F15

Glow-worm Low water pressure / pressure sensor fault

F15 error, furnace display shows low system pressure and locks out

Safety Warning

This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.

If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.

Possible Causes

Insufficient system water pressure, Leaking radiator or pipework, Faulty pressure sensor on Glow-worm hydraulic block, Blocked expansion vessel connection

How to Fix / Troubleshooting

Safety: Allow the system to cool before topping up. Hot water under pressure can be dangerous.

User steps:

Check the pressure gauge; if below the recommended level (typically 1.0–1.5 bar cold), top up using the filling loop as described in the user manual.
After topping up, bleed radiators and recheck pressure.
If pressure drops repeatedly, there may be a leak; contact an engineer.

Technician steps:

Inspect the system for visible leaks at radiators, valves, and under the appliance.
Check the expansion vessel pre-charge and recharge or replace if failed.
Test the pressure sensor output to the PCB and replace the sensor if readings are incorrect.

Repair Difficulty

Medium 3/5

Required Part

System Pressure Sensor

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