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.
Furnaces have become more energy-efficient over the years. A gas furnace made in the early 1970s typically has an AFUE of about 65 percent. The lowest efficiency allowed by law for new gas furnaces is 78 percent, and some new models achieve 97 percent--near-total efficiency.The price of a furnace generally rises in step with its fuel efficiency.fficiency also matters. Gas is currently the most common heating fuel and most new central-heating systems use gas. How efficiently a furnace converts gas into heating energy is reflected in its annual fuel-utilization-efficiency (AFUE) rating, which is measured as a percentage.