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.”

About the Experts

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