Medium Lockout
42 on Ideal Maximiser SE

Your boiler is producing more heat than your hot water tank can safely handle, causing the system to stop for protection.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

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 hot water tank is too small for this unit power (external hot water tank).

What causes this fault?

This fault occurs when the boiler generates heat faster than your hot water cylinder can absorb it, leading to a rapid temperature spike that triggers a safety shutdown. The most common reason is that the boiler's power output is too high for the size of your external water tank, or there is a restriction preventing the heat from transferring efficiently into the water.

Engineer required

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

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