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Outdoor wood furnaces: stainless vs. mild steel boilers

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Q. We are looking at outdoor wood burning furnaces. We where told there are certain elements if in your water they can corrode stainless steel. Some of these boilers are made with stainless steel. Should we go with mild steel over stainless or is this a dealers ploy for us not to buy stainless? Please give us an answer ASAP because we need to get something installed right away.

Thank you,

Mary S deleted
- Mooers, New York


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A. I have been a outdoor wood furnace dealer for 7 years and the problem with most is the warranty is worth a hand full of kindling. But I have seen some that will stand behind the warranty; they have a 20 year limited warranty and are made of 304 dairy grade stainless. Yes they will all work just fine, but the warranty sold me, and having dealt with 4 different brands in the last 7 years, the guys that stand behind them are what I look for.

Ed Meador
- Blair, Wisconsin


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A. About your question; if your water supply is "hard" or "sulfur," I'd recommend stainless steel. Sulfur water overtime tends to form deposits . My parents old farm house was our proof. Since you live in the North I'd recommend a [deleted by editor] unit over a [deleted by editor]. Unlike a [deleted by editor] made in the south, the northern made [deleted by editor] unit has many more features and is much better engineered. I speak from experience; I had to make repairs & re-engineer my [deleted by editor] that I purchased new. If I can help further free feel to ask.

Jeff W. Schuller
Owner of a [deleted by editor] Woodstove - Monroeville, Ohio

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Ed. note: We appreciate your suggestions, Jeff, but please try to restrict the discussion to generic issues rather than recommendations of specific brands. One problem is that distributors & sales managers go livid when a different brand is recommended so we're soon buried under spammy rejoinders. Even worse, due to the anonymity of the internet, we get fictitious postings pretending to be from satisfied users :-)



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A. GET THE BEST STAINLESS STEEL YOU CAN AFFORD! I PURCHASED A MILD STEEL MODEL 7 YEARS AGO AND HAVE HAD TO GET IT WELDED TWICE. THE PROBLEM IS CREOSOTE, AND ASH WHICH FORMS LITTLE WATER POCKETS IN THE CORNERS AND EATS AWAY THE MILD STEEL.

RUDOLPH BAJAK
- TURTLE LAKE, Wisconsin


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A. I HAVE SEEN MILD STEEL UNITS SIXTEEN YEARS OLD. UNDERSTANDING THE PHYSICS OF WHAT CAUSES CORROSION IN THESE TYPES OF METAL IS KEY. WATER ABOVE 175 DEGREES DOES NOT HAVE AVAILABLE OXYGEN FOR CORROSION. MOST OF THESE UNITS RUN AT 180-185 DEGREES. 304 STAINLESS IS EXTREMELY PRONE TO STRESS CORROSION CRACKING WHICH RESULTS FROM CHLORIDES IN THE WATER REACTING WITH THE NICKEL CONTENT OF THE STEEL. THIS IS NON-REPAIRABLE BY WELDING. 409 STAINLESS WILL GIVE YOU THE HEAT TRANSFER ABILITIES AND EXPANSION PROPERTIES OF MILD STEEL, IT IS A FERRITIC STAINLESS, SO THE SURFACE WILL TARNISH. THE CORROSION RESISTANCE IS STILL GREATER THAN MILD STEEL. THE MOST EFFECTIVE DESIGN HAS THE FEWEST PARTS. KEEP IT SIMPLE.

JERRY SOBOCINSKI
- MEMPHIS, Michigan


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A. Definitely go with the mild steel when you buy an outdoor wood furnace. I've seen both in use and the mild steel will not have the corrosion problems that the stainless steel has. I currently own a WoodMaster with mild steel and I can give my testimony that it has worked perfectly and has needed no maintenance or upkeep for any type of problems. I don't speak from a "metallurgy" knowledge, but from practical experience that has done me well.

Bill Kurtz
- Fort Wayne, Indiana



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Q. I am searching for an outdoor wood burning furnace. All brands claim to be the best. My question is which is really better? You also hear stainless over mild steel, and mild steel over stainless. Which should I get? So you prefer closed loop or open loop? I need some advice ASAP.

George R deleted
break press oper. - Flat Rock, North Carolina


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A. Which is really the better truck--Chevy, Ram, or Ford? There really are no objective answers to some questions, but I would suggest seeing if "Consumer Reports" [link is to product info at Amazon] or some similar testing outfit has ratings available for you. If that's not available, you have to listen to the proponents of both sides and decide what you believe and don't and what is important to you and isn't -- remembering that internet postings claiming to be from satisfied customers are sometimes actually from shills posting with fictitious names. Good luck.

