Gas, Electric, Oil: Getting to Know Your Furnace

Maybe your old furnace needs replacing, or maybe you're building your dream home and selecting all the right modern details. It could be you're just curious as to how furnaces work. Regardless, you want to know more about all the different furnace options there are today, and we can't blame you—there are a lot of choices out there, and the furnace you choose can have a major impact on your comfort, your energy bill, and your environmental footprint.

You won't exactly be an HVAC professional when you're done reading this article, but you will have a better understanding of how each major type of furnace works and what you can expect in terms of cost—both the up-front cost of the furnaces themselves, and the cost of operation and ongoing maintenance.

Electric Furnaces

Though there's a lot going on to make furnaces work safely and efficiently, all types of furnaces—including electric furnaces—are made of the same basic components. There's a source of heat, a "heat exchange" that allows for an efficient transfer of heat from the source to flowing air, and a blower or fan that moves air past the heat exchange and into the ductwork that carries the hot air throughout your home.

In an electric furnace, the heat source and the heat exchange are one and the same. Current is taken from the street supply through a dedicated connection—often a double circuit—in your electric panel, and passed through a bunch of thin pieces of metal. These pieces of metal heat up due to electrical resistance (we'll spare you the technical details, but it works exactly the same in your furnace as in your toaster), and the blower forces air through the mesh of heated metal where it picks up heat and carries it into your home.

Electric furnaces have been around for awhile, and are still frequently used in new constructions and as replacement furnaces--especially in rental units—because they tend to have lower up-front costs than other types of furnaces, and maintenance can also be cheaper and less frequently necessary.

Operating an electric furnace tends to be more expensive than other furnaces per unit of heat produced, however. Although electric resistance heating is itself 100% efficient—all of the electricity used is converted into heat—the process of making that electricity is often very inefficient, and it takes a lot of electricity to create a decent amount of heat.

Some argue that electric furnaces are better for environmental sustainability; both oil and gas furnaces require burning fossil fuels to create heat, while electricity can be reproduced in cleaner and "renewable" ways—wind, solar, and hydro, generally. Of course, much of our electricity is still produced in power plants that burn coal, so the difference your furnace choice makes to the environment really depends on your area and how your power is being generated.

If you have your own solar panels and/or a wind-powered generator, electricity might be more cost effective and more environmentally friendly, but not many households can produce enough electricity on-site to power their furnace. In general, electric furnaces are giving way to gas-powered furnaces in areas that have natural gas lines available.

Gas Furnaces

Though they work on the same general principle as electric furnaces, gas furnaces have a separate heat source and heat exchange. In gas furnaces, the heat source is a combustion chamber in which gas is ignited, and that heat is transferred to a series of thin metal tubes, similar to the setup used in electrical resistance heating. The materials used for the heat exchange in modern gas furnaces are able to absorb and store a great deal of heat, meaning they stay hot for a long time while requiring only a modest amount of gas to operate.

Then, as in an electric furnace, the blower forces air through the tubes of the heat exchange, where the air picks up heat from the surrounding metal before entering your home's ductwork and carrying the heat to your living areas.

If you look at the  EnergyStar  website, you'll see that all of the most efficient furnaces are gas-burning furnaces. Because of advances in materials and manufacturing processes, the heat exchanges have been made more efficient in terms of both holding heat and transferring it to the surrounding air as needed, and combustion chambers have become much better insulated, resulting in more heat making it to the heat exchange rather than being lost in the air around the furnace itself.

It's possible to get a gas furnace that operates with 97-98% efficiency, and because there's virtually no efficiency lost in transferring the natural gas to your home—unlike the energy-leaking process of generating and transferring electricity—these gas furnaces are considered more efficient overall. Natural gas is also much cheaper than electricity per unit of energy, in most of the country and most of the time, giving many homeowners another good reason to go with a gas furnace.

The latest and most efficient gas furnaces can be more expensive upfront than some of their electric counterparts, but they'll easily make up the difference in energy savings over their lifespan.

Oil Furnaces

Oil furnaces are waning in popularity, but they're still around. They work almost identically to gas furnaces (until you dig a bit more beneath the surface); the oil is brought into the furnace's combustion chamber, heating the heat exchange, and the blower forces air through to the ductwork.

Oil furnaces also require on-site oil tanks, though, and they need to be filled by an oil truck that comes to your property periodically and charges you the going rate for heating oil—essentially diesel fuel—to fill up. Prices can fluctuate widely, but generally the cost per unit of heat produced by an oil furnace is somewhere between the cheaper natural gas and the more expensive electric.

Because oil furnaces tend to be more costly to operate than gas furnaces, with the added inconveniences of an oil tank versus a dedicated gas line or electrical connection, and with the same maintenance requirements as a gas furnace, it's no surprise that these are becoming rarer. In areas where natural gas isn't available, however, they can be a more efficient and affordable option in the long-run than an electric furnace.

Which Furnace is Right for You?

Armed with a bit of information, it should be easier for you to find a new furnace that's right for your budget and your long-term comfort. And of course, remember that the cost of operating and maintaining your furnace depends on keeping it clean and performing basic tasks like filter changes regularly—no matter what type of furnace you settle on, take care of it and it will take care of you.