9.7.1 What Is a Furnace?
9.7.1 What Is a Furnace?A furnace is a heating system that warms air and distributes it throughout your home through a network of ducts and vents. It's the most common type of home heating system in the United States.
Text description of the Residential Furnace image.
The image features a furnace unit labeled "Lennox" positioned against a wall in a basement or utility area. The furnace consists of a gray metal body with a smooth texture and includes various components such as a large, curved duct that leads to the top of the unit. To the left, there is a metallic panel that holds a document with installation or operation guidelines, secured within a transparent holder. Below the furnace, there are several warning stickers and informational labels, including cautionary symbols related to safety. The surrounding environment appears to be unfinished concrete, with some visible piping and a glimpse of a red cooler in the background.
How Does a Furnace Work?
Most homes use gas furnaces. Here's the basic process:
- Ignition: Natural gas is lit in the burner
- Heat Exchange: The flames heat up a metal component called the heat exchanger
- Air Warming: Cool air from your home passes over the hot heat exchanger
- Distribution: A blower fan pushes the warmed air through ducts to rooms throughout your house
Energy Efficiency: What to Look For
When shopping for a furnace, energy efficiency saves money and reduces environmental impact. Here are three key features to understand:
AFUE Rating
AFUE (Annual Fuel Utilization Efficiency) measures how efficiently a furnace converts fuel into heat—similar to MPG for cars.
- Higher AFUE = Better efficiency
- Modern gas furnaces: 89–98% AFUE
- Example: A 90% AFUE furnace converts 90% of fuel into heat; 10% is lost
Heating Stages
Furnaces can have different operating modes:
| Type | How It Works | Energy Use |
|---|---|---|
| Single-stage | Runs at full power only | Less efficient |
| Two-stage | High or low power settings | More efficient |
| Variable-speed | Adjusts output continuously | Most efficient |
Think of it like a car: cruising at a steady (continuous) speed uses less gas than accelerating to full speed, coasting, and accelerating back to full speed.
ENERGY STAR Certification
ENERGY STAR is a government-backed label for energy-efficient products. Furnaces with this certification meet strict efficiency guidelines and can save you money on utility bills.
| Feature | Gas Furnace | Oil Furnace |
|---|---|---|
| Efficiency | 89–98% AFUE | 80–90% AFUE |
| Upfront Cost | Higher | Lower |
| Fuel Cost | Natural gas is cheaper | Heating oil is more expensive |
| Availability | Requires gas lines | Can be used anywhere (fuel delivered by truck) |
| Popularity | Most common today | Older homes, rural areas |
Bottom Line: Gas furnaces cost more upfront but save money long-term through better efficiency and lower fuel costs.
Furnace Maintenance Tips
A well-maintained furnace can last 15–20 years. Follow these basics:
Professional Installation: Most problems come from poor installation, not equipment failure
Annual Service: Have a technician inspect your furnace before each heating season
Filter Changes: Replace air filters every 1–3 months (dirty filters reduce efficiency and air quality)
Keep Vents Clear: Make sure air vents and the furnace area aren't blocked