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey

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A. I feel that I may be able to put a little spin on some of the comments. As you said, check Consumer Reports, etc. I personally have built 4 outdoor wood stoves, with the oldest being 7 years. Theses stoves are all simple to build and work quite well. You must remember that up here in Canada, we usually get a lot colder than most of the US. What I am trying to say is that like anything you buy, if you want it to last, treat it right. Do not burn green wood constantly. Do not let it sit all summer with the ashes still in it. Run the temperature higher, it will burn cleaner. Put a taller chimney on it, it will burn better and won't smoke you and your neighbors out. These are all factors which you should consider before deciding to buy a stove.

The stoves that I have built are all made from mild steel, but the thickness of materials can also determine life expectancy. I have used 1/4 inch fireboxes on three stoves, with #4 being 5/8 wall x 36 in. dia. pipeline pipe. The smoke pipe that travels through the water jacket is all 1/2 in wall pipe. These stoves will go for a minimum of 10 yrs. with no maintenance. Maybe some of you could build your own stove, and save a ton of money. By the way, I am not an engineer. Just a guy who likes to build stuff.

Thank you,

Dan Hurd
- ThunderBay, Ontario, Canada


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thumbsup2The Outdoor wood boiler discussion is interesting. The family business was tool and die and metal stamping, I've worked in the maintenance field for over 20 years, I even had my own welding business for a period; I'm presently an electrical systems support specialist...

I look at the quality of the work, and the price (now and down the road). The money savings is generally why one burns wood for heating. The quality and integrity is going to determine the longevity of the wood boiler. Good maintenance and storage in the summer months are very important!

I'm having my wood boiler built by a local fellow (One of the best welder fabricators I've ever seen) for $2900 without the shell and without the other bells and whistles because I'm going to put it in a small room in my shop and then run my lines to the house- heating my hot water, my house and even a setup attached to my clothes dryer (I have seven children- we do a lot of laundry). This fellow built one for another fellow and it works great.

Mark Sill
- Plymouth, Indiana

Woodburner's Companion


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A. Just to set the record straight on stainless steel, it doesn't work in an outdoor use. It can't account for the rapid temperature changes. I am only starting to get into the heating business and I already know that stainless steel is a nice concept but when you have a cracked water jacket and there is know way to fix it, don't come crying to me. Also I love reading about all these companies with there lifetime warranties who have only been in the business for 3-5 years, give me a break. As soon as one claim comes in they go bankrupt and your on your own once again. Mild steel is a metal that works and if you have half a brain to take proper care and daily maintenance of your furnace it will last you. Thanks for your time.

Tim Suderman
- Winkler, MB, Canada


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A. Titanium enhanced stainless steel is the preferred metal.

High nickel stainless expands and contracts too much.

Carbon steel will transfer the heat better than stainless though.

Hope this helps

Dale Andrews
- Dinwiddie, Virginia

Put Your Pig on a Diet


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A. I am looking for an outdoor furnace and researching the web. I found this discussion interesting. I have lived in the country for most of my life and there is one practice that has some implications with regard to the question of metal selection and water. That is the use of bleach on occasion to eliminate coliform contamination in drinking water. Bleach is an oxidizing agent and will speed up the general corrosion in circulation boilers and water heaters. You may want to consider glycol and an exchanger. In any event, if you have to treat your drinking water and are using bleach to do so, expect it to have a periodic negative effect on metal surfaces it comes in contact with.

Doug McPhee
- Cranbrook, BC, Canada


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A. I found this interesting. Looking at various stoves the companies that don't have stainless steel say it cracks and mild steel is better. The ones that do say it avoids corrosion better than mild steel. I personally would go with the stainless because I haven't seen much proof of the stainless cracking at wood burning temperature. I was told it would have to be much warmer and it won't get that hot with water around the firebox.

Andy Staupe
- Foxboro, Wisconsin


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A. We own a stainless model and after 5 years the 10 year warranty was a joke -- just try to collect. It's a discarded junk heap; is it the stainless? I have been welding lots of steel stoves for pitting but never seen one crack.

