How to Replace a Boiler Flow Sensor (UK)
Fit a new DHW flow turbine sensor on Worcester, Vaillant, Baxi combis. Symptoms: hot water lockout, cold flushes.
Worcester BoschVaillantBaxiIdeal
How the sensor works
A tiny impeller turns as DHW flows past — the Hall sensor sends a frequency to the PCB. Below 2.5 L/min the boiler ignores the demand (below the DHW minimum).
Common OEM numbers
- Worcester Greenstar — turbine 87161567150.
- Vaillant ecoTEC — 178988 (integrated in the aqua sensor block).
- Baxi 800 Combi 2 — 720855801.
- Ideal Logic — 174425.
Fit tips
Isolate the cold mains at the boiler service valve. Fit the O-ring dry, not greased with silicone (some kits ship with EPDM that swells with silicone contact).
Frequently asked questions
- Why no hot water but heating works?
- 90 % flow sensor or DHW NTC. Swap the flow sensor first — cheaper and more common failure.
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