Instructables
Picture of Woodstove Heat Exchanger
2013-03-09 16.40.30.jpg
      I wanted to build this heat exchanger to reclaim some of the heat that is lost up the chimney of my wood stove. I use the wood stove to heat my workshop during the winter months . I have seen heat exchanger units available in stores for around $160. Since I'm cheap, and I hate to pay money for things that I think I can build myself,  I set out to create my own heat  exchanger. 
 
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Step 1: Materials and Tools Used

Materials
  1. 2- 12" x 12" pieces of 19 gauge sheet metal
  2. 8- 1-1/4" diameter thin wall steel pipe pieces 11-1/4" long  ( I used Galvanized top rail from chain link fence)
  3. 1- 2-1/2" diameter thin wall steel pipe 11-1/4" long          ( I used Galvanized fence post)
  4. 1- 5 gallon steel bucket , the type that Tar, Asphalt roof coating or Driveway Coating come in.
  5. 2- 6" diameter black steel stove pipe unions
  6. 1 -Can of High Temp Stove Paint
  7. 1-Tube of High-Temp Fireplace Cement
Tools I used
  1. MIG Welder
  2. Bandsaw
  3. Chopsaw
  4. Bench Grinder
  5. Angle Grinder
  6. Rotary Tool
  7. Tin Snips 
  8. Various Drill bits
  9. 1-3/8 " diameter Knockout Punch
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Nicely done! Wondering if there's any way to use a thermoelectric module to power fan(s), so that no AC needed, so it'd be totally self-generating.

sgonzalez226 months ago
Wow 1st I wan to say thank you for all the information contained in this post im in the process of creating the same thing I was looking to make something like that but did not have any direction on how to get started thanks to you now I to can make one for my wood burning stove thank you its perfect
VertDude (author)  sgonzalez226 months ago

Im glad you can use some of my ideas to create your own project. Please post a picture when you complete it..

camping crazy7 months ago
Is that a ramp in the background or a building???
VertDude (author)  camping crazy6 months ago

Its a Vert Ramp and a building, I have storage under one side, and a 16' x 32' work shop under the other side

Respectfully, I disagree about ignoring holes and tolerating loose seams. I would recommend as tight a seal as possible on a stove flue if it is used in a modern building.

I was privileged as a youth back in the 1970s to meet many older rural people who came of age before WWII and who still used the old tech. My uncle's mother, who was then in her late 80s was born in the 1890s and distrusted electric heaters. She insisted on her potbelly stove right up to the end. The upshot is that I used these old stoves a lot. 

One of the dangers of pot bellied stoves or any tech burning carbon is that if you get incomplete air flow, you get incomplete combustion which creates carbon monoxide. If you have a stove whose fuel has burnt down to coals and you get an incomplete draft and you have holes, you can leak carbon monoxide. Carbon monoxide is colorless, odor less and its effects subtle. 

Wood burnt down to coals produces no smoke and even little smell (depending on the wood). It has become charcoal at that point. With an incomplete draft, the hot air produced by the coals isn't being sucked up the flue but is just slowly drifting up it by thermal expansion alone. It won't prefer exiting the chimney over leaking out a hole. Even the slots for lifting the cooking plates become an exit point. 

In the past, this was rarely a problem because the buildings, even houses, were drafty with lots of natural air exchange. CO or other gases couldn't concentrate easily. These days houses and even work buildings are insulated to the point of being hermetically sealed. Any gasses emitted inside the structure stay there and concentrate especially overnight when no one is opening and closing doors. 

All technology evolves in an "ecosystem" of other tech. When you move it out of that ecosystem it's associated risk change. Just because a technology has been used safely for decades in one environment doesn't mean it is safe in another.

When we put a pot bellied stove in a modern insulated structure we move it out of the well ventilated environment it was designed for and put in it a closed, air recycling environment, we can make them a hazard. Extra care must be taken in their use and maintenance.




plus u 4got 1 thing that it flow down down then upwards as well. u need a small amount of air inflow in to the structure as well.

VertDude (author)  shannonlove1 year ago
Good Point and thanks for all of the information. My workshop is definately not air tight, nor is the stove, and Im not spending the night out there while the fire dies down. My risk factors are low, although I am going to take the advice of Greenman48 and install a CO meter to be on the safe side..
If your shop is naturally drafty then there really isn't any risk but carbon monoxide detectors are cheap enough that they should be standard in any environment where carbon is burned for any reason.

astral_mage6 months ago

wat no work gloves.

astral_mage6 months ago

ok i c now the head. ok heres a thought try filling the void with sand 4 more therm mass soaking?

astral_mage6 months ago

u need to look at engineer775 channel an look at his water heater set up.

treed168 months ago
Cool! Great job.
SIRJAMES091 year ago
AWESOME IBLE!!
TY for sharing Sir.

