Another thing that people often point to as a negative thing about ketosis is that people can suffer symptoms such as headaches and dizziness in the first few days of a ketogenic diet. Also, athletic performance can suffer until the body adapts to using fat for energy instead of glucose.
Forced air furnace heat exchangers have an average life expectancy of 15 years, with a range typically between 13 and 20 years. Most heat exchangers used in models today today will last between 15 and 25 years and are usually made from aluminized or stainless steel.
A furnace will last for a long time. I Had a Rheem gas furnace installed in 1978 and it still works fine. Sure, it's had a few parts replaced over the years, but the price of repair is a whole lot less than the cost of replacing the furnace just because a fan motor failed.