Frequently Asked Questions

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By Steve Maxwell

Clean, green wood heat from an outdoor furnace

Green, sustainable, inexpensive space heating and domestic hot water is what I wanted for my house and workshop.

It was early 2010 when I began researching outdoor wood furnaces as an option for making this happen. A week before this past Christmas, I lit the first oak and maple fire in the unit I installed, and my hints at this project in my columns over the last few months have got people asking me for more specifics.

Outdoor wood furnaces have been around for decades, but it’s only in the last few years that new combustion technology means these systems now burn clean enough to be considered truly green.

All outdoor wood furnaces look pretty much the same from a distance. Imagine a small, metal building, like a half-size garden shed. There’s a door at one end that opens into a firebox. Energy from burning wood heats water to about 82C (180F) in an insulated tank within the building, where this water is available to be circulated to and from structures to be heated via underground pipes. The hot water can be used in radiators, in-floor heating pipes, or to warm air used in forced air heating systems.

Outdoor furnaces move the fire, ash and danger of heating with wood outside. They also have large fireboxes that handle big wood that’s easier to cut and split than smaller stuff. These are the two main advantages that got me thinking about outdoor wood furnaces in the first place. But as I soon discovered, there are two technical issues when it comes to assessing the quality of any particular unit. First, what percentage of heat energy from the wood actually makes it to the water in the storage tank? And second, how cleanly does the unit burn?

When the guy who works at the little country hardware store where I buy my stuff heard I was installing an outdoor furnace, he wondered why. “Don’t those outdoor furnaces burn a rainforest worth of wood each year?” Well yes, they do. Or at least some models do. The difference between efficient and inefficient outdoor furnaces means burning either 15 cords of wood a year or 30, and as I discovered, something called a fire-tube boiler is key to frugal operation.

The outdoor furnace I bought is made by a Canadian company called Portage and Main, and their fire-tube boiler design is one reason I chose them. Multiple hollow steel tubes run through the boiler tank itself, surrounded by water. Exhaust gases are directed through these tubes where they transfer huge amounts of heat to the water before heading up the chimney.

This fire-tube design originated in 1804, and was an enduring feature of marine and locomotive boilers in their time. Fire tubes are even used in modern industrial boilers today.

Most Trusted Outdoor Coal / Wood Furnace

since 1973

We have a 20 Year Limited Warranty, but we advice you to please burn smart. You could check out www.epa.gov/burnwise/bestburn.html

Portage and Main Boilers offer a comprehensive warranty and support program for their products. In addition, the company offers technical support and training to help customers get the most out of their furnace. Corrosion in the burn chamber, heat exchanger and water side is preventable and is usually not a warranty issue. Please do not neglect your furnace maintenance and ask for warranty.

Warranty covers workmanship and materials (i.e. welds that fail are usually repaired with no cost to the furnace owner.) Corrosion below the ash line is neglect and will not be covered. A hole in the side wall above the ash line would be considered a material defect and would be covered under warranty. Keep current with water chemical and testing to ensure that the water side remains corrosion free.

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