I just had a brain fart. :P
is there a way to connect a 2nd fan blade to the center of the blade that is already there??
My thinking is, with the 2nd fan blade, even the center pipe in the heat will put out some heat...
or am I out in left field with this?

static1 year ago
To be stating the obvious, simply getting black pipe would eliminate the need to deal with the zinc removal. unless the galvanized was free or already on hand.
LynxSys1 year ago
That supervising head needs to get some glass between him and that arc! And really, given his lack of regard for personal protective equipment, do you really want this guy--well, head--supervising anything around your shop?

Seriously though, it's a nice and well documented project. I'd be building something similar right now if I had a metal shop of the magnitude that you do!
VertDude (author)  LynxSys1 year ago
LOL, That Head has been hanging around the shop for quite a few years now(14) . He's kind of a shop mascott, by the way, that is not my personal metal shop, just the tool room at work. Thanks for commenting..
Quester551 year ago
First, Let me suggest that the Zinc That comes off after the ACID Bath, Is Now TOXIC & Baking Soda Has NO Effect on Zinc, After it's been Chemically Altered.
Otherwise, You have a Great Project , Glad it worked out for you.
To Avoid the Zinc problem in the future, I Suggest you purchase TAIL-PIPING or Muffler Piping from a local suppler, or Get it Cheap at an auto Pick-a-Part Yard.
Here's a Trick, Find some Indian Clay in the Ground, wash 7 Filter it so you get Just the Clay & Let it build up until you can fill a 5GAL. Bucket, Keep it moist, Using Sheet Metal Flashing make 2 Round Pipes or tubes, 1' about 20" across & one about 18"
Across, Drill Holes in the Lip of the 2 pipes & use 3" Bolts, Washers & nuts, As Spacers for the 2 pipes. Once finished, you should have a pipe Within a Pipe Mold,
Now comes the fun part.
Setting the Pipe mold on the Ground, ( Note using an old Burlap Feed Sack as a Ground pad) & Fill the space with the Clay. Now Cover it up with a Breathable Screen to keep bugs out & let it dry for a few days. When the ends feel dry, Move the Mold to a Hard Dry Surface & Let it Sun dry a week longer, Then with someone to Help you, Build a Fire big enough to surround the mold( Note or Place it on a Grill & ) Bake it Keeping the fire hot for 12 hours.
If All Goes well You'll have a Poor-Mans FIRE-Brick Liner for your Hot-Water-Boiler,,
Pot Belly Stove or Backers Oven.
Just Wish I had a way of Showing you the Steps.
Good Luck.
You can weld galvanized steel safely as long as you use the correct respiratory protection, so it might not have been strictly necessary to remove the zinc (of course, there are plenty of welders who think that "proper respiratory protection" is "keepin' yer melon outta the plume!" but let's not go into that). That said, I don't think that the zinc would be affected by the consistent high temperatures of the stove pipe, but I'm not sure.
VertDude (author)  Quester551 year ago
Cool Idea....By the way, the purpose of the baking soda is to neutralize the remaining Muriatic acid...So as not to corrode plumbing pipes as it is disposed of..
confu1 year ago
Pretty nice build!

If you would have used something with a larger wall thickness as a housing, I would have recommended to weld some fins to it to increase surface.
But I think that won´t work with that bucket and its wall thickness of probably about or less than 1 mm (?!).

And in my "very own perception of fluid dynamics" (since I´m not an engineer), it would be more efficient (for the overall air flow through the pipes) to duct the fan and/or make it "pull the air through" to avoid losses due to turbulences on all those edges/protrusions.
But my GF who actually "is" a mechanical engineer specialising in thermodynamics says that turbulent flows are best for dissipating heat. Hmmm. I really don´t know.
And if you are concerned about overheating the engine of the fan if it "pulls" air, you may extend the motor shaft through the center pipe and install the fan on the other side...

