If mainteined properly then on average most Heaters (or Furnaces) last no less then 15-20 years. Often Furnace is installed along with AC unit, but AC life expectancy is a bit smaller - air conditioning units usually last 10-15 years, so keep that in mind. Electric or gas water heaters has a life expectancy of about 10 years, while tankless water heaters last more than 20 years. Summary: Brand, type of a furnace and regular maintenance performed by professional technician will maximise efficiency and useful life of your Heating system.