High Warning
71:1 on Viessmann Vitodens 300-W

Your boiler is struggling to heat your hot water tank because it is reaching its internal safety temperature limit before the tank is actually full.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description

OFF during DHW loading to set value 1

What causes this fault?

This fault occurs when the boiler is unable to transfer heat to your hot water tank quickly enough, causing the internal water temperature to climb too high and trigger a safety shutdown. The most common reasons are a faulty sensor giving the boiler incorrect readings or a mechanical failure in the valve that directs hot water to your cylinder.

Engineer required

This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

Find a Gas Safe engineer
Search the official register →

Related fault codes

F2View →

Related guides you might find useful

No hot water but heating works

Almost always a stuck diverter valve on combis.

Read →

Banging / kettling / whistling noise

Each noise points to a different fault.

Read →

Radiators cold at the bottom

Sludge build-up — needs flushing, not bleeding.

Read →