Stainless steel vs. mild steel for wood boiler

I have had someone here tell me that s.s ain't. That may be true. But, I still think it is better than carbon steel in a boiler. Different people have different water quality. I am on a well, with a water softening and filtering system. I went with the stainless model hoping it would last longer. I plan on burning through it for as long as possible. I thought it was worth the extra expense.

F.Y.I. Dune posted this in response to another thread here https://www.hearth.com/econtent/index.php/forums/viewthread/81045/ Entire thread is short and an interesting read.

I had to look up 409.
Even as a fabricator with over thirty years experience, this is a new alloy to me.
From what I have read about it, 409 is likely suitable for wood stove construction.
It is not to be confused with the most common grade (18-8, the group wich includes 304 and 316) as the following commentary will show.
Couple factoids;
Cheapest alloy.
Chrome content approximate 10%
Nickel content .5 % ( for comparison the 18-8 group, which includes the familiar 304 and 316 alloys are 18% chrome, 8% nickel)
Because of the vast difference in chrome and nickel content, this S.S. (409) cannot be compared in any way to 304-316 alloys.
Chrome is hard and resists oxidation, 409 has just over half as much chrome as 304.
Nickel is tough and resists oxidation, 409 has 1/16th as much nickel as 304. Of course, this accounts for the marked difference in price.
In spite of my ignorance of this alloy, I have welded it; yesterday I welded a long crack in a 409 SS pipe connected to a catalytic converter from a BMW car.
It welded fine with 316 filler rod. (higher alloys usualy mix well with lower alloys). I didn’t know it was 409 until today when I looked it up.
The only thing I can say about it is that if it were 304-316 (18-8 SS group) it would not have needed welding, because it wouldn’t have cracked, especialy the way it did (linearly).
On the positive side, my woodhauler/service vehicle has the original exhaust pipe made from the same alloy, 23 years ago.
Overall, it should make a good alloy for stove construction (though not the best choice)
and if it does ever crack, any half decent welder can fix it.

The pros of SS are that it stands up to condensation and acid from the fire side much better than carbon steel, and will last much longer with oxygen in the boiler water than carbon steel will.

The cons are stress cracking, chemical corrosion (don't ask me to explain, I can't), poor thermal conductivity and different expansion rate (not really significant in itself, but causes cracking), cost and in many cases the SS will be thinner than a comparable carbon steel boiler. 304 is "food grade" SS so its much more corrosion resistant but less suitable for high temps than 409 which is more similar to exhaust system SS.

Is it worth the $ ? Carbon steel or cast has a long track record and works fine in closed systems, so I don't see any benefit for the $$$.

benjamin said:

The pros of SS are that it stands up to condensation and acid from the fire side much better than carbon steel, and will last much longer with oxygen in the boiler water than carbon steel will.

The cons are stress cracking, chemical corrosion (don't ask me to explain, I can't), poor thermal conductivity and different expansion rate (not really significant in itself, but causes cracking), cost and in many cases the SS will be thinner than a comparable carbon steel boiler. 304 is "food grade" SS so its much more corrosion resistant but less suitable for high temps than 409 which is more similar to exhaust system SS.

Is it worth the $ ? Carbon steel or cast has a long track record and works fine in closed systems, so I don't see any benefit for the $$$.

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Can you verify with some facts as to why you say the cons are stress cracking? And is that with 304 s.s. you are talking about? The boiler that he is talking about uses this. The 304 Stainless in the Wood Gun is 1/4" thick. In another boiler like an econoburn, the carbon steel is also 1/4" thick. As far as high temps are concerned, I believe that is why the manufacturers put ceramics in the boiler where those highest temperatures occur. ?

Rick S. said:

Alternate Heating Systems offer a stainless steel [304] option for the steel inside the boiler. I think the stainless option is about another $1200. What is the pros and cons of stainless steel? Is it worth the extra money? Thanks

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Rick, If you decide on a Wood Gun, they usually have a sale in the spring which amounts to 10% off. That will almost off set the S.S. option. I even bought a floor model they wanted to replace with the latest model and was given 15% off the price on that. Final price on a S.S. E100 model last spring - $8000 Cost has probably gone up on them now. Just F.Y.I.

Cover your eyes so you don't see the source, but this is what I was talking about. From what I've read and seen in person its a pretty accepted view of SS boilers.

http://www.centralboiler.com/stainless.html

That being said, I'd be much more inclined to go with a SS Woodgun, than a SS OWB from a fly by night operation. SS is not corrosion proof and is possibly much more crack prone than carbon steel.

Rick S. said:

So if money is not a factor 304 stainless steel may still be the best option. It makes me wonder if anything can be in the day to day usage of the boiler to reduce the chance of stress cracks. If I can longer boiler life with s.s., and can reduce the chance of cracking it looks like the best of both worlds.

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Alternate Heating has perfected ss welding a long time ago. I have never heard of a ss WG cracking & I believe they have been making them for about 30 years. Randy

Stainless Steel is a very unique material as anyone who has worked with it can testify.

With regards to the use of it in boilers, I would agree that in a closed system (with no leaks and no oxygen being introduced into the "dead, oxygen free" water) the use of carbon steel is much more popular due its lower cost and the properties of carbon boiler steel.

However, I know of a person who had a WoodGun in Northern Michigan for 20 years when it started to leak from corrosion (no storage was used on this system). He was very satisfied with the boilers past perfromance (and money was no issue) and thus installed a stainless steel version of the same boiler 2 years ago.

