# flue heat exchangers



## Would you like some Tea? (Aug 15, 2014)

I'm playing around with a shop-made outdoor wood boiler.  Obviously this is a fool's errand but so far it has been great fun.  

So far I have a relatively successful stove welded up that has preheated primary and secondary combustion air, lined with firebrick, and has forced air combustion (which can thus be controlled by varying fan speed).  Burns quite steadily, accepts a 3' log and will also burn wood chips or wood shreds which I can get free.  Depending on the fuel I have been able to adjust it to produce absolutely clear smoke, which I take to be a crude sign of pretty fair combustion efficiency.   Such a clean burn should also reduce the potential for flue fires.  I plan to burn this thing at full bore, and always when I've operated a wood stove this way I've had no problems with creosote.  No pesky regulations around here say I can't use something like this.  Gotta go outside in a little insulated hut  for safety as well as the fact it smokes until you get the draw right.  It will heat a tank of water which then heats the house hydronically. 

I plan on insulating the firebox with high temperature insulation (another thread) and then adding a heat exchanger, also homemade, to the top of the whole caboodle.  I have no illusions that this will be a long-lasting stove, or high efficiency, however it is certainly fun to build and so far cheap.  In a couple of years it will be fun to build again and improve.  Usually about the third time I build something it gets really good.  

Safety devices for such a heat exchanger are a whole NUTHER important discussion, but the short answer on safety devices is, I will plan to use an open, atmospheric system, which makes it much harder but not impossible to have a steam explosion, and a 500 gallon insulated tank to store the heat in, a pump to run the water around, an aquastat to turn the pump on and off, and temp/pressure relief valves at important points in the system.     

All flue heat exchangers in a wood stove will foul, and thus a plan to be able to clean them frequently and eventually replace them is in order.  Wood stoves that burn hot and fast and complete will have less but not zero fouling problems in the flue. 

Shop-made flue heat exchangers seem to come in about three classes:  copper watertubes wrapped around things (such as flue pipe), homemade water tube systems, and home made fire tube systems.  

Watertube systems of any kind, whether copper pipes wrapped around flues, or pipes running through flue gas, etc, seem like they would foul quickly and be really hard to clean.  Copper won't take the heat, steel corrodes fast.  Hilcoil sells stainless steel coils, not cheap however they might be a good idea if one was going to do a watertube heat exchanger that would last.  Perhaps any steel box (even an empty "barrel stove") could house a few of these coils, and have an access door for cleaning.    These stainless steel coils are certainly a good option, but I am not sure how many I'd need to transfer a reasonable amount of heat, and that system might get expensive fast.  

Now, a fire tube system seems to have some attractive qualities.  Designed right, if one can access the ends of the tubes, it should be relatively easy to run a brush up and down inside them.  This might be a frequent chore.  I also was just reading another thread about using an old chain as a turbulator in a vertical fire tube heat exchanger.  Good idea and rattling them around might extend the cleaning cycle.  If I built the heat exchanger, it would almost certainly be made of mild steel, a disadvantage in durability but an advantage in constructibility.  Not sure if such a system wouldn't tear itself to bits expanding and contracting.    

Years ago me and a buddy built a DIY fire tube wood-fired boiler out of a couple of old gas water heaters.  The flue ran right through the two 3" pipes in the centers of the water heater tanks, and the coily metal turbulators that came with the water heaters remained in place.  You'd fill up the firebox with small wood scraps, and the thing would roar once it began to draw.  Made 60 gallons of hot water in 20 minutes, enough for a few guys to take tub baths and do load of dishes with some to spare.  I have had little luck finding old gas water heaters that don't leak, but this might still be a simple option. 

A homemade fire tube heat exchanger is a possibility.  I'm imagining welding several tubes into a small 30 gallon drum, which goes inside an outer 55 gallon drum with a removable lid for cleaning.  16  1-1/2" tubes would be the same area as a 6" flue pipe approximately, that might be a design goal.  Flue gases go through the tubes (firetube design) and the 55 gallon barrel has a 6" flue on either end for connection to the stove and chimney.   Water is in the inner tank, and the inner tank is at atmospheric pressure.   An external pump moves the water through the inner tank.  Safety devices and controls and makeup water etc. will be discussed in another thread - I am trying to concentrate on the basic design of the flue heat exchanger here.  Obviously this would require watertight welding, a feat I am probably capable of with a few tries. 

Is there some thread or project folks can point me to that is similar, and was a successful heat exchanger?  Am I on another fools' errand with the firetube idea, and should abandon it and stick with water tubes?


