9.7 Furnaces and Boilers
9.7 Furnaces and Boilers mxw1429.7.1 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
9.7.2 What Is a Boiler?
9.7.2 What Is a Boiler?A boiler is a heating system that warms water (or creates steam) and circulates it through pipes to radiators or baseboard heaters throughout your home.
Text description of the Oil Heating Boiler image.
The image depicts an oil heating boiler system, illustrated in a diagrammatic format. At the center is a cylindrical combustion chamber filled with water, which exhibits a swirling pattern, indicating the flow of heated water. The water is color-coded, with the top half in shades of red representing hot water and the bottom half in shades of blue indicating cooler water. Below the combustion chamber, a flame is visible, symbolizing the burner mechanism that heats the water. Surrounding the boiler are several components labeled with clear text: an expansion tank is on the left, allowing for fluid expansion; a flue at the top releases smoke; a circulator pumps water; and heating pipes extend outward at the right, showing the flow of hot water into a heating system. Additional elements at the bottom include an oil tank, filter, fuel pump, and controls, alongside a thermostat for regulating temperature.
How It Works:
- Fuel (gas, oil, or electricity) heats water in a sealed tank
- Hot water or steam travels through pipes
- Radiators release heat into rooms
- Cooled water returns to the boiler to be reheated
| Feature | Furnace | Boiler |
|---|---|---|
| What It Heats | Air | Water/Steam |
| How Heat Moves | Through ducts and vents | Through pipes to radiators |
| Distribution | Forced air | Hydronic (water-based) |
| Also Provides | Can include air conditioning | Usually heating only |
Key Point: Both systems heat your home—they just use different methods to move heat energy around.
Repair or Replace? When to Make the Call
Heating systems typically last 15–20 years with proper care. Consider replacement if:
- Your furnace is over 10–15 years old
- AFUE rating is 80% or lower (modern units are 90%+)
- Frequent breakdowns or costly repairs
- Rising energy bills without increased usage
- Uneven heating throughout your home
- Unusual noises (banging, hissing, rattling)
- Visible rust, cracks, or leaks
Rule of Thumb: If repair costs exceed 50% of the price of a new system, replacement is usually the smarter investment.