It turns out the oil-burning colossus in the basement of my childhood home wasn’t a furnace, as I thought until recently.

My memories are full of instances of cozy gatherings around the cast-iron radiators on cold winter days, and drying soaked outdoor clothing on them. I thought that would only be possible if we had a furnace in the basement. I was too young to notice the array of hot water pipes — the telltale sign of a boiler.

“If you have heat that comes out of a vent, you have a furnace,” says John Gabrielli of Air Temp Solutions. “If you have heat that comes from radiators, you have a boiler. This is a tiny bit of a generalization, but for 99% of homes, it’s accurate.”

Any hot water heating system — including radiators, in-floor radiant heat or hydronic baseboard heaters — relies on hot water or steam from a boiler, not heated air from a furnace.

The house where I live now has a boiler of sorts. It’s actually a water heater that supplies radiant floor heat in a single room, while also providing hot water for the bathroom. Most boilers that supply radiators or radiant heaters in an entire house are more complex than this, however.

What Is a Boiler?

Tim David, CEO of Airlucent, explains boilers this way:

“A boiler is kind of like what you would think based on its name.” he says. “It produces hot water or steam which gets circulated throughout a building in a network of pipes. Boilers are typically used in northern areas that have colder climates, because of their reliability and consistency as a heat source.”

Because a boiler heats water with electricity, natural gas, propane, heating oil or wood pellets, it isn’t much different from a water heater.

In the past, boilers were enormous, like something you would find in a locomotive. Boilers for large buildings may still be as imposing. But most contemporary residential boilers are streamlined to fit inside a modestly sized square, rectangular or cylindrical housing. Some even hang from the wall and operate like tankless water heaters.

In a home with radiators, the boiler heats the water to produce steam. According to Energy.gov, circulating steam is a less efficient use of fuel than the alternative — heating the water just shy of the boiling point, then circulating it through radiant in-floor or baseboard heaters.

What Is a Furnace?

“A furnace warms air and then distributes it throughout the house or apartment via a system of ducts and vents,” David says.

Because furnaces heat the air directly, they raise the building temperature much faster than boilers. As a bonus, David says, they’re easier to work on. But they’re still complex, with lots of moving parts.

Besides a combustion chamber or electric heating element, furnaces need a blower to circulate the air. Those that burn a fuel need a heat exchanger to separate the gases in the combustion chamber from the air circulating in the building. Also, many furnaces have an inducer fan, also known as a draft inducer, to circulate air through the combustion chamber.

Because the blower and draft inducer need to run when the burners go on and off, a furnace needs a system of switches and controls. Malfunctions can often be traced to these electronic components.

Boiler vs. Furnace: Differences

Besides functionality, here are some key differences between boilers and furnaces, according to David:

  • Boilers provide more consistent warming: Because warm air rises and cool air falls, the temperature in the space heated by a furnace can feel uneven. Radiant heat from a boiler is more uniform.
  • Boilers don’t affect indoor humidity: Furnaces remove moisture from the air. Dry air can be uncomfortable to breathe and may cause problems with wooden furniture and floors. Radiant heat from a boiler system has no effect on humidity.
  • Boiler installation is more complex: That’s because it requires a piping network. In a house with ductwork for an air conditioning system, a furnace is easier and less expensive to install.
  • Boilers are more energy efficient: “Especially those that use condensing technology,” says David.
  • Boilers last longer than furnaces: That’s because they have fewer moving parts.

One last note from David about installation and operating costs:

“The initial installation costs of a boiler are higher than that of a furnace, which is usually a deciding factor for many,” he says. “But this can be offset by the boiler’s energy efficiency and longer lifespan.”

Furnace FAQs: Common Questions and Parts Explained

Whether you’re trying to understand how your furnace works or troubleshooting issues, here are the answers to some of the most common furnace questions.

About the Experts

  • Tim David, the CEO of Alabama-based Airlucent, is a retired HVAC tech with 25 years of experience.
  • John Gabrielli is the owner of Delaware-based Air Temp Solutions.

Author
Chris Deziel
Chris Deziel is a freelance service journalist who worked in landscaping and home improvement for more than thirty years. He has published over 10,000 how-to and expository pieces on home improvement and nature topics since he began writing in 2010. Deziel lives and works in an off-grid home in California where he enjoys recording original music an...
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Chris Deziel