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## Gearhead210 (Sep 18, 2014)

Would you like some Tea? said:


> I'm playing around with a shop-made outdoor wood boiler.  Obviously this is a fool's errand but so far it has been great fun.
> 
> So far I have a relatively successful stove welded up that has preheated primary and secondary combustion air, lined with firebrick, and has forced air combustion (which can thus be controlled by varying fan speed).  Burns quite steadily, accepts a 3' log and will also burn wood chips or wood shreds which I can get free.  Depending on the fuel I have been able to adjust it to produce absolutely clear smoke, which I take to be a crude sign of pretty fair combustion efficiency.   Such a clean burn should also reduce the potential for flue fires.  I plan to burn this thing at full bore, and always when I've operated a wood stove this way I've had no problems with creosote.  No pesky regulations around here say I can't use something like this.  Gotta go outside in a little insulated hut  for safety as well as the fact it smokes until you get the draw right.  It will heat a tank of water which then heats the house hydronically.
> 
> ...


 


9/18/14- Hello and you are not the only one interested in such a project. The fire-tube HX in the flue gases seem like the most user-friendly for the reason of cleaning and for durability. I have been pondering the idea of adding one of these HX's to my homemade boiler that I have been using for 3 years now. After burning almost 30 face cord last winter I need to try something to get some more efficiency out of it. (My choices in quality of firewood were partly to blame for the amount burned).
I am likely going to use the idea from the "Magic Heat" flue heat exchangers- but rather than running air through the tubes I would have several jumper lines with water running through them, and just run a second pump to the whole thing on my chimney as its own little unit up top. 
The tubes would have the water running through them, and the flue gases passing around them to help create more turbulence, with the tubes running horizontally.
As far as cleaning goes, I was planning on copying the principle from the Magic Heat again, and flame cut a thick steel plate that would be slid back and forth to "scrape" any buildup off the pipes. If I make it easy to do then it shouldn't ever get built up bad enough to become difficult to scrape.

My boiler was built out of a 500gal propane tank that I shortened the length of, and I used a 3/8" wall 36" diameter structural steel pipe for the firebox that I fitted ends on. Should take it about 20 years to rust through all that steel I hope. I'll upload some pics later once I am back on the other computer that they are saved on.

I have yet to purchase some steel to get going on this, but I may draw the whole thing up in Solidworks to see what it may look like before making sparks.

Richard from SE Michigan


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## TCaldwell (Sep 18, 2014)

consider the extra fluegas resistance you are creating in your design, any reduction in fluegas cfm directly reduces heat output, weather or not what you are creating will capture more heat than the reduction you created with the heat exchanger would be a good documentation.


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## maple1 (Sep 19, 2014)

If I was going to start on some kind of project like this, I think I would start with a used boiler that had tubes and work on the firebox end of it.

So maybe you could find an old fire tube boiler with a burned out firebox and mod it to fit?

I guess you guys just have more get-up and ambition for this kind of stuff than I do.

But on the overheat stuff - maximize convective flow, don't depend on just pumps & controls.


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## Would you like some Tea? (Oct 19, 2014)

Great suggestions, guys.  

I have made a couple of attempts at welding up a firetube heat exchanger, fortunately with junk steel, and have learned mostly how poor my welding skills are.  That was no surprise.  The challenge here is to weld up a watertight tank.   I find I can make about 95% of my welds watertight, but I haven't achieved 100% yet.  Good practice.  Been reviewing a bunch of youtube videos by this guy  called "How not to weld", a great compilation of how welding errors look after you've made them.  Since I learned to weld by grabbing an acetylene and MIG welder about 30 years ago, with no formal training, I am trying to go back to basics and unlearn many bad habits.  

I'm using a MIG with mild steel.  The first (obvious) lesson was that the steel for the tank and the tubes needs to be about the same thickness.  Since I started with scrap steel, they were widely dissimilar thicknesses and the thin stuff kept burning through.  I can weld two pieces the same thickness without burning through, but it gets dicey if one is thin.  I had an old water tank and some schedule 40 steel tubing, but the water tank wasn't much thicker than sheet metal.  

The second important lesson is a welder's mantra:  "tack, tack, tack equals crack, crack, crack."  Whenever you stop and start welding, there is likely to be a crack or a pinhole.  

Burning through thin metal is really problematic.  If you do burn through, you have to stop, let it cool, and weld it up again.  Stopping makes a crack.  Every time you burn through, there is almost no way to make that watertight.  Good thing this was all free scrap steel.  These objets de Art will sell pretty good at my estate sale, I am thinking.  

A buddy welded up an old boiler watertight, (Heck he is a worse welder than I am! and was using a stick welder!) and he suggested that, whenever I find a leak (10 PSI air pressure and soapy water method) to grind it off and reweld it, instead of trying to weld over the top of an already pinholed weld.  That works better, and is pretty obvious once someone mentions it.   

In both tries it took about an hour to weld up the tank and tubes, but then four hours later I was still chasing pinholes, rewelding the same places over and over again, and gave up.  

