OUTDOOR WOOD FURNACE: Heating Technology Offers Energy Security

Posted on February 2nd, 2024

If you’ve ever wondered how you could heat your entire home without hauling firewood inside and ash outside, this article about outdoor wood boilers is for you. There’s a fair amount of work and expense to installing this kind of thing, but outdoor wood boilers make it easy to heat large spaces with less work, especially if you’ve got multiple buildings to heat. That’s a picture of my first boiler going in at my place in December 2011. I’ve since changed this boiler for another, as I’ll explain.

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This December 2011 photo was taken as I was installing my first outdoor wood boiler. Notice the insulated water lines sticking up from the ground behind the furnace, ready for connection to the boiler. The other ends of the lines go to my house and workshop.

As energy prices trend upwards, more and more people are looking to wood as a sustainable, affordable source of heat. Wood heat makes sense for any able-bodied person living near trees, and while firewood can be burned in a freestanding stove, a masonry heater or a basement furnace, outdoor wood boilers are a little-known option that’s worth looking at. Just before Christmas 2011, I fired up an outdoor wood boiler (aka “furnace”) I installed at my own place and it addresses the kind of energy issues that more and more people are concerned about. Living through more than a decade of heating with an outdoor wood furnace, I really like this technology. This article is an introduction to how it all works, with links to related content.

How Outdoor Wood Boilers Work

Outdoor wood boilers (sometimes called “water furnaces”) are made by a handful of companies and all models look like a small metal garden shed with a short chimney. Feed wood into the firebox behind a door on one end, and the fire heats water in an insulated tank that’s also inside the structure. Pipe this hot water to buildings through insulated underground pipes, extract the heat, then use it for space heating and to heat domestic hot water. This is the outdoor wood furnace proposition in a nutshell, and it offers unique advantages. By moving the fire outside, you sidestep the work of bringing firewood indoors, the potential hazards of burning it there, and the work (and mess) of hauling ash back out. You can also burn very large pieces of wood, letting you save at least 50% of the work of making firewood. Canada and the US are at the leading edge of outdoor wood furnace technology, and as I’ve discovered for myself, this heating option offers a range of benefits.

Need help learning to stack firewood? Click here to learn more about how we stack wood in round piles at our place. You’ll definitely want to see how we build what you see below.

Here are some of the round wood piles we build at our place. Much faster stacking, greater resistance to falling over, and interesting appearance are the advantages.

Outdoor Wood Furnace Benefit#1: Easier Firewood Prep & Handling

Outdoor wood furnaces make wood cutting and splitting more efficient, because the work can happen right next to the place where the wood will be burned. The best setups include a wood storage shelter near the furnace, creating an all-weather location to keep wood as dry as possible over winter. Some users even have their wood splitter permanently parked in the shelter. Splitting and stacking happens all in one place, minimizing the chore of handling wood.

The fireboxes on most wood boilers are larger than you’ll find on even the largest indoor woodstoves, and this is a big advantage. The smallest outdoor wood furnaces can accommodate wood 20 inches long, but many larger boilers burn wood twice that long. The Portage and Main BL-3444 I’m using now is a case in point. It can take pieces of wood up to 40″ long. Bigger wood makes for much less cutting time and more efficient use of your wood splitter. The bigger your wood, the less time it takes to prepare it. In practice it comes down to how large you care to handle your wood.

Massive heat output is another unique feature of outdoor wood boilers, and this offers another efficiency gain. Even the smallest models are capable of heating a full-size home. Larger units are capable of heating multiple buildings as well as heating domestic water, all from a single location. The work of tending one centrally located fire that’s outdoors is safer and easier than maintaining two or three fires inside several buildings. If you want to heat your whole realm with wood (and perhaps an outbuilding and the domestic hot water you use), you can’t do better than an outdoor wood boiler.

This open door reveals the fire tube heat exchanger in the first of two outdoor boilers I’ve used at my place. This design of heat exchanger was first used with steam locomotives in the 1800s.

Outdoor Wood Furnace Benefit#2: Lots of Heat

Life with my outdoor furnace is great, in part because of the design of the unit I chose to buy. I’ve owned two boilers over the last decade, both made by a Canadian company called Portage and Main. The first one I bought was what they call a “gasification” boiler – the Optimizer 250. Instead of letting smoke waft upwards and out through the chimney, combustion gases are directed downwards, through the burning coals and into a secondary chamber. Here they burn with a roar that looks and sounds like a large, blue natural gas flame. The result is performance that’s clean enough to meet EPA Phase 2 standards for wood burning appliances, some of the toughest in all of North America. You can see my gasification boiler in action below, including that impressive blue flame.

**Click to watch a detailed tour of a gasification boiler in action at may place.**

Outdoor Wood Furnace Benefit#3: Burn Multiple Fuels

A few years ago my Optimizer 250 developed a water leak because of an engineering issue that has since been corrected, and Portage and Main offered me any boiler they make as a replacement. I opted to try a non-gasification model that was made to burn either wood or coal. The BL-3444 I’ve been running for years now has advantages and disadvantages. On balance, I like it a lot.

The non-gasification BL-3444 is easier to maintain and keep clean than the gasifier, and it does a better job burning wood that’s not completely dry. The shaker grate on my BL-3444 also allows me to burn coal instead of wood. I’ve burned 60 tons of anthracite coal in this outdoor “wood” furnace over the years and it burns long and smokelessly on each filling. On the downside, the BL-3444 does produce somewhat more smoke than the Optimizer 250 did when burning wood, and it probably burns a little more wood, too. All things considered though, as much as I liked my 250, I like the BL-3444 more because of its flexibility and reduced maintenance time.

Outdoor wood furnaces aren’t for everyone, but they do have a growing place in the rural and semi-rural heating scenes. They make direct use of an energy resource that’s abundant and renewable across many parts of the world, and the fact that it’s likely to stay this way may be the best benefit of all.

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Speed is only one advantage of an electric saw chain sharpener. Excellent cutting results are another benefit. With some practice you can sharpen a chain so it cuts better than a new chain.

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