What does boiler pressure of 0 mean?

Most boilers show a red zone below 1 bar. At 0 bar, the boiler's safety switch prevents it from operating to avoid damage.

Use the filling loop (usually a silver braided hose under the boiler) to add water until pressure reaches 1–1.5 bar. Turn both valves, watch the gauge, then close them.

If you need to repressurize frequently, there's a leak in the system. Schedule a professional inspection to find and fix the leak.