Furnaces work by blowing heated air through ducts that deliver the warm air to rooms throughout the house via air registers or grills. This type of heating system is called a ducted warm-air or forced warm-air distribution system. It can be powered by electricity, natural gas, or fuel oil.

Furnaces have AFUE (annual fuel utilization efficiency) ratings. A 95 percent AFUE means 95 percent of the energy in the fuel becomes heat, while the other 5 percent escapes.

The minimum AFUE number required by the federal government for most furnaces is 80 percent. However, for a gas furnace to meet Energy Star standards, the AFUE must be 95 percent or more in the northern United States and 90 percent or greater in the South.

Furnace replacement:
There are three main signs that your furnace needs to be replaced. The average life of a furnace is 12-18 years. After that they start to become less energy efficient and work harder to heat your home. If your furnace is professionally maintained, it will have a longer life. No matter the age of the furnace, if it begins to break down frequently, and needs costly repairs, a good rule of thumb is if the cost to repair it is 50% of the cost of a new furnace, you should replace it.

A squealing or screeching sound could indicate a problem with the blower motor. It typically costs a few hundred dollars to replace the blower motor, but it may not be worth it to replace the part if your furnace is getting old. Your HVAC technician will discuss various options with you.