Clayton Brinkman
- Kenora, Ontario, Canada


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A. We have been in the business of outdoor units for many years. We sell all three, mild steel, 409 stainless and 304 dairy grade stainless. Yes, they will all do the same job for a while. No, I have never seen 304 grade crack. Mild steel is the cheapest and 304 is the most expensive. 409 is middle of the road. There is a reason for that. If you are serious about your decision, go to a neighbor that has a mild steel unit and get a water sample from their stove, yuk. Then go to your other neighbor with the 304 and get a sample. You will see the difference. In my years of experience, like one other said, buy the best stainless you can afford. If you are choosing mild steel because that is all you can afford, take out a partial loan and get stainless. As for 304 cracking, that is horse sh*t. We have hundreds of units in 304 grade working perfectly. You must remember that this is not a cremator, it is an outdoor wood furnace with 180 degree water surrounding it. I have sold, installed and serviced many hundred units and plain mild steel will never be a more cost effective decision in my book. Cheaper up front yes, better overall value, not a chance. KISS keep it stainless steel.
Stay warm my friends. '

Douglas Weed
- Lafarge Wisconsin


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A. TRY THIS.FOR THOSE OF YOU THAT WANT TO BUILD YOUR OWN. HERE IS WHAT I DID. I WENT TO A METAL SCRAP YARD AND BOUGHT FOR 10 CENT A POUND A PAIR OF ROLLERS OUT OF A PAPER MACHINE. A LOT OF SCRAP YARDS HAVE THESE SINCE THE PAPER INDUSTRY IS NOW IN MEXICO OR ELSEWHERE. A PAPER MACHINE HAS 8 TO 16 ROLLERS IN IT FROM 18" TO 48". I BOUGHT A 48"L X 48"W AND A 48"L X 36"W. BOTH HAVE .500" THICK CHROME PLATED FACED WALLS. ONCE I (CAREFULLY) TORCHED OUT ONE END OF THE 48" ROLLER AND TRIMMED THE SHAFT I (ALSO CAREFULLY) SHORTENED THE OTHER TO 40" BY TORCHING OUT THE INSERT AND SHORTENED THE TUBE, THEN REINSERTED THE INSERT AND WELDED. THESE ROLLERS ARE PLUS OR MINUS .010 IN SIZE, WHICH IS REAL NICE TO WORK WITH. AFTER WELDING TANK IN TANK I CUT OUT A 30 X 30 DOOR AND FRAMED IT WITH .500 PLATE STEEL. DOOR IS MADE OF .500 PLATE AND 1" INSIDE DIMENSION .375 CHANNEL STEEL FOR THE FRAME SIDES. 1" FIRE SEAL ROPE IS INSERTED IN THE DOOR GROOVE AND IS ADJUSTABLE ALL 6 WAYS BY SHIMS. WITH .500 STEEL, ADJUSTMENT HAS NOT BEEN NEEDED.

I GOT A FUEL OIL BURNER FROM A 150000 BTU FURNACE AND USE IT AS THE FORCED AIR FOR THE WOOD FIRE ON A 185 TO 165 STAT. I ALSO BUILT IN A DUAL FUEL BACKUP BY WHICH WHEN THE SECONDARY STAT HITS 155 THE PUMP AND IGNITER FIRE FOR FUEL OIL HEAT. THE FUEL OIL TANK IS BUILT IN UNDER THE WATER JACKET. AT 185 THE FUEL OIL CUTS OUT TILL THE NEXT FIRING. I MODELED THIS FURNACE AFTER THE WOODMASTER 4400 SINCE I WAS TOLD IT WAS THE MOST EFFICIENT IN THE INDUSTRY, INCLUDING THE HEAT FIREBOX PIPES AND THE SMOKE BYPASS DOOR.

THE UNIT HOLDS 300 GALLONS OF WATER AND HAS BURN TIMES OF 24 TO 28 HOURS IN WINTER +20 °F AND 18 TO 22 FROM 0 TO -10 °F. I HEAT A 1600 SQ FT HOUSE AND HOT TUB AND INTERMITTENTLY A 32 X 40 POLEBARN WHEN NEEDED. I BUILT MY OWN TWIN SIDEARM HEAT EXCHANGERS WITH 2" COPPER AND .750 COPPER INSIDE FOR A 60 GALLON WATER HEATER. I PLUMBED MY HOUSE FOR HYDRONIC BASEBOARD HEAT WITH 6 ZONES AND HAVE IN TOTAL AROUND $3800.00 IN ALL OF IT. I WAS PAYING 185.00 A MONTH IN PROPANE AND ABOUT 600.00 + - PER YEAR FOR ELECTRIC FOR THE HOT TUB WE USE DAILY.

