Block 85 Medium

Atag Block 85 Fault Code

Also written as BLOCK85 — same fault.

The Atag Block 85 fault code means: your boiler is currently unable to sense water moving through it, so it is running an automatic internal process to clear out any trapped air bubbles before it will allow the heating to start again.. It appears on 8 Atag 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: The control has not detected a water flow. The venting cycle is started. If during this cycle water flow is detected, the venting cycle is ended and the burner is released.
Typical repair cost £120-250 (including labour)

Engineer required

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

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Atag models that show the Block 85 fault code

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

Source

Atag Block 85 fault code information on this page is sourced and cross-checked against the official Atag service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026