Furnaces 101

Furnaces fueled by natural gas are by far the most common heating system for homes in the United States. Newer models are very efficient. They tend to last 20 years on average, but they do require annual inspections and some basic maintenance to function safely and well.

How a Furnace Works

Furnaces heat your home by circulating heated air through a series of ducts. That air is heated by the burning of fuel within the furnace. Furnaces can be fueled by natural gas, oil, and even propane, but natural gas is the most prevalent. As the fuel burns, the hot gases that are generated move through curved metal tubing called a heat exchanger, and then exit your home through a vent pipe. Simultaneously, the air circulating through your ducts passes over the exterior of the heat exchanger, and gathers heat from the hot metal. That warmed air is then circulated throughout your house. The heat exchanger keeps the combustion air and the supply air separate, ensuring that the air in your home is heated without risking exposure to the toxic gases that are the by-products of combustion.

Furnaces can vary widely in efficiency. That efficiency is measured by the furnace’s annual fuel efficiency, or AFUE rating. This number calculates the percentage of energy that is returned to your home as warm air.

Standard Furnaces have an average AFUE rating of 80-85%. This means that 80-85% of every dollar you spend on heating energy is returned to your home as warmed air. Standard furnaces have only one heat exchanger to collect the heat produced by the gas burner. An AFUE rating of 80% means that 20% of your energy is lost as water vapor that's exhausted outdoors through the flue pipe.

High-Efficiency Furnaces go one step further, and use a secondary heat exchanger to evaporate the water vapor a second time, to extract even more available heat. High-efficiency furnaces can have ratings as high as 96 AFUE, which translates to an energy loss of only 4%. High-efficiency furnaces are rapidly becoming the standard as the preferred choice for new construction and as older furnaces fail and are replaced by newer models.

Older Furnaces, or gravity-flow models, functioned without a blower fan, and had efficiencies of as little as 30%. These are no longer manufactured, but there are still some in use. If you have one, you’d be well advised to replace it. Newer models are vastly more efficient, and offer energy-saving features such as electronic pilot light ignitions, sealed combustion units, and vent dampers. They’re far safer as well.

How They Wear Out

Most furnace deaths are due to cracked heat exchangers. Cracks can occur over time due to expansion and contraction, as the burners turn on and off to regulate the heat in your home. That expansion and contraction eventually fatigues the metal to the point where it cracks. This is a serious issue, as even a small crack can allow for deadly carbon monoxide gas to enter your home.

Maintenance and Safety

Every year, hire a licensed professional to test your furnace and conduct an inspection to check for wear, damage, and possible carbon monoxide leaks, as well as to lubricate parts and ensure that  safety mechanisms operate properly. Annual tune-ups typically run anywhere from $50-100, but they can reduce your heating costs by as much as 5%, and more importantly, help guard your family against exposure to deadly carbon monoxide gas.

In addition, here are some steps you can take to help ensure your safety, keep your furnace healthy and efficient, and save you money along the way.

Change your filters once a month, or clean your reusable filters every other month. A dirty filter decreases the airflow and prevents the system from working efficiently, results in higher heating bills, and can cause possible damage to the motor. More than half of furnace service calls are caused by ordinary dust and dirt clogging up vital components.

Purchase and install a carbon monoxide alarm near your furnace, or better yet, in several locations around your house. Carbon monoxide gas is odorless and colorless, so you can’t detect its presence on your own. If you already have a carbon monoxide alarm, and it is more than 5 years old, replace your existing alarm with a new, improved model. The Consumer Product Safety Commission, in cooperation with Underwriter’s Laboratories, has advanced the standard for reliability of these detectors since 1999.

Install a programmable thermostat. By automating your heating system, you can set it and forget it for long periods, and help save energy by not heating your home when you're away.

Check your interior heating vents to ensure that they are not obstructed in any way. You’ll promote more even heat flow throughout the house, and save energy.

Don’t forget the outdoor exhaust vents. In winter, keep snow and ice away from the outdoor intake and exhaust vents. If those vents become blocked, dangerous carbon monoxide fumes can back up into the house, and the furnace could shut down.

A Word About Filters

Not all filters are created equal. When selecting your filters, you may be tempted to buy the least expensive one to save some money. If you have allergies, asthma, or other respiratory ailments, you’ll be much better off purchasing a higher quality filter.

Here are the three common types of furnace filter.

  • Disposable fiberglass filters are certainly the cheapest, but they’re also the least effective. They block only the largest particulates to protect your furnace. Smaller particles, like pollen and mold, sail right through, and can make sensitive individuals miserable.
  • Washable electrostatic filters contain a static charge that attracts dust, dirt, and other matter. These filters are more effective than disposable models, but still screen out only 15-20% of airborne particles. They can also be a chore to clean.
  • Pleated filters use a more dense mesh material to trap particles, further enhanced by a pleated design to increase the surface area. This helps eliminate large allergens like pollen and mold and more than doubles the efficiency of cheaper models. While they are more expensive per filter, they are designed to last 3 times longer, so you can replace them less often.

Keep the Home Fires Burning

If you do the little things and dutifully look after your furnace, you can prolong its life span, and keep your energy bills low. More importantly, you help ensure your family’s safety, and that thought should warm your heart like no furnace can.


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Author: Robert Bundy