I HAVE THOUGHT ABOUT BUILDING THESE COMMERCIALLY BUT I DIDN'T TRUST THE THINGS I WAS READING ON THE INTERNET SO I DON'T THINK ANYONE WOULD BELIEVE WHAT I WAS SAYING AFTER I WENT INTO BUSINESS. SO I WILL JUST DRIVE MY SEMI WHICH IS WHAT I DO 10 HOURS A DAY :-)

Bran Thomas
- Kalamazoo, Michigan


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A. Stainless or mild steel ... I have seen both leak in less than 3 years and I have seen both last longer. They are all built cheaply....come on, fire boxes 3/16 and water jackets as thin as 1/8 inch -- warranties that fall apart faster than the stoves. I have personally seen one rot a hole in less than 3 years and the "rock solid 20 year prorated warranty" was a free patch of 1/8 rolled plate that the guy had to pay me to weld on at his expense!

Stainless fire boxes on all models I have seen still have plain steel water jackets (so steel has to meet stainless somewhere) -- where is the advantage of stainless?

William Gordon
- Watford, Ontario, Canada


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Q. I am possibly looking to purchase a rather large outdoor wood boiler to be used in a commercial application (>10,000 sf floor area, pool, hot tub and domestic hot water). Before I make the investment, I would like to find someone (or two) out there who has been relying on a boiler of this size for more than a few months. Hopefully there are boilers that have been in use for at least five years with known track records and of how well the unit has performed, specifically relating to type of metal used.

Every manufacturer/dealer tells you what you want to know and that their metal is the best. It appears there are basically four types of metal to choose from: 304 SS, 409 SS, mild steel and boiler plate. I hope there is someone out there who can shed some light on the durability, or lack thereof, of some of these metal choices.

Chris Paffi
hotel - Lewiston, Michigan

Self Sustaining Homes


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A. Chris I am no expert on these things (as I am sure someone else will likely point out, lol) and I have never seen any of those "gasification " models in my area yet. I have seen numerous of the other kind in operation. As for their durability, it does seem to have a lot to do with the owner (although I personally think they are built way too thin) and the ones I have seen rot out early tend to sit outside in the elements and are surrounded by a steel siding of some sort. Here I think lies much of the problem, the driving wind pushes snow through the tiny crevices in the steel siding, this snow collects on the water jacket, melts, runs to the bottom of the stove and is trapped in the insulation, and rust out prematurely -- again my opinion, but I now have welded 2 stoves in the same spot.

The ones I have seen are not as efficient as the sellers claim them to be. I was told I could not build one as good; I did build my own, and in comparison to others in my area it is just as efficient (although I am certain they can be improved on). I heat a home of about 2,200 sq ft., a small 30 x 40 shed (at lower temps that the house), and my hot water. I use between 8 and 10 bush cords of wood (when I had an indoor wood furnace I used about half that, but I was not heating any hot water or the shed then). I do believe it is not as efficient, but I traded that for no more smoke or ashes or open fires in the house. I think you will burn a lot of wood to heat the area you are speaking of, but my wife likes to keep the house over 70°

I light my stove in October and usually run it until the first of June, anywhere from 1 fill per day to twice daily in the heart of winter.

If you are serious about a stove, check all the ones you can find; ask not just a dealer, maybe write a manufacturer and tell them how big an area you are going to heat. Then when you have done all that, do as I did: build it heavier yourself. These things are built like any appliance: to wear out so they can sell you a new one.

Like I started out saying, just my opinions -- I am sure you will hear many more.

William Gordon
- Watford, Ontario, Canada

Gasification technology
wood/coal boiler


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A. I have owned a stainless stove 9 years and after 5 years it was cracked like spider web; warranty covered a new water jacket. I paid $600 to ship old and new units and did all the work myself -- they said cracking was due to salt in the water and I should use only rain water, ok!?

4 years later the stove is cracked and leaking again! Stainless is junk that will not hold up to stress of heat/cool cycles of these boilers; mild steel can be repaired by any good welder. A friend got a [deleted by editor] unit of mild steel: half the cost, burns less wood, and going strong. I will be installing a new indoor wood boiler and no stainless steel on the wood stove; the outdoor units are too hard on the wood supply.

