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How to Isolate the Water Supply to a Boiler
Important Notice: This guide is intended for general informational purposes only and outlines basic steps for isolating a boiler’s water supply.
Before attempting any work, ensure you understand your system and follow all manufacturer instructions. If you are unsure or the issue involves leaks, faults, or internal components, you should contact a qualified heating engineer.
Gas components, internal boiler parts, and repairs must be handled only by a Gas Safe-registered engineer. We are not liable for any damage, loss, or injury resulting from reliance on this information.
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To safely stop water from flowing into the boiler, for maintenance or in an emergency (e.g. a leak).
Turn off the boiler
Switch the boiler off at the control panel
If present, turn the boiler’s electrical isolation switch to OFF
(Only Certified Gas Safe Engineers should touch the gas isolation valve or remove the front casing from the boiler).
Locate and close the boiler filling loop / cold water inlet
This is usually under the boiler, as a silver-braided hose with one or two small valves, or a rigid pipe with a lever or screw-type valve.
Note: A Heat only boiler usually cannot be individually isolated would need to turn off supply to your property (note the boiler may continue to leak whilst the system fully drains)
Common valve types
Lever valve (quarter-turn)
Turn the lever 90 degrees so it is perpendicular to the pipe
Screw/slot valve (turned with screwdriver)
Use a flat-bladed screwdriver to turn the slot 90 degrees (a quarter turn) so it is horizontal or perpendicular to the pipe. The water supply is OFF when the slot runs across the pipe.
Knob valve (twisted by hand)
Turn the slot 90 degrees (a quarter turn) so it is horizontal or perpendicular to the pipe. The water supply is OFF when the handle runs across the pipe. (Sometimes there are two of these close both)
Then confirm that you have successfully isolated the boiler
Open a radiator bleed valve briefly; the pressure as visible on the gauge on the front of your boiler should drop (confirming isolation).
Isolate your Mains Supply
If you are unsure on actioning the above or have a significant leak you can isolate the mains water to your property using:
The internal stop tap (stopcock) this is usually in the kitchen
The external stop valve which is usually on the pavement outside the property in a small metal or plastic cover marked along the lines of “WATER” “Stop Cock” or “SV”
Once you have isolated your mains supply open the lowest cold tap in the property as this helps drain remaining water and confirms isolation the water should:
Slow down
Then stop completely
If water continues to run, the stop tap may not be fully closed or may be faulty.
Next Steps After Isolation
Isolating your boiler is an essential first step in managing a leak or preparing for maintenance. Now that the water supply is off:
Contact a Professional: Do not attempt to repair internal boiler leaks yourself. Use our Find an Installer tool to locate a local expert.
Assess Your System: If your boiler frequently leaks or loses pressure, it may be a sign of wear and tear.
Explore Your Options: If your boiler is beyond economical repair, it might be time for a boiler replacement.
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