Find Local Contractors

1. I need a new:

2. My zip code is:

How an Outdoor Coal Furnace Works

Outdoor coal furnaces can be a good alternative or supplement to a traditional gas furnace or boiler. They may seem quite complicated, but in reality are quite easy to understand and use.


An outdoor coal furnace is typically designed to be installed 25 to 200 feet away from the building it will heat, but often can be installed as far as 500 feet away. Because they are located outside and do not vent through a chimney, these furnaces contain their own vent.

How the furnace produces heat

Coal burns at extremely hot temperatures, in excess of 3500° Fahrenheit. A very thick firebox, where the coal is loaded and burned, is required for the intense heat. The firebox usually has a firebrick lining, which adds protection to the firebox while allowing it to hold heat much longer.

Oxygen is fed through a damper into the coal furnace’s firebox, which is surrounded by a water reservoir, also known as a water jacket. Many manufacturers use a rounded firebox to reduce the risk of cracks and leaks, as well as allowing the water to be heated more efficiently. The combustion of coal heats the water jacket. The amount of coal needed per day depends on factors including the size of the building, how well it is insulated and the outside temperature, but 25-50 pounds per day appears to be the average.

How it heats a building

Depending on the furnace and how the system is set up, hot air or hot water is transferred by pump through insulated underground pipes from the outdoor furnace to the building. Some coal furnaces have tubing running through the water jacket through which air passes, is heated, and then distributed through the underground pipes, while others have an inlet and outlet on the water jacket which circulates hot water to and from the building.

Water-to-air or water-to-water heat exchangers or direct circulation carries the heat into the home or building’s forced-air furnace, radiant baseboard or radiant floor heating system. The heat is circulated through a loop to heat the building and then returned to the coal furnace via underground pipe, where it is mixed with the heated water in the water jacket and again circulated. As long as the furnace is fed, the water jacket will remain hot and able to supply heat. Coal furnaces can be used for domestic hot water as well as heat.

Taking care of the furnace

A coal furnace usually needs to be filled twice a day, once in the morning and again in the evening. The water level should also be checked daily. Burning coal produces a lot of ash, much more than wood. Coal furnaces are usually equipped with shaker grates, which help clear the ashes into an ash pan and allow the coal to burn efficiently. Most manufacturers suggest cleaning out the ashes at least once a week, and the vent should be inspected monthly. Although they do take a little more work than a traditional furnace, many people find that an extra few minutes a day filling and cleaning an outdoor coal furnace is worth the savings and quality of heat it provides.

Copyright 2002 - 2019