Duane Ingalls
- clymer, New York


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Ed. note: We had to delete a couple of postings from this thread when the IP addresses proved them to actually be from manufacturers and suppliers trying to leave the impression that they were from satisfied customers :-(
The internet is largely anonymous -- and we certainly haven't done police background checks on the remaining posters to determine if their names are real, and their possible vested interests :-)
Don't put great stock in testimonials from people you don't know!


October 14, 2010

A. To any one interested: As for a boiler, there is a reason that every single industrial boiler is pressurized, it prevents corrosion, period. As for stainless vs. carbon, I own a stainless works great, been working great for years, probably more susceptible to cracking if over heated; I have overheated, might have got lucky, but no problems. My dad built a pressurized carbon steel boiler 31 years ago, still works great. Pressurize them, carbon probably a safer bet but stainless in mine works great and it is also pressurized. Oh, by the way, one manufacturer will try to scare you by telling you you have bomb in your back yard. Tell them I also have one in the propane tank also! If you install pressure relief valves and even temperature relief, you will be fine.

Jim Erickson
- South Range, Wisconsin USA

October 14, 2010

Hi, Jim. Thanks for the several interesting points. However, I don't think the reason industrial boilers are pressurized is to prevent corrosion. In fact, isn't the situation a bit backwards: the heating the water pressurizes the boiler? I'm not trying to make a semantic point; rather I am saying that you can't create really hot water without keeping it from expanding because it will boil away: if you want 220 °F water in an industrial boiler you can only get it by keeping it from expanding (i.e., allowing it to pressurize). Similarly with steam heat: the more steam pressure you allow to build up, the hotter the steam.

Regards,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey

October 23, 2010

A. Hi Ted, I guess I was not focused on my response on pressurized boilers. I was simply trying to cut around the corners. In the industry I am in, my company and others who I have worked for, go to great lengths to prevent corrosion in their boilers, On several occasions the introduction of nitrogen is used to maintain pressure and prevent corrosion. I can provide several maintenance manuals that highlight this procedure which, their goal is to prevent corrosion by reducing the amount of oxygen into the system. Corrosion is a chemical reaction usually but not always caused by oxidation. Oxidation of metal components can be slowed down by proper pH levels and reducing the amount O2 in the system. Basically what I was trying to say to others is that by using a pressurized boiler you reduce the effects of oxidation on the components of the boiler.

Jim Erickson
- SouthRange, Wisconsin

October 25, 2010

Thanks, Jim. You're obviously highly versed in this subject.

Regards,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey

February 10, 2011

A. My [deleted by editor] was built 1992 and has worked ever since. It has been welded twice but the stove is 19 years old. I have only had to replace the dampener switch for draft twice and welded it twice. I can say for mild steel for 19 years I don't think I need to tell you what to buy. I have 10,000 square foot heating and heat the house, garage, hot water and have no issues.

Bill Mdeleted
Government Employee - Dryden, Ontario, Canada

October 27, 2011

A. I installed an outside wood boiler furnace 15 years ago to avoid the $600 a month gas bills racked up heating this 100 year old farm house. The type I have looks like a small metal shed, it has a 350 gallon water jacket surrounding the box and is all made of mild steel.
The first 11 years were great; they sell a conditioner to add when topping the tank back off every year that is supposed to act as a corrosion inhibitor and prolong the boiler life. I did this the first 2 years but at over $140 a pop I decided to take my chances without it.
I will say that it did begin to show some signs of minimal leakage after 11 years, but they do sell quart bottles of BOILER STOP LEAK. On the 11th year I added that and the water stopped leaking, I've added a bottle every year when re-firing around October and 15 years later it's outside smoking right now and 75 degrees in my house and garage.
I will say to those who have no experience with a boiler that, depending on your circumstances, keeping wood stocked is NOT easy. I usually burn 30-40 full size pick up loads a year. My burner firebox is 3' x 3' x 4' deep. It's great because I heat the house, garage, water and a shed and it costs me nothing. But cutting 30-40 pickup loads of 4' logs will take several weekends even if you have friends helping you.

Dean Hornberger
- Clarion, Pennsylvania, United States

November 18, 2013

Q. We have a Wood Doctor that we just finished repair of leaks inside. We had to do welding and patching on it. We found we have 2- tiny drips still. How can we repair these leaks with taking it all apart again. Any suggestions to to seal the leaks possibly?

Dean Hunter
- Port Perry, Ontario, Canada


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