Solid Fueled Furnaces and Their Types

What is a Solid Fuel Fired Furnace?

Solid fuel furnaces burn wood, pellets, coal or even biomass in a combustion chamber to generate heat. They don’t rely on oil, natural gas or electricity like the other furnace types. The furnace is usually located away from common living spaces and distributes heated air throughout the home through a network of ducts. This makes them different from appliances such as pellet stoves or fireplace inserts, which are placed in living areas such as family rooms and emanate heat directly from the appliance.

Since these furnaces use combustible materials as fuel, solid fuel furnaces require chimneys to carry off smoke and gasses generated from the combustion process.

They are an excellent choice for secondary heat sources or primary heat sources for homes or outbuildings that aren’t connected to the energy grid. Modernize created this guide to solid fuel furnaces to help homeowners understand the unique benefits these types of furnaces offer, potential drawbacks, costs, and more. Read on to learn more about how solid fuel heaters can be used to offset energy costs and increase the comfort of your home in colder months.

Types of Solid Fuel Furnaces

As noted earlier, homeowners who are considering adding a solid fuel furnace to their homes have several different options from which to choose.

Coal Burning Furnaces

Coal has been used as a fuel source for heating and cooking for centuries. In the mid 1880s, coal actually surpassed wood as the primary heat source in American homes. Coal is still used today in homes in the eastern U.S. because it is readily available, inexpensive, and easier to use than wood.

Coal fired furnaces generate heat by burning either Anthracite or Bituminous coal inside the combustion chamber. The former is between 86 and 97 percent carbon, resulting in greater efficiency. The latter is between 45 and 86 percent carbon, but it is the most common type of coal mined in the U.S.

Wood Burning Furnaces

Forced-air wood burning furnaces work just like gas, oil or electric furnaces except they burn wood as a fuel source. Wood furnaces must be loaded manually by placing wood inside the combustion chamber. Heat is distributed throughout the home’s ducting system or by a blower installed in the appliance.

Other Types of Solid Fuels

Wood and coal aren’t the only fuels used in solid fuel fired furnaces, but they are the most common. Here are two other alternative fuel sources:

• Corn. Corn stoves use dried corn kernels as a fuel source. Corn kernels contain both ethanol and oil, so they are a clean-burning fuel source.
• Biomass. Biomass is an umbrella term that describes a range of plant- or animal-based organic fuel sources. Biomass could include corn cobs, wood chips and sawdust, or it could be a mix of animal, agricultural, household and industrial waste.

Biomass fuels have grown in popularity because they are renewable, environmentally friendly, inexpensive, and easy to source. They generally require larger storage space, however, as well as larger furnaces.

How Much Solid Fuel Fired Furnaces Cost?

Solid fuel furnaces typically cost much more than common gas or oil fired furnaces. Homeowners also have to take into account the ongoing costs of buying wood unless they have access to a readily available supply on their properties.

Wood Burning Furnace and Boiler Costs

On average, homeowners can expect to pay between $9,000 and $16,000 to have a new outdoor wood furnace installed. Wood fired burners that use the highly efficient gasification process to generate heat are likely to cost between $10,000 and $20,000. These prices are for models that can heat a 2,000 to 3,000-square-foot home, so costs could be lower if your heating needs are less. A new wood fired indoor furnace or boiler, meanwhile, could cost between $2,000 to $5,400 installed.

Coal Burning Furnace Costs

Homeowners who have easy access to coal to power their furnaces and are eyeing a new coal fired furnace can expect to pay between $3,200 and $9,500 to have a unit installed. Cost could be toward the lower end of this range for smaller furnace units.

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Coal Versus Wood Fired Furnaces: Pros and Cons

A lot of factors come into play when considering the decision to use coal or wood for your solid fuel fired furnace. Here are 10 primary considerations:

• Storage space. Wood takes up way more room than coal. One single cord of wood measures 4’ wide by 4’ high by 8’ long. That’s a big pile totaling 128 cubic feet. A ton of coal, however, will take up about 40 cubic feet of space. 

• Heat production. Heat generated from wood depends on the species. Red oak is the hottest, burning at about 1,600 degrees Fahrenheit. Softwoods such as pine burn at less than 1,000 degrees Fahrenheit. Anthracite coal, though, burns between 2,500 and 3,500 degrees. Bituminous coal, however, represents more than 90 percent of the coal burned in the U.S. and burns at 980 to 2,600 degrees.

• Seasoning – Wood needs to be properly seasoned in order for it to burn at its optimal temperature. Seasoning requires air-drying to remove moisture. Bituminous or Anthracite coal do not have to be seasoned. 

