OUTDOOR WOOD BOILER DESIGN:
Stainless for Outside Wood Furnaces?
In our previous article about choosing the right metal for an outside wood furnace we discussed all the characteristics we have to be aware of, to select THE "perfect" metal for trouble free operation. We also know that finding the perfect material is not realistic unless we won the lottery and price was no object.
We're all old enough to understand life is about trade-offs, both financial and technological.
So what is the best metal for an outdoor wood boiler? Some might suggest stainless steel construction wins the lottery, but even stainless has its issues and trade-offs, so read on....
First let me define stainless steel. It is an alloy with a minimum of 12% and up to 30% of chromium added to the mix to create a steel alloy significantly more resistant to rust than just plain steel. The chromium forms a thin layer of chromium oxide on the surface of the steel to protect the layers of metal below from rusting.
It comes in a number of grades depending on exactly the mix of metals, like nickel, manganese and carbon that are added to the soup pot. So someone telling you that their outside wood furnace is made of stainless steel is kind of like telling you that I'm making dinner. Obviously we're talking about food but what type, is the question. (further research on the various types of stainless steel)
That being said no one will dispute that stainless steel has significant advantages over traditional carbon steel, but here's the key ....only IF the design of the outside wood furnace capitalizes on its strengths and accommodates its weaknesses.
- Corrosion: With the addition of chromium stainless offers much greater corrosion resistance than any other traditionally used metal for outdoor wood furnaces. Canada and other cold northern climates .experience extremes of temperature and humidity and can promote rapid development of rust, making corrosion resistance an important consideration. Stainless wins over traditional mild steel in every way.
That being said, if the stainless is too thin, than any "corrosion advantage" is lost to the fragility of the components.
- Type of Stainless: There are a WIDE range of different types of stainless steel so don't assume that all are great. Think of the stainless steel mufflers that rot out in under a year. Some suggest that a titanium enhanced 409 is the preferred option for a hot water wood boiler... traditional 300 series stainless might be better but the prices make it most often out of the range of possibilities. ... feasible for small surgical utensils but an outside wood furnace uses a lot of metal and the market is not willing to pay for that volume.
- Thermal Transfer: The tech sheets suggest that stainless is less efficient at transferring heat.
That is true, but might be overshadowed by more significant issues of creosote build-up on the inside of the firebox and lime scale buildup on the opposite "boiler water" side of the metal making this difference in thermal transfer relatively insignificant.
.Also in comparing thermal transfer data, the tables will rate them based on a set thickness, but the stainless used on traditional outside wood furnaces is typically thinner relative to the thickness used in traditional mild steel boilers. Thus the "less-thick" stainless components, that are less thermally efficient may come out of the race equal to the efficiency of thicker mild steel components making "thermal transfer efficiency" a mote point!.
- Thermal Expansion: As the temperature rises, stainless steel expands like all metals, but does it at a much higher rate than traditional steel.
As an example a 100' length of pipe may expand over 1" in length with a 100 degree change in temperature... a situation that is not unusual in outdoor wood furnaces by the time it cools down during cold Winters in Canada and then heats back up again when it gets restocked with wood in the morning. Compared to mild steel, this represents an expansion of 25% more than traditional steel, so seams and welds must accommodate this movement.
This is one of the KEY components that I refer to when suggesting that what makes the best outdoor wood boiler is when the material of choice is matched with the design of the boiler to optimize the strengths of both.
- Fabrication Challenges: Stainless steel is difficult to cut and weld.
If controlled and sophisticated fabrication methods are not in place then stainless steel loses it's corrosion resistance in the heating process during welding of the joints. Re-treating after welding can restore ductility and corrosion resistance but many manufacturers simply don't do that so the benefits of stainless on a outside wood furnace is lost.
- Expensive stainless steel is expensive and can be extremely expensive depending on grade.
If you are interested in looking at a stainless steel boiler try the Heat more Wood Furnaces
- I have never seen titanium used as a stand alone material in outdoor boiler plans as it is significantly more expensive than even relatively costly stainless,
- It is much more difficult to weld, cut and machine
- But it is used as an additive to stainless to help improve the corrosion resistance at the welded seams.
I hope that this small review of "stainless steel metallurgy" helps you in understanding what salesman are trying to impress you with in your shopping efforts of trying to figure out "what is the best outdoor wood boiler". In the next section I've written an article on outside wood furnaces that are created from traditional mild steel.