How to Replace the Expansion Vessel on a Baxi Platinum Combi 40 (UK)
Step-by-step UK guide to replacing the internal 8L / 10L / 12L expansion vessel on a Baxi Platinum Combi 40. Part numbers, torques, commissioning.
The part
Replace or top-up the internal expansion vessel on Baxi Platinum Combi 40 when the pressure gauge climbs above 2.5 bar on heat-up.
Isolate safely
Fully drain the system to zero bar, disconnect the Schrader valve and check pre-charge — should be 0.75 bar for Baxi Platinum Combi 40.
Fit and commission
If the pre-charge holds but the diaphragm is perished, fit a new OEM vessel or an external 12 L wall-mount vessel on the return.
Recommission checklist
Refill the Baxi Platinum Combi 40 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
- External vessel size?
- For a system up to 100 L water content, an external 12 L vessel supplements the internal one on Baxi Platinum Combi 40.
- What pre-charge?
- 0.75 bar for a 2-storey property, 1.0 bar for 3-storey. Charge with the system fully drained.
- Do I need to notify the manufacturer?
- Only if the Baxi Platinum Combi 40 is in warranty — Baxi require log of any part swap for labour claims.
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