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.
Just because you called an HVAC company and it performed an estimate doesn't mean you are obligated to purchase from it. Natural gas furnaces generally last 10 to 15 years, so don't budge on getting the best furnace your budget allows. When you call for a quote, ask the HVAC company about its licensing. A suitable contractor needs to be licensed and certified to install your new system.