F84 Medium

Glow-worm F84 Fault Code

The Glow-worm F84 fault code means: your boiler has detected an unusual and lasting temperature difference between the water leaving it and the water returning to it, so it has safely shut down to prevent damage.. It appears on 8 Glow-worm models in our database.

Book a service

This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description: Permanent temperature difference between flow and return heating sensors
Typical repair cost £150-£300 (including labour)

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Glow-worm models that show the F84 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Glow-worm F84 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 31 August 2026