• Availability. Coal is not readily available as a solid fuel source outside of regions where it is commonly mined. Wyoming produces about 40 percent of all coal mined in the U.S. Another third comes from the Appalachian region of Kentucky, Tennessee, Virginia and West Virginia. Wood fuels, however, are more readily available in most parts of the U.S.

• Cost. The average cost for a cord of wood is about $300. Seasoned wood and hardwoods could cost much more, though, and sellers also usually charge delivery and stacking fees. Wood pellets, meanwhile, typically cost between $5 and $9 for a 40-pound bag. You might be able to purchase one ton of pellets for $250, with delivery fees ranging from $50 to $100. In 2021, bituminous coal cost about $61 per short ton, while anthracite coal cost $107 per short ton.

• Chimney fires. Burning wood emits creosote, a potentially harmful substance that stays in your chimney and requires regular cleaning to avoid a chimney fire. Coal doesn’t have that issue.

• Pollution/carbon emissions. Wood combustion releases carbon dioxide, methane and nitrogen dioxide into the atmosphere. Coal combustion also emits carbon dioxide, though how much depends on the carbon, hydrogen, oxygen and sulfur content present in the coal.

• Ash pit and cleaning. Both fuel sources leave ash residue in the ash pit and require regular cleaning. Softwoods leave less ash than hardwoods. Generally speaking, burning a cord of wood – 128 cubic feet – will leave in six to 10 percent ash. Coal combustion can leave behind fly ash and bottom ash. Amounts depend on the type of coal being consumed.

• Burning rate. The average home using only wood for heating can consume as much as six cords of wood per year. Homeowners using coal will consume about three tons annually since coal burns much hotter.
•  Easy to load and store. Expect to load a wood fired furnace each morning and night. Expect to load a coal furnace every 12 hours.

Understanding Indoor Versus Outdoor Fuel Fired Furnaces

Both wood and coal fired furnaces can be placed either inside in remote locations of the home, such as a garage or basement, or outside. Outdoor wood furnaces are typically larger than indoor models and can be loaded with more wood so they don’t have to be restocked as often. These furnaces may also require heat exchangers to exchange heated water for hot air that’s blown throughout the home.

Indoor forced-air solid fuel-fired furnaces should adhere to strict installation requirements that include:

• Installation in a one-hour fire-rated room with an entry door leading to the outside.
• Hot air ducts and cold air returns that end no less than eight feet above the floor.
• Air for the combustion process should be drawn from outside sources.
• Chimneys and vent pipes should be inspected and cleaned annually.

Clearances for Fuel Burning

Solid-fuel furnaces require minimum clearances around the appliance to ensure safety. Here are some specifications to keep in mind:

• Create a minimum three-foot safe zone around the appliance for child safety.
• Minimum of 36” from the nearest combustible surface unless using heat shields.
• 18” from the flue to wall surfaces, and also along the length of the chimney stovepipe in all directions.

A licensed installer will be familiar with the exact specifications for your appliance and region.

Optimizing Solid Fuel Combustion

Just because you have installed a solid fuel furnace does not necessarily guarantee savings on energy costs – you need to ensure the furnace is optimized for fuel flow and combustion. Follow these tips to ensure your solid fuel furnace is operating at its fullest potential:

• Continuous fuel. Load fluctuations can reduce furnace efficiency. Always make sure the furnace has a continuous fuel source for uniform feed and combustion rates.
• Regulate under-fire and over-fire air flow. Controlling the air/fuel ratio is crucial to achieve maximum efficiency. The best approach for under-fire air is to have a control system in place that regulates air and fuel input. Similarly, introducing too much over-fire air results in heat loss and decreased efficiency. Find the balance between your system’s oxygen and carbon monoxide levels by taking readings or have a professional optimize your system.

Solid Fuel Furnace Maintenance

You will have to perform routine maintenance to keep your solid fuel fired furnace clear of ash.

Regardless of whether you burn coal or wood, expect to clear the firebox at least once a month unless your appliance has an auger system that pushes out the ash. Even then, you will still likely have to perform semi-regular cleanings to remove ash from the sides of the firebox.

You may want to wear a dust mask to avoid inhaling any ash particles as you shovel ash out of the appliance. If your unit also heats water, you will have to inspect the anode rod annually. Lastly, wood stoves generate creosote during the combustion process, so you will want to have your chimney cleaned on an annual or semi-annual basis to reduce chances of a chimney fire.