capture heat from pellet stove flue?

I wonder if anyone has any experience with this:
I have a nice pellet stove, and the inside flue is about 6 inches long, before it goes through the wall and blows the hot exhaust outside. When it is vented outside, it is hot enough to start a fire, even when it is in the 20s outside.

The exhaust from the pellet stove is blown by an electric fan inside the pellet stove.

I am in the process of building a rocket stove mass heater for my green house, and suddenly yesterday, I saw that pellet stove exhaust in a different light.

I have a wood frame house with a slab floor. Seems like if I moved the pellet stove, and had the stove pipe inside for 10 feet, maybe had it run along the slab floor, under the windows, with proper spacing, and / or insulation for the wood frame wall, and devise a thermal battery of some kind, I could capture at least some of the heat from the pellet stove before it goes outside.

Any ideas suggestions perspective advice experience on this?

Thanks

Thekla

Thelka, I completely agree but cannot find any information.

Post why can't the exhaust off a pellet stove be captured like in a rocket stove?
ok, passive is great (no electricity, amish style living is admirable) but my question is still:
why can't the exhaust off a pellet stove be captured like in a rocket stove configuration?
why not recapture this heat?--for certain, the exhaust is quite hot.
is there a danger in the exhaust flow being (a) too rapid?, (b) too hot?...too much heat build up, or (c) too gaseous?....
would an exhaust fan be needed at the end of the exhaust tubing to mitigate the above items?

as a minimum, why not use heat exchangers as used on flues to wood stoves to recapture the heat? see http://www.northerntool.com/images/product/images/172781_lg.jpg

anyone with ideas?

yes you are right in my opinion to want to recapture that energy and you can so easily do it by the methods you mention. I am thinking there might be a problem routing the exhaust down to the floor before the chimney is heated I ran into this two days ago and i am just stunned at the simple efficiency they get by lighting the stove and warming the chimney first before rerouting to a thermal mass...

http://www.geopathfinder.com/9597.html

look at their exhaust diagram about half way down the page.

How about using a container of water or thermatic oil to capture and store that heat from the flu?

A coil in the flu could carry the liquid in for heating and then back into the container. The hot container would continue to radiate heat after the fire went out. Circulate it to containers in the bedrooms and your stove in one room would continue to heat those bedrooms even after the fire was out, assuming the stove runs long enough to effectively heat all that liquid.

If the containers were surrounded with a thermal mass, maybe a RMH-type bench or piece of furnature built around the containers, then perhaps that would increase the thermal storage and extend the amount of time the heat radiates.

Anyway, I guess this would depend on how many hours per day you use the stove. The more liquid you have, the longer it would take to heat it.

Thekla McD. : Just a thought, it is possible that your Pellet Stove was supposed to exhaust to the outdoors through a heat exchanger! I would find the Manufacturer,
Model Number, Serial Number, and see if the output in B.T.Us is listed as a single number or a range . You should be able to go online to the Manufacturer and see your
installation instructions, owners manual and a parts/repair parts numbers listing, possibly with an exploded picture/diagram of your exact model and its related models
all without going back to the dealer who installed it FIRST !

While the odds are actually slightly against you having a Pellet Stove with an uninstalled heat exchanger, it is worth checking into ! The E.P.A. should have a listing of
what their tested model had! It would be interesting to find out the exhaust gas temperature you are experiencing ! Having said all that, it is a little sad that your pellet
stove required a powered exhaust fan, especially if there was/is not Heat Exchanger ! I would like to see a picture of the Exterior Wall thimble kit for your pellet stove.

If I was faced with your situation and checked out the possibilities above, I would have no hesitation in building a Cob bench running near the exterior wall but not touch
ing! I am assuming an outside wall with sheet rock over 2 by X studs and pink insulation .

Hint - Ernie and Erica Wisner who are the Wood and Rocket stove Moderators here at Permies, have a site with detailed plans to deal with building Rocket Mass Heaters
near Exterior walls of that type ! You can reach them at ernieanderica.info check out their 'shop for plans' section !

