All furnaces and boilers have a life expectancy, so as yours gets up in years, deciding if you should repair it or replace it can be a tough choice.
As a baseline, we recommend following the 50% rule - if the cost of repairing your unit is more than 50% of the cost of replacing it, it's going to be more cost-effective in the long run to go ahead and replace it.
But before you pale at the thought of that expense, keep in mind that newer furnaces and boilers have advanced features, additional comfort capabilities, and are much more energy efficient - so the investment will likely save you money over time.