HV30 Aladdin Self Bleeding Automatic Radiator Valve 1/2" BSP Single Valve
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Close the drain valve and add fresh coolant, generally a 50/50 mix of antifreeze and distilled water (not tap water, which can form mineral deposits.) Do not overfill the coolant when you do this.  X Expert Source Duston Maynes
Autovent Products - Aladdin Products
You can also use a container to catch any water spills when you’re bleeding your radiator. Use your bleed key to open the radiator bleed valve
Do Aladdin autovents work with any heating system?
It’s always a good idea to use an actual radiator bleed key to bleed your radiators, as this reduces the chance of damage to the valve (and a potentially costly repair or replacement).
Aladdin Autovents: Self Bleeding Radiator Valves | UK Radiators
The bleed valve is one of the easiest valves to change on a radiator - no need to drain the system or the radiator - and shouldn’t take more than 10 minutes. You just need to know a few trade secrets. Be prepared: It’s likely that there will be a little bit of water loss so preparation is key. Make sure you have all of the tools you need before you begin.
If you can’t find the bleed valve on your radiator then keep looking because there needs to be one.
Aladdin HV30C Autovents Chrome 2 Pack - Screwfix
Some modern radiators are equipped with valves that are designed to be turned with a simple flathead screwdriver.  X Research source
If the water doesn’t stop coming out after isolating the radiator then one of the radiator valves must be broken and not closing fully.