The five fluid categories as defined, from wholesome water to a serious health hazard, with the everyday example to know for each.
Fluid categories grade the harm if water from a fitting or appliance flowed back into the supply. They rise with risk, and the protection required rises with them.
- Category 1: wholesome water supplied by a water undertaker. Wholesome means it meets the standards for drinking water. It needs no backflow protection.
- Category 2: category 1 water whose temperature, taste, smell or appearance has changed, with no health hazard. Water warmed in a hot water cylinder, or passed through a domestic softener, is an example. A single check valve is the minimum protection.
- Category 3: a slight health hazard from the concentration of substances of low toxicity. In a house, water in a washbasin, bath or shower tray is category 3. Domestic heating water with a usual low-toxicity inhibitor at its intended dose is commonly assessed as category 3.
- Category 4: a significant health hazard from the concentration of toxic substances, such as toxic chemicals or pesticides. It includes environmental organisms of potential health significance.
- Category 5: a serious health hazard from the concentration of pathogens, radioactive or very toxic substances, including human and animal waste. Water in a WC pan is the everyday example.
Using the categories
Judge the category by what the water could contain where it could flow back, not by its appearance or the appliance's name. Stored wholesome hot water is normally category 2; it differs from heating-circuit water. A domestic heating circuit is commonly category 3, but its actual fluid, dose and use must be assessed. The same appliances can be a higher category outside a house. A heating additive or an outdoor location does not settle the category: check the actual substance, its concentration and the foreseeable use, and confirm with the water undertaker where needed.
Then choose protection rated for that category and for the forms of backflow that can occur. A category 5 arrangement that protects against backsiphonage only does not cover a connection where backpressure is possible.