Your boiler is reminding you that the water pressure inside the system has dropped below the recommended level and needs to be topped up.

DIY fix possible

Locate the external filling loop (a flexible metal hose) connected underneath your boiler.Open the valves on both ends of the filling loop to allow cold mains water into the system.Watch the pressure gauge on the front of the boiler until the needle reaches the green zone (usually between 1 and 1.5 bar).Close both valves tightly and remove or disconnect the filling loop if required.