OUTDOOR WOOD BOILER DESIGN:
Best Steel for Outdoor Wood Furnaces?

When you're contemplating building or buying a hot water wood boiler there is NOTHING that is more important in its fabrication than the material that is used in the construction of the box.

Yes design is important, plumbing is essential, and controls to turn it off and on efficiently as the needs present themselves are critical as well, but all this is somewhat irrelevant if you can't create heat from the unit in the first place because it corrodes right through and you have nothing left but a leaky sieve and a box full of rust flakes.

So here are a number of considerations In making the decision about what type of metal you ultimately think makes the best hot water wood boiler..... a few ideas to get the brain going.

  • Corrosion is number one. Different metals have different resistance's to corrosion. Most outdoor wood furnaces, especially in Canada will be exposed to extremes of temperature and humidity, thus corrosion resistance will become the key consideration.

    So what affects the speed or extent of rust?

    Type of metal and the mix of metals that are added to the soup to create "alloys" will affect a material's corrosion resistance. Some additives like chromium and nickel can increase steel's resistance to rust and in fact this is what they do to make stainless steel. Some additives make the metal perform better at high temperatures which is equally important. Understanding the exact composition is well beyond your and my knowledge base and interest, but asking questions and feeling comfortable with the answers will give you a gut reaction to their attention to detail and honesty.

    The quality of the product is significant as well, especially these days where much of the industry is recycling what they can. Recycled metal is much less likely to be consistent in content or behavior than virgin material. Impurities in a metal's composition can often create erratic pitting on both the water and fire side of outdoor wood furnaces. Canada AM news is always reporting on old ships being towed to offshore recycling centers to produce new steel sheets so it is not illogical to imagine that some of this is finding its way back to North America as imports. Knowing what's in the material you are using will make its corrosion behavior more predictable and more controllable.

    Fabrication: The science of metal treatment is quite fascinating AND complex. If you take hot molten metal and quench it in oil it slows the cooling process down and creates one set of characteristics, and if you flood the original hot metal quickly with cold water it can create quite a different structure. If either process is done uniformly then the finished product exhibits a predictable set of characteristics. But as we strive to reduce costs much of our fabricated products are coming from overseas factories where quality control is not quite as prevalent.

    Like recycled metal where exact composition is unknown, poor quality control in production can create a metal that behaves in unpredictable ways. We have seen this happen in a big way with outdoor barbeques that rust within the year. It is undoubtedly a combination of cheap recycled metal, poor fabrication methods and thinner steel... but barbeques are significantly cheaper to replace when they rust out.
  • Thermal Transfer: Since we ARE talking about a hot water wood boiler it is important that the material that we make the furnace out of is able to efficiently transfer the heat from the combustion chamber to the boiler water that flows around the burn chamber.

    Different metals have different levels of success in moving this heat. and it is this efficiency that we are after. Greater efficiency equals less wood that needs to be purchased, longer burn times between loading and less smoke and associated pollution that goes up the chimney.. all good things!

    Thermal transfer is a function of the thickness of the materials being discussed. This should be obvious. After all we, R-value in home insulation increases as we make thicker walls. Metal is no different. If you want to compare apples to apples then you not only need to know the types of metal used in a particular hot water wood boiler, but also the related thicknesses of material in the two units you are trying to compare. Only then will this conversation have any comparative relevance.
  • Machineability: if you looking to BUY one of the best outdoor wood furnaces then you are not as concerned with how easy the material is to cut, bend or weld. Canada and the United States have a number of national manufacturers, that make boilers out of stainless, mild or boiler plate steel in which you can choose from.

    But if you are more interested in learning how to build an outdoor wood furnace then this may be a KEY issue. Some metals require specialized equipment and experience to create a leak proof container. Some metals are incredibly hard to cut. Some make after-the-fact repairs almost impossible, or just really expensive. So in making a "metal" decision the first step may be a fair evaluation of your fabrication facilities and talent!

So I hope that this gives you a general understanding of the critical importance of the raw materials that go into the construction of an outdoor wood furnace, and generates a few ideas on the questions you might ask in the buying or planing process to determine if the fabricator has considered corrosion, thermal transfer and fabrication in their design decisions.

The next article in this series will discuss some of the advantages and disadvantages of using stainless steel in hot water wood boilers

Main Index: Design Considerations for the Best Outside Wood Furnace