Drain-off cock
In plumbing · also called Drain valve
A small valve fitted at a low point so that a system can be emptied. A hose is pushed onto its outlet and led to a drain, and the valve is opened with a key or spanner. Opening a tap or vent higher up lets air in behind the water, so that it drains instead of holding in the pipe.
Using Drain-off cock in a plumbing system
Updated
A distinction worth remembering
A system can retain water, pressure or heat after a supply is closed. Recognising these conditions comes before opening a connection. The learning aim is to understand which part of the system is controlled and when a competent person must take over.
Questions to take further
- Does turning off the stop tap empty the system?
- No. Pipes and storage vessels can still contain water, and separate systems may remain supplied. Isolation and draining are different operations.
Sources
- Skills England: plumbing and domestic heating technician — The systems, materials, planning and customer responsibilities in the England occupational standard.
- GOV.UK: Approved Document G — Guidance for England on hot-water safety, including stored hot water; guidance is not itself the legislation.
Where Drain-off cock fits in the learning path
The Isolation, Hot Water Safety and Simple Repairs section covers these topics:
- Isolating and Draining Down
- Hot Water Safety
- Everyday Plumbing Repairs
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- Isolating and Draining DownIsolation, Hot Water Safety and Simple Repairs
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In plumbing, turning off the water to the part of a system you are working on, then proving it is off by opening an outlet and checking that the flow stops. Isolate as close to the job as possible, with a service valve where there is one and the stop tap where there is not, and drain the section down before opening it up.
Isolation
Read the Isolation entryCompare related terms
Open a meaning to compare its role with Drain-off cock, or follow its full entry.
Isolation
In plumbing, turning off the water to the part of a system you are working on, then proving it is off by opening an outlet and checking that the flow stops. Isolate as close to the job as possible, with a service valve where there is one and the stop tap where there is not, and drain the section down before opening it up.
Read the Isolation entryAirlock
A pocket of trapped air that stops water flowing through a pipe, often after a system has been drained and refilled. It is most likely where pipes rise and then fall. Refilling slowly, and opening outlets one at a time from the lowest upwards, helps push the trapped air out.
Read the Airlock 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.
A definition is a start. Using it in a real question is where it sticks.