For the Good of the Craft ! Be safe, keep warm ! PYRO AL - As aways, your questions and comments are solicited and are Welcome ! A. L.

Theka McD. : Your slab floor is a potential problem in any attempt to spit ball a working lay out, could we have a simple sketch of the present layout
and what you would be happy with ?
Of course any pictures you want to share about your Rocket Mass Heater would interest us all though maybe asa separate Thread so it doesn't get lost ?

Every Time I run into a potential heat flow problem I ask myself if a lattice wall of bricks would aid in storing more heat for slow release !

I just checked the date of your original post, I am sure you have solved your problems by now ! Color my face red !

For the good of the craft ! Big Al

My bro has a 5th wheel trailer with a propane furnace, vented thru the wall. The exhaust is really hot outside! This is waste heat, just like your pellet stove, or a rocket stove, a rocket mass heater, etc...

So, I set up a platform, topped with a styrofoam sheet insulator. Used a one gallon aluminum cook-pot full of water as a test, about an inch or 2 in front of the exhaust. We measured it at 125 f. degrees, it may have gotten hotter if left longer. You could thermo-siphon (a passive, energy-free way of moving hot water heat) into an equal-height, long trough of some sort (a metal rain gutter?) inside your house. You might want something enclosed, more like a big horizontal pipe, or tank, so water vapors wouldn't evaporate indoors. But don't trap the steam, it could over-pressure (doubtful but you never know) so vent the top, to indoors or outdoors, that's up to you.

This (hot water) large thermal battery could also be used to heat an outdoor shed, or greenhouse, etc... It will recover exhaust heat, but without the toxic fumes. You could even electrically pump from the large hot water reservoir, into radiant heating. A distant room (insulated water pipes/hose for the distance) or in-floor hydronic heating system (pex tubing, a hot-safe hose, Home Depot) Many, many options here! Good wishes all, thank you open source attitude!

AFTERTHOUGHT - - Pellet stoves have a concentric exhaust pipe, the inner pipe is the hot exhaust, the outer circle is the air intake. This is to keep the hot, inner pipe safe and fireproof, as well as to pre-heat the (outer pipe) incoming air, for a more efficient combustion. You shouldn't put a coil inside the flue, nor should you pack cobb around it.

Also, instead of the open pail like I used outside, you could have some copper pipe or tubing in front of the outside exhaust. This would feed into an indoor tank, just like the old school wood-fired kitchen cook stoves with the copper coil in the fire box, thermo-siphoning to a large, slightly elevated hot water tank. The secret to this set-up was, an extra piece of vertical pipe, tee-d in next to the hot water tank, externally connecting the hot side with the return side. This "kept things moving". Make sure the tank is widely, openly vented at the top, so pressure doesn't build up. You'll probably have to add some water once in a while. Send the boy out to the well! haha - The thermo-siphon effect is caused by the hot molecules in the water rising upwards in the water, towards the hot water tank. The water isn't moving like a pump, but the hot molecules move. Some folks call this a phenomenon, but the best thing is, it is passive, requiring no power. A final caution, don't let the water freeze, drain it out, or keep it hot. I bet it will run upwards of 150 f. degrees. Remember the longer you run it, the return side will keep getting hotter and hotter.

Wow, folks thanks for all the ideas-- I have not been getting the notifications when someone posted so I have not looked prior to today, when Roger Purple Moosed me. Thanks Roger.

The stove has a heat exchanger, a second fan blows room air through the heat exchange tubes which then blows into the room.

I especially did not realize the configuration of the stove pipe, with inner and outer tube. Possibly the funtcion would not be changed by the addition of mass around the pipe, and additional lenght of pipe, still it does not make it easy to get the heat from the inner tube. I did know there was outside air to feed the fire. Not having an air tight house, not living in a plastic bag, I am not sure I want the outside air for the fire, but a pellet stove is not something I want to tinker with... My thought had been to cob in the stove pipe, or using Paul's portable rocket stove design, build a wood structure that would house mass such as sand, or even move the stove and run a longer pipe along the floor, right next to the concrete floor, let it soak up what it could, but I did nothing so far. And now, knowing about the concentric pipe design, I think it might be easier to capture heat from the discharge, maybe.

