Cost wise there is nothing that will give a payback, an auto flue damper
that closes on shut down should have saftey lockouts to insure the
boiler does not run should the vent become stuck or Co can poison you.
This might cost 750-1000, a guess and save 5%-7 %. Making sure it is
maintained and burning right is about it. Your old units may be at
50-70% efficient new units can go to 96%. Best is insulation, low E
Argon windows and a new boiler. The two most cost effective things are
R-5 cellular window shades and attic insulation to above code.