Backflow
In plumbing
Water flowing the opposite way to the one intended, for example from a hose left in a bucket, a cistern or an appliance back into the pipe that supplies drinking water. It can happen when the supply pressure drops, or when the system downstream is at a higher pressure. The water fittings rules in each UK nation require protection against it that matches the risk, judged by fluid category.
Using Backflow in a plumbing system
Updated
A distinction worth remembering
Backflow can arise through back-siphonage or back-pressure. Identifying the mechanism helps explain why a device suitable for one application may be unsuitable for another.
Questions to take further
- Is a double check valve suitable for every backflow risk?
- No. Protection must match the fluid category and the pressure conditions. Higher-risk uses can need an appropriate air gap or other suitable arrangement; do not choose a device merely because it fits the pipe.
Sources
- Water Regs UK: backflow protection — Contamination risks and why protection must match the fluid category and conditions.
Where Backflow fits in the learning path
The Cold Water Basics section covers these topics:
- The Incoming Water Supply
- Cold Water Storage and Backflow
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- Cold Water Storage and BackflowCold Water Basics
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A scale of five categories that grades water by the harm it could do if it flowed back into drinking water. Category 1 is wholesome drinking water and category 5 is the most serious risk, such as water that may contain human waste. The higher the category, the stronger the backflow protection needed between that water and the supply.
Fluid category
Read the Fluid category entryCompare related terms
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Fluid category
A scale of five categories that grades water by the harm it could do if it flowed back into drinking water. Category 1 is wholesome drinking water and category 5 is the most serious risk, such as water that may contain human waste. The higher the category, the stronger the backflow protection needed between that water and the supply.
Read the Fluid category entryAir gap
A physical gap between the outlet of a pipe or tap and the highest level the water below it could reach, so that water cannot be drawn back into the supply. Because there is nothing in it that can fail, an air gap is the protection used against the highest-risk water, such as a category 5 source. A tap whose spout sits well above the spill-over level of a basin is a familiar example.
Read the Air gap entryBefore you start: some plumbing work is regulated or notifiable
Some water fittings work needs notice to, and consent from, the water supplier before it starts. Check the rules where you live.
Gas work is for competent workers covered by a Gas Safe registered business in Great Britain. Northern Ireland has separate gas regulations. Do not work on gas pipes or appliances yourself.
In England, installing an unvented hot-water system over 15 litres is notifiable building work: use a competent installer registered with a competent-person scheme, or tell building control before work starts. Leave this work to a suitably skilled person.
Wales, Scotland and Northern Ireland have their own water and building rules; check which apply before work starts. Electrical connections belong with an electrician.
This guide does not qualify you or make you competent, approved or registered for plumbing, gas or unvented work.
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