legionella: The Control of Legionella Bacteria in Water Systems — ACOP L8

• unvented cylinders — Unvented cylinder selection and installation detail

• shower trays — Shower installation including hot water supply

• part j combustion — Part J requirements for gas-fired water heaters

• heat pumps — Hot water cylinders in heat pump systems (larger volumes, higher temperatures)