Overview
Vaillant is Germany's most prestigious residential boiler brand. Baxi is a longstanding British manufacturer. Both have excellent reputations — but at different price points.
Price
A comparable Baxi 800 combi is typically £150–£300 cheaper than a Vaillant ecoTEC equivalent. Baxi wins on price.
Reliability
Vaillant has a stronger long-term reliability record, particularly after 10 years. Both are well above average compared to budget brands.
Warranty
Vaillant: up to 10 years. Baxi: up to 10 years. Equal.
Parts
Both have good UK parts availability. Vaillant has a slightly larger European parts network; Baxi's UK manufacturing base gives it good domestic supply.
Verdict
Vaillant ecoTEC Pro/Plus for homeowners who want the best long-term reliability and efficiency. Baxi 800 for those who want strong quality at a more competitive price point.