Unvented Hot Water Systems: What Part G3 Requires in England

This page covers England. It is based on the Building Regulations 2010 and Approved Document G (2015 edition incorporating the 2016 and 2024 amendments), as at 30 September 2026. Wales has its own version of Part G and its own approved document, and Scotland and Northern Ireland have separate building regulations. None of those are covered here.

What the law says

Requirement G3 in Schedule 1 to the Building Regulations 2010 is short. For a hot water system with a storage vessel, it requires:

• the system, including any vessel that supplies it or takes its expansion water, must resist the temperatures and pressures it may meet in normal use or in a malfunction that can reasonably be expected, and must be adequately supported (G3(2));
• precautions that stop the stored water ever going above 100°C (G3(3)(a));
• any discharge from safety devices must be carried safely to where it is visible but will not endanger people in or around the building (G3(3)(b)).

G3(3) does not apply to systems that heat or store water only for an industrial process. G3(4) limits the hot water delivered to a fixed bath to 48°C, but only where a dwelling is newly built or formed by a material change of use.

Everything else on this page comes from Approved Document G. Following it is one way to show you have met G3. Paragraph numbers are given so you can check them.

Safety devices

An unvented system should have at least two independent safety devices, in addition to any thermostat that controls the stored water temperature (paragraph 3.17). An acceptable approach is a non-self-resetting energy cut-out plus a temperature relief valve or a combined temperature and pressure relief valve (3.18). Other approaches are acceptable if they give an equivalent level of safety.

• Up to 500 litres and under 45kW, the system should be a proprietary unit or package meeting an appropriate standard (3.22).
• If a boiler provides the indirect heat, the energy cut-out may be on the boiler (3.21).
• Each heat source should have its own non-self-resetting energy cut-out (3.33).
• The relief valve should sit directly on the storage vessel (3.38). In packaged units it should be factory fitted, disconnected only to replace it, and not moved to another fitting (3.39).
• Relief valves should not be used where the stored water is not automatically replenished, such as unvented primary thermal stores. Those need a second independent energy cut-out instead (3.35).

Water heaters of 15 litres or less that have suitable temperature and pressure safety devices will generally meet G3(3) (3.19).

Approved Document G does not set the incoming pressure, the pressure reducing valve setting, the expansion vessel charge or a strainer. Those come from the manufacturer's installation instructions for the unit you are fitting.

Labels on the unit

The unit or package should be indelibly marked with the manufacturer's name and contact details, model, rated capacity, operating pressures, data on each safety device and, for indirect units, the maximum primary circuit pressure and flow temperature (3.23). It should also carry a warning label, visible after installation, telling the user not to adjust any part and to switch off and call the installer if hot water comes from the discharge pipe. The label has spaces for the installer's name, address, phone number and completion date (3.24). Fill them in.

Tundish and discharge pipework

D1: valve to tundish

• The safety device discharges through a short length of metal pipe (D1) to a tundish, directly or by way of a manifold (3.50).
• D1 should be no smaller than the nominal outlet size of the valve (3.51).
• The tundish should be vertical, in the same space as the cylinder, as close as possible to and lower than the valve, with no more than 600mm of pipe between the valve outlet and the tundish (3.54).
• The tundish should have a suitable air gap to comply with the Water Supply (Water Fittings) Regulations (note to 3.54).
• Any discharge should be visible at the tundish. Where occupants might not notice a discharge, for example people with impaired vision or mobility, consider an electronic warning device (3.55).

D2: tundish to termination

• At least 300mm of vertical pipe below the tundish before any bend, then a continuous fall of at least 1 in 200 (3.56).
• Metal, or another material shown to withstand the discharge temperature and permanently marked with the product and its standard (3.57).
• At least one pipe size larger than the valve outlet, and larger again as the equivalent length grows, counting bends (3.58).
• A common discharge pipe serving more than one system should be at least one size larger than the largest D2 connected to it (3.59).

Table 3.1 gives the copper sizes:

The table is for copper. Plastic pipe has a different bore and resistance, so size it from that pipe's own data.