An additional approach for maxing out the efficiency of your stove;
My neighbour uses an almost similar woodstove and he attached an array of steel pipes to the walls of the burning chamber to increase the surface. Seems to work pretty well (as he says).
If you like, I will ask him to take a photo of that build.
VertDude (author)  confu1 year ago
These are some good ideas, I was planning on building the original housing out of thicker steel, but then came across the bucket and it seemed to simple, so i used the bucket...a trained welder with better equipment could surely weld some fins on the bucket...I am interested in your neighbors set up, if you do get a picture..
confu VertDude1 year ago
Took a while, but here you are. He also added a "humidifier" on top now :)
IMG_1929_1.jpg
VertDude (author)  confu1 year ago
Cool.I will have to add some pipes to my stove!. Thanks for the picture.
Switching the fan to a squirrel cage type(like off of a junked van) would solve the problem of the center not getting enough air. for version 2 i would recommend to make it longer and turn it on it's side (so the hot air goes in the little tubes and you blow cold air inside the shell). this way you could make it as long as the vertical rise above the stove. and by forcing air in the top and out the bottom you start differential cooling allowing you to increase efficiency by several times. better yet it wouldn't matter what kind of fan you use as the path is no longer a straight line. so go nuts on fan size. for a better description just google vertical fire tube boiler. Great job.
VertDude (author)  absolutekold1 year ago
Thanks, I will check it out
wudwerks1 year ago
Nice work. I just bought an identical woodburner as yours, and the instructions say to keep a 36" clearance from any combustibles (including 2x4's behind wallboard). I thought about that cement board, too, but don't have much faith in it, as far as keeping the heat off my wall studs. What is your experience with this stuff, as I see your clearance is only about 12" from the walls and your workbench.
VertDude (author)  wudwerks1 year ago
Thanks wudwerks, actually my stove is about 18" in the closest spot, to the wall. the stove is sitting at about 45 degree angle to the wall. The cement board is 5/16" thick James Hardy siding, left over from my house. I used 3" long pieces of 1/2" metal tubing as stand offs,or spacers, between the cement board and the wall. I measure the temp of the board as hot as 375 degrees F , while the wall behind the board remains at 80 degrees, So I am quite confident with it, I also use a piece as an exit flange for my chimney and it remains rather cool as well. I have been using the stove in this location for 4 winters now with no problems
VertDude said: >>I also use a piece as an exit flange for my chimney and it remains rather cool as well.<<
Brilliant! I was wondering what to do about that, too! Thanks.
VertDude (author)  wudwerks1 year ago
Sorry wudwerks I had a typo in my first reply. My stove is only 8" away from the cement board in the closest spot, rear corner of stove...
zawy1 year ago
Thicker steel pipes would reduce heat exchange slightly. You want the steel to transfer the heat actually cool off as much as possible with a larger fan, not retain heat.
zawy1 year ago
The pipes need to be vertical and parallel and in front of the fan and having the hot air flow inside instead of around them, and no outside "drum". The reason is that your fan air flow rate is a lot higher (and should be) than the exhaust airflow rate, so the internal net effective duct width should be smaller. As it is, not much air as able to flow through compared to what the fan is capable of. Heat extracted will be sort of, in a wild sense, proportional to the air flow rate you can get next to the pipes. Since you're dropping the heat only about 30 F on 400 F, you're getting about 10% of what's theoretically possible. My suggested change will cause a lot more creosote and need a good bit of pipe to maintain good exhaust airflow to keep a good fire with low CO. Another option is a benjamin franklin stove (a sideways barrel on top with the inlet and outlet on opposite ends to give the exhaust air a lot more surface area than a simple pipe) . Pumping water over such systems from a larger tank (50 gallon barrel for 400 sq feet) and raised to 140F is more effective than air at extraction and releases the heat energy all night after an evening fire. Use heat capacity of air, estimate exhaust flow rate, and measure the temperature difference to calculate heat output of the exchanger. (13 watts for each liter per second exhaust flow for each 10 C drop in temperature.
VertDude (author)  zawy1 year ago
Thanks for the comments zawy. Actually, I have excellent air flow through the unit, When the stove is really fired up , it feels just like standing in front of a torpedo heater. I mentioned in my "ible" about weak airflow, only through the center pipe of the exchanger. Thanks for your ideas about heating water, I have been considering the possibilities of heating a mass of water for release into the night..
zawy VertDude1 year ago
