This study presents the design, thermodynamic analysis, and experimental validation of a condensing economizer dedicated to bi-omass-fired heating boilers. The proposed device incorporates a centrally arranged flue-gas distribution chamber and a water-cooled tube bundle that enables recovery of both sensible heat and the latent heat released during water-vapor condensation. The economizer, protected under Patent No. PL247342B1 was evaluated through analytical modelling and laboratory testing per-formed on pellet boilers with rated outputs of 25–100 kW. The heat-transfer model, which couples convective and condensation mechanisms, demonstrated that the presence of water vapor in the flue gas significantly increases the effective heat transfer coef-ficient, thereby enhancing the overall energy recovery potential. Prototype units with heat-exchange surfaces ranging from 0.63 to 2.90 m2 achieved thermal outputs between 1.8 and 7.8 kW, with stable condensation occurring when the boiler’s return-supply temperature difference was maintained at ΔT ≥ 25 °C. Experimental results confirmed a substantial reduction in flue-gas tempera-ture (down to 59 °C at nominal load and 34 °C at reduced load), which translated into an increase in gross boiler efficiency to 107% and 119%, respectively. All emission indicators complied with the Ecodesign Directive (EU) 2015/1189, while seasonal effi-ciency analysis yielded an Energy Efficiency Index of 167.8 (A+++). The findings demonstrate that the integrated counterflow–crossflow economizer significantly improves waste-heat recovery, reduces fuel consumption, and lowers particulate and gaseous emissions. Owing to its modular construction and corrosion-resistant design, the system is suitable for practical implementation in modern biomass heating installations.