Atag Boiler Review

iCO Modulation Mastery

Atag’s iCO patented technology achieves a 1:10 modulation ratio – the widest in the industry – allowing a 30kW boiler to fire as low as 3kW, eliminating short-cycling in super-insulated new builds. This is critical for UK properties meeting 2022 Part L regulations where heat loss can drop below 5kW. Their iCon stainless steel heat exchanger uses a single-piece welded construction (no O-rings) that eliminates leak points we commonly see in Viessmann and Vaillant after 8 years. The Atag OneZone smart control natively supports weather compensation and OpenTherm, integrating seamlessly with Tado° and Nest – a flexibility Vaillant’s closed ecosystem lacks. For British homeowners, the compact 595mm height fits inside standard kitchen wall units, a crucial advantage in space-constrained London flats where acoustic boxing isn’t an option. Atag holds 3.2% UK market share, operating in the shadow of Worcester and Vaillant. Their sweet spot is self-build and custom homes where architects specify efficiency over brand recognition. We’ve installed 150+ units in Graven Hill self-build village (Bicester) since 2016, where Passivhaus standards demand sub-6kW minimum modulation. The brand’s Achilles’ heel is resale value: UK buyers recognize Worcester; Atag raises eyebrows during property surveys, occasionally triggering mortgage valuers to question long-term parts availability. Their pricing sits 10-15% below Viessmann but 20% above Worcester, targeting informed buyers who research modulation ratios rather than relying on brand heritage.