H.01 – .21 High

Baxi H.01 – .21 Fault Code

Also written as H0121 — same fault.

The Baxi H.01 – .21 fault code means: your boiler is heating up too quickly when you turn the tap on, which usually means water isn't flowing through the system fast enough to take the heat away.. It appears on 24 Baxi models in our database.

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: Temporary error – flow temperature increasing too fast in domestic hot water (DHW) mode, insufficient circulation or sensor error.

What causes the Baxi H.01 – .21 fault?

This fault occurs when the water inside your boiler heats up much faster than the system expects, causing it to shut down to prevent overheating. The most common reasons are a blockage in the internal heat exchanger or a pump that isn't able to push the hot water away from the burner quickly enough. It can also be caused by a faulty sensor giving the boiler an incorrect temperature reading.

This fault is slightly more common in summer when the boiler has been idle for long periods, which can lead to the internal pump sticking or debris settling in the heat exchanger.

Typical repair cost £120-£350 (including labour)

Engineer required

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

Find a Gas Safe engineer

Search the official register for a qualified engineer in your area.

Gas Safe Register →

Baxi models that show the H.01 – .21 fault code

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

Source

Baxi H.01 – .21 fault code information on this page is sourced and cross-checked against the official Baxi service / installation manuals for each affected model.

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