Low System Pressure Warning

Symptoms

• The pressure gauge reads below 0.5 bar
• The boiler display shows a low pressure warning or error code F1
• The heating does not come on when the thermostat calls for heat
• Radiators remain cold even though the boiler appears to be in standby

Likely Causes

• Small leak in the system — a weeping radiator valve, loose fitting, or corroded joint somewhere in the pipework
• Recently bled radiators — releasing air from the system also releases a small amount of water, which reduces pressure
• Pressure relief valve has discharged — if the system was overpressurised, the safety valve may have released water through the external discharge pipe (usually a copper pipe exiting through an outside wall near the boiler)

Step-by-step Fix

1. Locate the filling loop. This is a braided flexible hose located beneath the boiler. It connects the mains cold water supply to the central heating circuit. You will see two small valves — one at each end of the hose.
2. Open the first filling valve. Turn it a quarter turn so that the handle aligns with the pipe. You will not hear anything yet.
3. Slowly open the second filling valve. Turn it gradually. You will hear water flowing into the system. Open it slowly — fast filling can introduce air pockets.
4. Watch the pressure gauge. The needle will begin to rise. Fill the system until the gauge reads between 1.0 and 1.5 bar (the green zone).
5. Close the second valve first, then close the first valve. Both handles should be perpendicular to the pipe when fully closed.
6. Check the pressure holds. Turn the heating on and run it for 30 minutes, then check the pressure again. A slight rise during heating (up to 2.0 bar) is normal. When the system cools, it should settle back to 1.0–1.5 bar.