I'm about ready for the next try with some expensive purchased steel.  Here is the plan:  1. Use thicker steel, all of the same thickness - 1/8" to 3/8 would be ideal, with a couple of practice welds on scrap pieces.  2.  I can't weld continuously, gotta stop every once in a while.  I'll have some soapstone handy and mark the spot where I had to restart, then reheat that spot with the acetylene torch to get it good and melty.   3. Air pressure test at a low pressure - say 10 PSI - so I don't blow the thing to smithereens before it is finished, and use soapy water to find pinholes.  4.  Grind off welds with pinholes, and weld them back with the acetylene torch instead of the MIG. It is slower, but much more controlled.  Make sure the air pressure is relieved when rewelding so the pressure doesn't blow out the melted steel (another obvious idea that didn't occur to me at first).  

If I ever achieve an airtight tank, I'll take it way out in the yard, and pressurize it to 30 PSI whilst standing a LONG way away in case something blows. I have 100' of air hose, that ought to be enough for a chicken like me.  Maybe leave it for a few hours to check for more leaks.   

The basic design I am going to use is this:  

A 6" flue enters the middle of the end of a 50 gallon drum.  Inside the drum is a smaller tank with about 1" clearance on all sides.  The inner tank has 4-3" firetubes inside it, but the neat transfer surface on the outside of that inner tank is significant.  I'll arrange some removable flue gas turbulators inside the tubes, and the outside of the inner tank will also have some turbulators that serve to support it as well as mixing flue gas flow.  

 The total heat exchange area should be about 24 square feet.  The open area (gap between tanks plus 3" tubes) is about 88 square inches, versus the area of a 6" pipe which is about 28 square inches, so this should not obstruct flue gases significantly.  Turbulators will reduce the area somewhat.  
The inner tank should hold about 35 gallons, with 286 lbs of water.  The whole heat exchanger could weigh 400-500 lbs, so it will need some support as it is intended to go above the stove.   I have a nice CAD drawing of it but the CAD program keeps crashing today, so I'll post a drawing later.  

Water connections at the inner tank come in at the top and bottom.  Bottom (entry) is arranged so it circulates water spirally, increasing turbulence.


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## Buzz Saw (Oct 19, 2014)

Also check out Jodi at weldingtipsandtricks.com

So you talk about burning through but did not mention any details of your settings. What voltage, Amps(inches per minute), gas mixture.  If your welds are cracking at tacks you need to fill the crater longer when ending the weld.  At the beginning of the weld let the arc stabilize and burn in well before you start advancing. Be careful using to much water to test for leaks, water causes cracked welds because there is hydrogen in water.

If you have to weld different thickness of metal focus the arc on the thicker section and let the puddle wash over into the thinner metal. The molten puddle will melt the thinner metal.  Also if you are burning through move faster.  Hope my thoughts make sense. It's hard to proof read on my stupid smart phone.


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## Would you like some Tea? (Oct 20, 2014)

Thanks for the advice, Buzzsaw!   Water is definitely another problem (obvious) that I didn't foresee.  As you mention, putting water on a hot weld causes hydrogen embrittlement, which is deadly to strength and causes cracks, something I was unaware of until reviewing some of those welding basics videos I linked above.  Getting in a hurry with my soapy water probably caused more problems than it solved.  Weld the whole thing, let it cool, test it, mark and grind off bad spots, weld again.    The other suggestions you make are good ones, and are also lessons I have learned over time just by making (pretty much) every welding mistake that can be made. I'm the Murphy of the MIG.  Good, shiny weld prep, right amps and speed and feed for the thickness, etc.  Plan to spend more time cleaning and prepping the weld, clamping the steel, cutting the steel, etc. than actually welding.  Take a few practice welds on scrap, and grind them off to inspect penetration.  Use a freaking magnifying glass to look at the weld edges.  Set up the weld in a position that isn't excrutiatingly uncomfortable, so I can stay on it without straining or taking a break.  (Usually I am down on the floor of the shop in a position resembling a yoga instructor).  

One of the keys, though, it is just a lot easier for me to weld something that is thick (within the capacity of the welder) than paper thin, and easier to weld nearly identical thicknesses than wide-ranging thicknesses.  I can make a passably strong weld in dissimilar thicknesses, using the techniques you mentioned, but not a perfectly watertight one.   Maybe a more skilled welder can adjust for that problem, I am not there yet. 

All of these are lessons I sort of knew, and can ignore somewhat if I am fixing a shovel, but still have a weld that won't break but won't win any beauty contests either.  Those sloppy mistakes won't fly if I am trying to make, essentially, a perfect weld that is watertight.   I'm about 90% there, so the next session in the shop will likely be a better one.


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## Gearhead210 (Nov 11, 2014)

My solution for leaking welds: put the mig welder away. Unless I'm welding light sheet metal everything I do is with a stick welder. It's much harder to get a good hot penetrating weld with a wire welder than a beefy old stick welder with some 6010 or 6011 rod. Plus you can weld in any position you want a little easier. And clean metal isn't a necessity like with wire welders. Been away from the boiler project a few months, gonna re visit in the spring so that it's ready for next winter. Can't start cutting and welding on something that's heating my house right now


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