Can I clean rather than replace?

Yes for a limescaled unit — soak the turbine in citric acid, but replace if the Hall-effect PCB is corroded.

What minimum flow triggers?

The Worcester Bosch Greenstar 4000 25 typically ignites at 2.5 L/min. Below that, check the aerator on the DHW outlet.

Do I need to notify the manufacturer?

Only if the Worcester Bosch Greenstar 4000 25 is in warranty — Worcester Bosch require log of any part swap for labour claims.

How to Replace the Flow Sensor on a Worcester Bosch Greenstar 4000 25 (UK)

The part

Swap the DHW turbine on Worcester Bosch Greenstar 4000 25 when hot water runs cold at low flow or the boiler fails to fire on demand.

Isolate safely

Isolate cold mains at the boiler's blue isolator. The sensor on Worcester Bosch Greenstar 4000 25 clips into the hydraulic block with two retaining clips.

Fit and commission

Fit new O-rings and align the turbine so the flow arrow matches the cold mains direction.

Recommission checklist

Refill the Worcester Bosch Greenstar 4000 25 to 1.2 bar cold. Bleed high-point radiators and the internal AAV.

Run a full flue-gas analysis after 15 minutes. Log the reading against the OEM commissioning envelope in the Benchmark book.

Frequently asked questions

Can I clean rather than replace?: Yes for a limescaled unit — soak the turbine in citric acid, but replace if the Hall-effect PCB is corroded.
What minimum flow triggers?: The Worcester Bosch Greenstar 4000 25 typically ignites at 2.5 L/min. Below that, check the aerator on the DHW outlet.
Do I need to notify the manufacturer?: Only if the Worcester Bosch Greenstar 4000 25 is in warranty — Worcester Bosch require log of any part swap for labour claims.

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