Preventing water backflow

Below is another article from my monthly contributions to the training section of a leading heating and plumbing magazine, written on behalf of a leading manufacturer serving those industries.

Preventing water backflow

The Water Supply (Water Fittings) Regulations 1999 (or Byelaws 2000 in Scotland) states that where water or water-using equipment is used with fluids or materials which could contaminate it, there must be adequate protection to stop backflow of potentially contaminated water into other parts of the system, especially drinking water.  The regulations define Fluid Risk Categories by the type of contaminants present and specify the appropriate type of prevention device which must be fitted to guard against backflow.
The Regulations provides for five Fluid Category types, as follows:

Fluid Category 1: Wholesome (mains) water.
Fluid Category 2: Water that is safe to drink, but whose aesthetic quality is impaired due to a change in its temperature, or presence of substances or organisms causing a change in taste, odour or appearance.
Fluid Category 3: Represents a slight health hazard because of the concentration of substances of low toxicity.
Fluid Category 4: Represents a significant health hazard due to the concentration of toxic substances, including chemicals, carcinogenic substances or pesticides or environmental organisms of potential health significance.
Fluid Category 5: Represents serious health hazard due to the concentration of pathogenic organisms, radioactive or very toxic substances, eg containing faecal material or other human waste; butchery or other animal waste or pathogens.

The simplest means to prevent backflow is an air gap.  For example, water can be discharged (into a hose pipe, for example) through a fitting that includes a gap to the atmosphere.  A cistern (header tank) provides an air gap provided the water inlet is above the overflow, so water always falls through air.

This is usually satisfactory for most dwellings, but a more robust and secure solution is required in industrial and commercial premises and in apartment blocks, particularly because header tanks need regular attention to guard against Legionella.

In all these cases the “reduced pressure zone” (RPZ) valve is widely accepted for backflow prevention.  Its triple security provides optimum protection against backflow and back siphonage: as the diagram of a Honeywell BA295 valve shows, assurance is provided by two check valves and a discharge valve which divide the unit into three chambers, each providing a separate pressure zone.

An RPZ valve can be installed to provide protection against backflow from a fluid category 4 risk.

The installation, use, maintenance and testing of RPZ valves is covered in an “Approved Installation Method” document, reference AIM-08-01, issued in April 2008 by the Water Regulations Advisory Scheme (WRAS).  The document can be downloaded free of charge from the WRAS web site at  It incorporates the previous guidance note 9-03-02, which has been withdrawn.

All proposed installations of RPZ valves must be notified in advance to the local Water Supplier.  Document AIM-08-01 includes a form for this purpose.

The design of Honeywell’s BA295 valve simplifies servicing and greatly minimises spares holding over earlier RPZ designs. There is unrestricted access to all internal components and the discharge valve is mounted on the underside.  The unique design minimises the number of spares required to service all models in the BA295 rang.  They have a dezincification resistant brass housing with a choice of DN15 to DN50 (½ to 2 inch) screwed connections.

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Royer Slater

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