Boilers are measured by annual fuel utilization efficiency (AFUE). An AFUE of 80 means 80% of the energy from fuel is transformed into heat and 20% is lost. New gas and oil units are required to have an AFUE of 78 or above, and energy-efficient units need a rating of at least 85.
Any 20-year-old boiler may only work at 60% or 50% heating efficiency, or even less. On top of that, the government regulations for energy consumption were different 20 years ago than they are today, so it probably didn't start out with an equivalent heating efficiency rating as a modern boiler.