How to Replace a Boiler Gas Valve (UK, Gas Safe)
Fit a new gas valve on Worcester, Vaillant, Baxi and Ideal boilers. Honeywell VK4105, SIT 848 Sigma cross-refs.
HoneywellSITWorcester BoschVaillantBaxi
Gas Safe only
This is a notifiable gas-work job. Only Gas Safe registered engineers may fit and commission. Full FGA is mandatory post-fit.
Common OEM part numbers
- Worcester Greenstar — Honeywell VK4105Q (87161424040).
- Vaillant ecoTEC plus — SIT 848 Sigma (053464).
- Baxi 800 — 720078201.
- Ideal Logic — 175594 SIT valve.
Set the CO₂
Every gas valve swap requires a CO₂ set to manufacturer spec at max and min rate. Target ranges — Worcester 8.7–9.4 %, Vaillant 9.0 ±0.5 %, Baxi 8.8–9.2 %.
Frequently asked questions
- Can I re-use the old gas valve gasket?
- No. New copper crush washer or fibre gasket every time.
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