How to place a connection in the right category: judge the water that could flow back, know the everyday examples, and watch how downstream use changes it.
What the scale measures
The fluid-category scale ranks the harm if water were drawn back into the wholesome supply. Category 1 is wholesome supply water. Each step up is a greater reduction in quality and a greater potential hazard to health, and the backflow protection must become more stringent with it, up to a recognised category 5 arrangement such as a suitable air gap. The grade follows the hazard, not pipe size, pressure, fitting type or how clean the water looks.
Judge the water that could flow back
Assess the water at the point where it could return, not the water leaving the tap. Downstream equipment, its contents and its processes set the hazard. The same supply pipe can feed a harmless tap or a high-hazard process, so an assessment made only at the stop tap misses where contamination arises. Assess first, then choose the device; choosing a familiar fitting first can under-protect.
The examples to know
- Category 2: water that has changed only in taste, odour, appearance or temperature, with no health hazard, such as warmed water or water passed through a domestic softener.
- Category 3: a slight health hazard. In a house, water in a washbasin, bath or shower tray is category 3, because after use it can contain soap and other low-toxicity substances. Domestic central-heating water with a usual low-toxicity inhibitor at its intended dose is commonly category 3. Confirm the fluid, concentration and water undertaker’s assessment.
- Category 4: a significant health hazard from the concentration of toxic substances, for example certain chemical or process fluids. Very toxic substances can fall into category 5; a chemical’s name does not settle the category.
- Category 5: a serious health hazard, such as water in a WC pan, which can contact faecal matter, or a bidet whose outlet can be submerged.
The same appliances can be a higher category in non-domestic premises.
Apply it
Rank the points in a house bathroom: a washbasin, a single-feed bath mixer and a WC. The WC is the most serious, category 5, because its water can contact faecal matter, and it needs the strongest protection, typically a physical air gap rather than a mechanical valve. Basin and bath water are category 3.
A hose used by hand for domestic gardening or washing a personal vehicle, with a flow-control device and disconnected when not in use, is typically category 3. That is not a blanket rating for every hose tap: reassess if the hose can reach a pond, pool, animal waste or other higher-risk water.
A hose left in a bucket creates a continuous path from the bucket to the supply. If the pressure falls, backsiphonage can occur even though the tap normally flows outwards.
Chemical dosing points
Dosing equipment puts chemicals at the outlet. A pressure loss or backpressure could draw them towards the main, so simple check protection may not be enough; the protection must match the chemical's hazard.