Your boiler has detected that the water pressure inside the heating system has dropped too low, so it is running at a reduced level to protect itself.

DIY fix possible

Locate the filling loop (usually a silver flexible hose) connected to your boiler pipes.Open the valves on the filling loop slowly to let water into the system.Watch the pressure gauge on the front of the boiler until the needle reaches the green zone, typically between 1.0 and 1.5 bar.Close both valves tightly and remove the filling loop if it is not a permanent fixture.