d.20 Low

Heatline d.20 Fault Code

Also written as D20 — same fault.

The Heatline d.20 fault code means: your boiler is currently monitoring or adjusting the temperature limit for your hot water supply, and this message indicates that the system is functioning within its set parameters.. It appears on 7 Heatline models in our database.

Monitor it

This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.

Technical description: Maximum temperature setting for domestic hot water

Engineer required

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

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Heatline models that show the d.20 fault code

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

Source

Heatline d.20 fault code information on this page is sourced and cross-checked against the official Heatline service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026