Smell gas? Leave the property and call the National Gas Emergency line on 0800 111 999

F.23

Glow-worm fault code

Glow-worm F.23: temperature difference too great between flow and return

Engineer only

The water leaving the boiler is much hotter than the water returning. Glow-worm lists a blocked pump, low pump output or air in the boiler as causes.

What you can safely do

There is nothing you can safely do yourself with this one. Leave the boiler off and book an engineer.

Find a Gas Safe engineer near you Opens the official Gas Safe Register search.

Applies to: Energy range, Betacom A range. Codes can differ between models, so check your manual.

From Glow-worm's own fault tables. Your model name is usually on the front of the boiler or in the user manual.

Model	Meaning	Causes listed for engineers
Energy range	Safety switch-off: temperature difference too great	Pump blocked, insufficient pump output, air in the boiler, flow and return sensors connected the wrong way round
Betacom A range	Water circulation fault	Not listed

Can I reset F.23?

Resetting won't fix a pump or circulation problem. Book an engineer.