The facts about stainless steel are:

1.) The coefficient of thermal expansion is much greater with stainless steel than with carbon steel (meaning that when SS is heated it will move/grow much more than with that of carbon steel. It is very common that when trying to repair stainless steel that the the welds crack and it seems like you are trying to "chase the cracks" around). Obviously, there are those few well trained individuals that specialize in the repair of stainless steel and have learned how to overcome the cracking problems.

2.) The thermal conductivity of SS is much less than that of carbon steel (meaning that it does not transfer heat as well as carbon steel). Nomally, it is common for stainless steel heating applicances to use much thinnner material in an effort to be able to transfer heat efficiently. For example, the Hardy brand of OWB has been around a long time and uses a much thinner stainless steel material.

One could argue for days about that use of stainless steel in wood boiler construction vs. carbon steel. Obviosuly stainless steel is used in many factories and production plants making food items and also in nuclear plants making energy for specific reasons. There are individualts out there that specialize in the fabrication and welding of stainless steel and are very good at it. But for most, it is a very difficult material to work with and repair.

Hope this information helps!

Brian

One thing is that old rusty burned out waterlogged mild steel or cast iron would s##k to weld compared to 304 S.S. that had endured the same punishement. Therefore if it did ever crack, repairs would be easy.

304 is subject to work-hardening, however, that is usualy overcome by extra thickness. It is hard for me to imagine that 1/4" 304 would be subject to work-hardening (stress cracking) in a residential boiler aplication,
but I am certainly no expert, these are just my thoughts.

Rick S. said:

Alternate Heating Systems offer a stainless steel [304] option for the steel inside the boiler. I think the stainless option is about another $1200. What is the pros and cons of stainless steel? Is it worth the extra money? Thanks

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fwiw; When I ordered my WG and asked them about the SS option, they asked me if I would be heating DHW all summer, I'm not. They said that I would not need it (SS) and to run my "Gun" hot, set the turn off at 210. If I get 20 years I'll be real happy but I'm thinking that in 10 to 12 years I'll be old enough to park the saws and burn a Coal Gun next go round, or maybe pellets.
The guys there seem to be really up front and honest in their business, ask them! example; While standing in their (old) showroom/office, they actually recomended that rather than ordering their oil back up, to install a small seperate oil boiler in line because the oil burner itself, is in a real nasty environment (wood smoke) and should be serviced and cleaned much more than annually to be dependable. They sold me right there with "unselling" 2 of their upgrades!

I am going to try heating my hot water year round. It may be too warm in July and August though. I hope to burn cordwood for another 20 years at least. If my health holds out, maybe 30. I turned 44 yesterday. And man did I eat to much cookies and ice cream last night and tonight! :lol: But they were damn good. :lol:

I think the comparison of steel is not done well when we are discussing outdoor wood boilers and indoor wood boilers. I would bet that those people on the www who hate SS have had a bad experience with an outdoor wood boiler. Many of the outdoor wood boilers had problems with rusting and leaking. Then, some of the manufacturers tried to go to S.S. Some did not use the right kind and/or thickness, etc. Most did not have adequate insulation to prevent exposure to extreme cold. Have you ever seen the insulation on an outdoor wood boiler? Not much there. The whole idea of putting a boiler outside in cold temperatures and elements just seems inherently wrong to me. Unless you enclose it properly. (Then it is not outside anymore. :lol Some of the outdoor wood boilers have failed, the stress put on the steel was too much for it to withstand. Hot temperaturs on the inside, cold on the outside, with a thin layer of insulation in between? How long is that insulation going to last before it starts to deteriorate? Even a few areas of leaking. And when it does, hello -10F, then the exposure of the steel to the cold and moisture from outside gets worse. Then, throwing unseasoned (wet) and frozen wood into the boiler also seems inherently wrong as well. But that is one of the benefits they are sold on. Keep the mess outside. So the boiler is out there and the wood, often not even covered, is out there with it. Now they they don't even need to split and season it, they can throw in logs that are wet and now frozen. This further subjects the boiler steel to extreme temperature stress.

I am not trying to bash the outdoor wood boilers. But comparing the steel in a boiler that is subjected to the outdoors is not at all like a good quality S.S. boiler that will be inside it's entire life. I am glad to see that some of the OWB man. are starting, slowly, to move towards gassification. A better burning of the wood. I hope the outdoor wood boiler industry starts to push burning seasoned wood. The entire wood burning industry needs to educate people on the advantages of burning seasoned wood. The amount of people out there who still burn wood that was just cut in the late summer or fall that year is amazing. I would not hesitate to say that 75+ % of the people around here burn wood that is not even seasoned from the spring before. Seems strange to me. With the amount of Ash that is up here, it seasons well from early spring to late fall. I am also glad to see that some of the outdoor wood boiler manufacturers are going to closed cell spray foam for their insulation. At least the boiler steel will be better protected from the outdoor temperatures. And with the better insulation, the boiler should stay warmer, longer, resulting in less wood burned. And because of a better seal, the boiler life should be longer. I would think.

First, get it out of the weather. Second, get and keep the oxygen out of the system. With an enclosed system, and proper air purging equipment, there is less chance of deterioration through chemical reaction. Especially with the difference in water quality, and water system quality, from place to place. Third, use the steel with the least chance of corrosion.

Rick. I would suggest a completely closed system. No water testing required once it is up and running. Simpler. I would also suggest at least some storage for the purpose of less idleing of the boiler, the ability to use the boiler longer during the shoulder seasons of spring and fall, and the ability to heat your DHW for a longer period of time throughout the year. Think long term. With the world we live in today, the price of oil could double at any time. It will affect you less if you can heat everything with wood. My 222 cents. :lol: