In the process of fuel combustion not all energy of combustion gets delivered to the radiators. Some energy is lost in the process, particularly is vented trough the chimney.
Standard efficiency atmospheric boilers have continuous air flow trough the boiler hot surfaces to the chimney and heat is lost outside.
Heat losses in the atmospheric boiler amount to 12% of fuel costs and in condensing boilers are just a few percent.
So, if he heating bill is around $5,000 a year , losses trough the chimney are about $500 per year alone comparing standard efficiency boilers to condensing boilers.