Double check valve
In plumbing
Two check valves in one body, one after the other, so water is still held back if one of them fails to seal. It is common on appliances and outside taps where the backflow risk is moderate, and it is no substitute for an air gap where the water could be seriously contaminated.
Using Double check valve in a plumbing system
Updated
A distinction worth remembering
Backflow can result from a pressure drop upstream or higher pressure downstream. An arrangement suitable for one fluid category may be unsuitable for another. Water Regs UK explains the differences between backflow devices and air-gap arrangements; the water supplier can advise on the applicable case.
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 Double check valve 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 valve that lets water pass one way and closes if it tries to flow back. A single check valve gives protection where the backflow risk is low; where the risk is higher, a double check valve or an air gap is used instead, matched to the fluid category of the water involved.
Check valve
Read the Check valve entryCompare related terms
Open a meaning to compare its role with Double check valve, or follow its full entry.
Check valve — Non-return valve
A valve that lets water pass one way and closes if it tries to flow back. A single check valve gives protection where the backflow risk is low; where the risk is higher, a double check valve or an air gap is used instead, matched to the fluid category of the water involved.
Read the Check valve 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 entryBackflow
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.
Read the Backflow 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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