Furnace shows H1; may continue to run at reduced output

Safety: Allow the system to cool before adding water. Hot water can cause scalding.

• Check pressure gauge: On Bosch hydronic furnaces/boilers, system pressure should typically be around 1.0–1.5 bar when cold (check your manual).
• Top up system: Use the filling loop or built-in fill valve to slowly add water until the correct pressure is reached. Close the valve securely afterward.
• Bleed air: Bleed radiators and high points, then recheck pressure and top up again if needed.
• Inspect for leaks: Look for visible water around the boiler, radiators, and pipework.

If pressure drops repeatedly, a leak or expansion vessel fault should be investigated by a technician.