Worked example from the approved document: a G½ valve with 7m of 22mm D2 and four elbows. 9m less 4 x 0.8m leaves 5.8m, which is shorter than 7m. At 28mm, 18m less 4 x 1.0m leaves 14m, so 28mm is satisfactory.

Soil stacks

The discharge pipe should not be connected to a soil discharge stack unless the stack can be shown to withstand the temperature of the discharge. If it can, the connection should have a mechanical seal with no water trap, be a separate branch with no sanitary appliances on it, use PB or PE-X if the branch is plastic, and be continuously marked to say no sanitary appliances may be connected (3.60).

Where D2 ends

D2 should end in a safe place with no risk to people nearby (3.61). Paragraph 3.62 gives acceptable examples:

• into a trapped gully, with the pipe end below a fixed grating and above the water seal;
• downwards at low level, up to 100mm above an external surface such as a car park, hard standing or grass, with a wire cage or similar guard that prevents contact but keeps it visible;
• at high level into a metal hopper and metal downpipe with the pipe end clearly visible, or onto a roof that can take high temperature water and is 3m from any plastic guttering that would collect the discharge.

The discharge is very hot water and steam. It can damage asphalt, roofing felt and non-metallic rainwater goods (3.63).

Temperatures

• Where stored water in a dwelling can go above 80°C in normal use (for example heat stores, solar, or solid fuel boilers without intervening controls), fit a device such as a tempering valve so the water supplied to the hot water pipework does not exceed 60°C (3.64).
• Where G3(4) applies, limit the supply to a bath to 48°C with an in-line blending valve or similar device that has a maximum temperature stop and cannot easily be altered by users (3.65, 3.66).
• For legionella control, HSE's guidance is to store hot water at 60°C or more and distribute it so it reaches 50°C at outlets within one minute (HSG274 Part 2, paragraph 2.6). Landlords have a legal duty to assess and control legionella risk, and setting the stored water to 60°C is one of the simple controls HSE lists (2.138 to 2.142).

Who may install it

The law does not name a qualification. Approved Document G says building owners should choose installers with the skills for this work, and gives as examples registration with a competent person scheme for unvented systems or a current registered operative skills certification card for unvented hot water systems (3.40). No training course is named.

Notifying the work

Installing an unvented system is notifiable building work and must be notified to building control before work starts (3.41). You do this with a building notice or a full plans application (regulation 12). A hot water storage system with a vessel of 15 litres or less is exempt, provided any electrical work with it is itself exempt from notification (Schedule 4).

If you are registered with a competent person scheme for this type of work, you do not notify in advance (regulation 12(6)(a) and Schedule 3). Within 30 days of completing the work, you must give the occupier a certificate that the work complies and give building control a notice or the certificate (regulation 20(3)). The certificate is usually issued by the scheme operator (3.42).

Contravening the Building Regulations is an offence (Building Act 1984, section 35).

What to keep on record

The approved document asks you to fill in the installer's details on the unit's label (3.24). Beyond that, a record that lets building control or the next engineer check the paragraphs above would note the unit and its safety devices, the D1 length to the tundish, the D2 size, length and number of bends, where D2 ends, and how the work was notified. Settings such as pressures and thermostat temperatures should be recorded against the manufacturer's instructions for that unit.

CertBox has an unvented hot water storage vessel certificate for recording the installation.

Sources

Checked against these on 2026-09-30.

• Building Regulations 2010, Schedule 1, requirement G3
• Approved Document G, 2015 edition incorporating 2016 and 2024 amendments (England)
• Approved Document G publication page, gov.uk
• May 2024 amendments to Approved Document G
• Building Regulations 2010, regulation 12
• Building Regulations 2010, Schedule 3
• Building Regulations 2010, Schedule 4
• Building Regulations 2010, regulation 20
• Building Act 1984, section 35
• HSE, HSG274 Part 2: the control of legionella bacteria in hot and cold water systems

Published 2026-07-27. Corrected and updated 2026-09-30. This article is for general guidance only and does not constitute legal or professional advice. Always refer to the relevant standards and consult qualified professionals for definitive requirements.