How to Replace a Boiler Heat Exchanger (UK)
Replace primary or plate heat exchangers on Worcester, Vaillant, Baxi and Ideal combis. Torque, gaskets, flushing.
Worcester BoschVaillantBaxiIdealGiannoni
Primary vs plate
Primary HEX handles combustion side (Giannoni stainless on most UK boilers). Plate HEX transfers heat from primary to DHW. Blocked plate = weak hot water; corroded primary = whistling and F28.
OEM part numbers
- Worcester Greenstar 30i — primary 87186859290, plate 87172060270.
- Vaillant ecoTEC — primary 065131, plate 065153.
- Baxi 800 — primary 5114707, plate 720873201.
- Ideal Logic — plate 173418, primary 174485.
Flush before refit
Powerflush the system before fitting a new primary. New HEX + old sludge = warranty void inside 6 months.
Frequently asked questions
- How long should a heat exchanger last?
- 10–15 years on treated water with inhibitor at 500 ppm. Untreated systems fail in 3–5.
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