High Warning
No DHW function (CH OK) on Worcester Bosch 24si II

Your boiler is providing central heating but is failing to detect when you turn on a hot tap, meaning you have no hot water.

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

Flow sensor failure. DHW sensor (LED 4 flash/second). Board fault.

What causes this fault?

This fault occurs when the boiler's internal sensors fail to detect water moving through the system when you open a tap. The most common reason is that the plastic turbine inside the flow sensor has become stuck with limescale or debris, preventing it from sending a signal to the boiler to fire up for hot water. In some cases, the temperature sensor has simply worn out and is providing an incorrect reading that tricks the system into staying off.

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.

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