BoilerFaultCodesGlow-worm F23 Fault Code
The Glow-worm F23 fault code means: your boiler has stopped working because the water isn't moving through the system correctly, which is often caused by air bubbles or a blockage.. It appears on 2 Glow-worm models in our database.
Technical description: Water circulation fault
What causes the Glow-worm F23 fault?
This fault occurs when the boiler detects a significant temperature difference between the water leaving the unit and the water returning to it, indicating that heat is not being carried away fast enough. The most common reasons are a failing internal pump that can't push the water around, or a build-up of sludge and debris that has created a physical blockage in the pipework. It can also be triggered by a large pocket of air trapped within the heat exchanger, preventing smooth water flow.
This issue often surfaces in autumn when the heating is turned on for the first time in months, as pump components can seize up or debris can settle during the summer period.
DIY fix possible
Check that all radiator valves and isolation valves are fully openBleed your radiators to remove trapped air from the systemCheck the boiler pressure gauge and top it up to 1.5 bar if it is too low
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Avoid the next call-out bill
A cover plan would handle faults like F23: no call-out fees, no surprise bills.
Affiliate links. We may earn a commission at no extra cost to you.
Glow-worm models that show the F23 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Glow-worm F23 fault code information on this page is sourced and cross-checked against the official Glow-worm service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026