If there is no leak anywhere in the system [pipes & radiators] Check the emergency pressure relief valve. It usually is at the base of the boiler and drains to outside [15mm pipe] where, if the valve leaks, water drips can be seen.. Looks similar to the picture with your query Renew the valve You can take it apart and clean it but I've never found that to be successful in the long term
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If there is no leak anywhere in the system [pipes & radiators] Check the emergency pressure relief valve. It usually is at the base of the boiler and drains to outside [15mm pipe] where, if the valve leaks, water drips can be seen.. Looks similar to the picture with your query
Renew the valve
You can take it apart and clean it but I've never found that to be successful in the long term