If the stove pipe temperatures above and below the exchanger are only 30 or 40 F different, then about 5 times more heat output is possible. The white fan you use can supply about 100 liters per second but the exchanger's "room-air" pipes only allow probably around 5 through. It think the external drum is where most of the exchange is occuring. I think a fan on either side of it would do at least as well, not using the internal pipes at all. Airflow through pipes for a given pressure difference is a 3rd if not 4th power of the diameter. So if you had one or two 4" pipes in the exchanger you can get a lot more airflow at a much smaller cost in surface area and no change in draft restriction. Mpja sells 4" fans for $8 that supply 40 liters per second but they are a little noisy. I assume the picture of the red center was taken while the fan was off, otherwise it is obvious from that very little air makes it through. If you're copying professional designs (which i am not familiar with) in terms of pipe sizes and airflow methods then I must be wrong and missing something about what I see. I did a lot of DIY work with air flow and heat exchange on solar hot air boxes, which is why I am opinionated.
zawy zawy1 year ago
The 6" vertical stove pipes give 28 sq in. If the 5 gal bucket is 11" in diameter, two 4" pipes side by side in it can have a 1" gap along the horizontal length of the bucket in the middle and at each side. For the approx 10" useful length of the bucket, this gives a gap with an area of 3x10=30 sq in which will not reduce exhaust flow while forcing the exhaust maximally into the bottom side of the two 4" pipes. Air flow against the steel surface is everything in heat transfer on the inside and outside of the 4" pipes, assuming the fan airflow is fast enough to extract enough heat. Minimal paint on the outside helps because paint does not conduct nearly as well as metal. Minimal creosote build up on the bottom of the 4" pipes is crucial for good heat transfer. If the ambient airflow through the 4" pipes is poor, then it does not matter as the exit air can only get as hot as the exhaust flow. Very high airflow will reduce the 4" pipes' outlet temps but you want this for higher heat transfer. The higher volume of air and its heat capacity more than make up for the reduction in air temp. The temp of the ambient air heated coming out from the exchanger is meaningless without knowing the airflow rate. I do not think complex designs with fins as in cooling CPUs is feasible. This 4" pipe design has an exposed area of 2xPIx11"x(4"/2)^2=276 sq in whereas your design has 8xPIx11"x(1.25"/2)^2 + PIx11"x(2.5"/2)^2 = 210. It should allow at least 8 times more airflow per pipe area (this is from (4/2)^3 = 8 see my previous post), and since the exposed surface area is more, this means 8 times more heat exchange if the ambient air outlet temp of the exchanger did not decrease due to extracting more heat, but it will, so my estimate is 3 to 5 times more heat from the exchanger with this design.
zawy zawy1 year ago
Oops, I made an equation error at the bottom. Two 4" pipes have 552 sq inches exposed and VerteDude's design has about 800, about 50% more, not less. But the improvement in airflow through the exchanger is still overwhelming. But as i hinted at the beginning, just splitting the 6" pipe into two exposed 4" pipes and putting a box fan on it to flow around the pipes instead of inside will do as well. The advantage of having the air flow inside is that a high velocity of air stays against all sides of the pipe instead of just half, and gives the opportunity for a longer pipe run horizontally. A box fan on the side of the 5 gallon bucket might double the efficiency of VertDudes design, and help my design by 50%.
I was a certified chimney sweep for many years. Your craftsmanship and thinking are good here. However, there is one problem with this sort of technology.

I am sure that you are familiar with creosote,the causes and the danger. You say you send too much heat up the flue. The heat you lose keeps the creosote down. You may find yourself clogging the flue with the heat exchanger you installed, although, that would normally happen with motorized exchanger. If I were doing this in my shop I think I would prefer a "schoolhouse run" to avoid the sudden cooling. However, it looks like a really good job.
VertDude (author)  wstarvingteacher1 year ago
Thanks for your comments wstarvingteacher, The heat exchanger only drops flue temp by 20 to 30 degrees, so Im not too concerned with excess creosote, I clean my flue completly each fall before burning season. Plus I probably only use the stove about 3 days a week on average. What is the "school house run" you are referring too?
A school house run is where the stove pipe is run from one end to the other to give off the most heat possible. The picture of our stove tells me that it isn't all that efficient. This is to dump the heat in the room and not up the flue. You can actually use the heat exchanger and do that as well.

With the run you can take the pipe apart, throw it on the ground in the yard and clean it there. No mess that a water hose doesn't cure. Still makes a lot of creosote till the flue warms up.
ANDY!1 year ago
Looks fantastic, I'll let my friend's family know about this 'ible so they don't have to spend so much time lugging wood around every day!
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