Under the Water Supply (Water Fittings) Regulations 1999, which apply in England and Wales, a permanent mains connection to a closed heating circuit must include an approved backflow prevention device, and a heating system fitted by an installer must also meet the wider requirements for water fittings.

Why backflow matters for a heating circuit

Schedule 2 to the Water Supply (Water Fittings) Regulations 1999, which the legislation marks as applying in England and Wales, says every water system must contain an adequate device or devices for preventing backflow of fluid from any appliance, fitting or process. The device must be appropriate to the highest applicable fluid category downstream before the next such device, and it is adequate if it complies with a specification approved by the regulator. Scotland and Northern Ireland have separate water fittings rules that are not covered here.

Filling a closed heating circuit

Paragraph 24 of Schedule 2 provides that no supply pipe or secondary circuit shall be permanently connected to a closed circuit for filling a heating system unless it incorporates a backflow prevention device in accordance with an approved specification. The Water Regulations Approval Scheme (WRAS) explains this in its installation requirement IRN R360. Where the level of risk is no greater than fluid category 3, the circuit may be filled or topped up through a temporary connection between the primary circuit and the supply pipe, with an approved double check valve at the point of connection to the supply pipe. Alternatively, a permanent connection may be made through a CA backflow prevention device or another no less effective device, fitted in accordance with IRN 510.

Where the level of risk is no greater than fluid category 4, WRAS says a permanent connection may be made through a BA (RPZ) backflow prevention device, which is notifiable under Regulation 5. In practice, the category that applies to a particular heating system depends on the system and the water company’s classification, so it is a question for the installer.

Notification and consent

Regulation 5 requires a person who proposes to install a water fitting in connection with certain listed operations to notify the water undertaker and not begin without its consent, which shall not be withheld unreasonably. The list includes a reduced pressure zone valve assembly or other mechanical device for protection against fluid in category 4 or 5. The requirement does not apply where the fitting is installed by an approved contractor in certain listed cases, including that type of device.

Quality and workmanship

Regulation 4 requires every water fitting to be of an appropriate quality and standard and suitable for its circumstances, to comply with Schedule 2, and to be installed, connected, altered or repaired in a workmanlike manner. Workmanlike means work carried out to an appropriate British Standard or equivalent national specification, to a specification approved by the regulator, or by a method approved by the water undertaker.

Unvented hot water and the boiler

Paragraph 17 of Schedule 2 says every unvented water heater, other than an instantaneous heater of not more than 15 litres capacity, and every secondary coil in a primary system, must be fitted with a temperature control device and either a temperature relief valve or a combined temperature and pressure relief valve, or be capable of accommodating expansion within the secondary hot water system. An expansion valve must be fitted with provision to ensure the discharge is directed correctly if the expansion vessel or system fails. Building Regulations Part G contains separate requirements for the same equipment, covered in a companion guide.

Offences and defences

Regulation 7 makes it an offence, punishable on summary conviction by a fine not exceeding level 3 on the standard scale, to commence a listed operation without notice or consent or in breach of a condition. In proceedings against an owner or occupier for an offence based on installing or altering a water fitting, it is a defence to show the work was done by or under the direction of an approved contractor who certified that the fitting complied with the regulations. A householder therefore has good reason to ask the installer for confirmation of compliance for the work.

FAQs

Is a permanent filling loop always allowed?

Not automatically. WRAS IRN R360 says a permanent connection needs an appropriate backflow device, and the device depends on the fluid category of the circuit.

Do these regulations cover Scotland?

The legislation marks the 1999 Regulations as applying in England and Wales. Scotland has separate water regulations that are not summarised here.

What should I ask my installer?

Ask which backflow prevention device is fitted to the filling connection, who is certifying compliance and whether the water company was notified where regulation 5 required it.

Bottom line

The Water Fittings Regulations 1999 require a backflow prevention device wherever a heating circuit is filled from the mains, notification for higher-risk devices, workmanlike installation and safety devices on unvented hot water systems. Compliance sits with the installer and, for offences, the person who commissioned the work unless an approved contractor certified it. This is general information for England and Wales, not advice on a specific installation.

Sources

This guide draws on the following primary sources, current as of 20 September 2026:

• legislation.gov.uk, “The Water Supply (Water Fittings) Regulations 1999, Schedule 2: Requirements for water fittings”
• legislation.gov.uk, “The Water Supply (Water Fittings) Regulations 1999, regulation 4: Requirements for water fittings”
• legislation.gov.uk, “The Water Supply (Water Fittings) Regulations 1999, regulation 5: Notification of proposed work”
• legislation.gov.uk, “The Water Supply (Water Fittings) Regulations 1999, regulation 7: Offences”
• Water Regulations Approval Scheme, “IRN R360”

Related guides

Best Boiler Deals is an independent guide. We may earn a fee from some links; this never affects what we write. This article is general information, not regulated advice on any specific installation — always confirm the details with a Gas Safe registered engineer.