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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Use this guide if you need to stop water flowing through the boiler because of a leak or maintenance.
COMBINATION BOILERS
Before you start
Where it is safe to do so; turn the boiler to the OFF position, this is located on the front of the appliance on a manual dial, or within a digital menu. Please refer to your user guide for boiler specific guidance.
Where it is safe to do so; turn the boiler off at the isolation point or fused spur.
Do not remove the boiler casing.
Step 1: Find the isolation valves
Look underneath the boiler.
You should see several pipes and isolation valves.
Step 2: Isolate the heating system
Find the two central-heating isolation valves beneath the boiler:
The CH Flow valve is on the far left, as shown in the diagram below.
The CH Return valve is on the far right, as shown in the diagram below. Please note: There may be two taps close together: one at the front and one at the back. Turn the tap at the back, directly underneath the boiler.
Turn both valves so that they are across the pipe, in a horizontal position.
This stops water moving between the boiler and the central-heating system, including the radiators and heating pipes.
Step 3: Isolate the mains water supply to the boiler
Locate the valve labelled DHW Inlet as shown in the diagram below, this valve is to the immediate right-hand side of the yellow gas isolation tap.
Turn the DHW inlet isolation valve so that it is across the pipe, in a horizontal position.
Please note: Water already inside the boiler or external pipework may continue to leak for a short time after the valves have been closed whilst the system drains.
HEAT ONLY & SYSTEM BOILERS
Before you start
Where it is safe to do so; turn the boiler to the OFF position, this is located on the front of the appliance on a manual dial, or within a digital menu. Please refer to your user guide for boiler specific guidance.
Where it is safe to do so; turn the boiler off at the isolation point or fused spur.
Do not remove the boiler casing.
Due to their design, heat-only boilers do not have an isolation valve for the mains water supply beneath the appliance.
To isolate the water supply, you must turn off the mains water entering the property using a stop valve located away from the boiler.
Typical places include:
Under the kitchen sink
Inside a kitchen cupboard
In a downstairs toilet or utility room
Under the stairs
Near the front or rear external wall
In a cellar, garage, or meter cupboard
If the isolation tap is inside the property, they are generally located against an outside wall especially where the mains water supply enters the property from the street.
This may vary for houses of multiple occupation, or flats as these types of properties can be on a communal, or separate supply.
It may also be boxed in or hidden behind removable panels. There may be a separate external stop tap outside the property, usually under a small cover near the pavement or boundary.
Please note: Water already inside the boiler or external pipework/system may continue to leak for a short time after the valves have been closed whilst the system drains.
SYSTEM BOILERS
Before you start
Where it is safe to do so; turn the boiler to the OFF position, this is located on the front of the appliance on a manual dial, or within a digital menu. Please refer to your user guide for boiler specific guidance.
Where it is safe to do so; turn the boiler off at the isolation point or fused spur.
Do not remove the boiler casing.
Step 1: Find the isolation valves
Look underneath the boiler.
You should see several pipes and isolation valves.
Step 2: Isolate the heating system
Find the two central-heating isolation valves beneath the boiler:
The CH Flow valve is on the far left, as shown in the diagram below.
The CH Return valve is on the far right, as shown in the diagram below. Please note: There may be two taps close together: one at the front and one at the back. Turn the tap at the back, directly underneath the boiler.
Turn both valves so that they are across the pipe, in a horizontal position.
This stops water moving between the boiler and the central-heating system, including the radiators and heating pipes.
Due to its design, system boilers do not have an isolation valve for the mains water supply beneath the appliance. The mains water supply goes to the hot-water cylinder, rather than passing through the boiler.
To isolate the domestic water supply, you must use an isolation valve located away from the boiler, such as using the mains water supply stop valve, or a separate isolation tap situated on, or by the hot water cylinder.
Typical places for the property stop valve include;
Under the kitchen sink
Inside a kitchen cupboard
In a downstairs toilet or utility room
Under the stairs
Near the front or rear external wall
In a cellar, garage, or meter cupboard
If the isolation tap is inside the property, they are generally located against an outside wall especially where the mains water supply enters the property from the street.
This may vary for houses of multiple occupation, or flats as these types of properties can be on a communal, or separate supply.
It may also be boxed in or hidden behind removable panels. There may be a separate external stop tap outside the property, usually under a small cover near the pavement or boundary.
Please note: Water already inside the boiler or external pipework/system may continue to leak for a short time after the valves have been closed whilst the system drains.
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.