I think I gave confusing information re the greenhouse, which is some distance from the house and pellet stove. Last January, I began building a rocket stove in the greenhouse, and built a heat storage mass. Simply doing that during the time of year that I use the pellet stove made me think of some kind of heat storage mass for the heat that is discharged from the pellet stove, but it is a physical impossibility to use the heat discharged from the pellet stove to warm the greenhouse.

The situation with the pellet stove is just where it was when I posted on January 18th last year. As I said I have not been getting the notifications of posts, and have a lot of projects. Still room for something grand and beautiful when I get the idea design and inspiration. I'll re read all your posts and start cogitating.

The rocket stove in the greenhouse: (I could put this in a different thread, and may, but the rocket stove topic is pretty thoroughly covered)
I built built and rebuilt that thing and learned a lot. At this point it functions well enough, with help from permies dot com and Ianto's book, and Eric W. This summer I got the permies 4 disc DVD set on rocket stoves too. My stove does need modification. It is just not rockety enough to roar. It smokes sometimes. It doesn't draw with conviction or enthusiasm. Lots of possible things to do: longer riser(I'd have to dig deeper into the dirt under the floor and mass, because the top is close enough to the wood frame roof), or fewer turns in the exhaust tube, or larger diameter riser (presents the same challenge as increasing the length of the riser) and or get the exhaust pipe riser into the interior of the greenhouse and through the roof. (To do that I need to rebuild the south wall of the greenhouse and add a new section of roof.) Another option would involve rebuilding the whole mass bench as well as all of the above. The modifications will be worth it, because I just think with the current configuration the temperature does not reach those of the perfect rockets.... lower temperature means slower heating of the mass.

But, it is working, and when cold weather came a month ago, I began to get the hang of it. I've been able to keep the overnight temperature in the greenhouse in the 45 F range. The thermometer on the heat mass is about 60 to 65 in the morning, before the sun adds any warmth. The soil in the 15 gal Moringa post has a soil thermometer and it has been steady at 60 as well.

And the discharge is all steam about 85 % of the time. I like that, and this is what convinces me that it is working. But I want it to work better.

Priority for modification and perfection of this system was lower than my soil development project and participation in the soil carbon challenge. Just briefly, I'm starting out with documented less than 1% organic carbon in my soil. Plenty of room for more!

Thanks again for your posts and ideas.

Thekla

Not all pellet stoves have the intake designed around the exhaust.

My dad has a Harman and it does have a doubled wall exhaust for safety. However, it does require an additional intake pipe for combusting outside air.

I think pellet stoves pre-heat the intake air to increase combustion temps. If you cobb the pipe, you'll be cooling the intake air. Might not be a problem, but it could change how the pellet stove burns. That hot exhaust blasting outside is where it's at!

If the pipe exterior inside the house is quite hot, yes more length indoors (moving the stove) would help. Thermal mass, close to, but not touching the pipe, shouldn't affect the intake air temp.

Another experiment I did a few years ago; Woodstove water coil plumbed to slightly elevated 60 gallon stainless steel water tank, no pumping required. All that hot water in the tank (thermal mass) gave me a warm living room in the morning, unattainable previously. It really really works! I have removed the woodstove, replacing with rocket mass heater, that firewood thing was just way too hard on my back.

Back to the concentric exhaust pipe with preheated intake air - I think this is true for pellet stoves, gas water heaters, propane stoves (the woodstove style ones) or anything where the exhaust may go out through the wall, excepting rocket mass heaters

thank you Johnny, good to know!

I think it was 'hooked up wrong', but that is something that my father and I disagree on. I think it is possible to have a 'direct vent' option/feature, but for some reason his wasn't installed this way.

Hi Roger,
The idea of the concentric intake outgo stove pipe fascinates me. Seems like you could cob around it without cooling the ingoing air that much. I am imagining it this way: all the walls are metal, conducting heat through the metal walls from hot to cold, if you remove heat from the outer air, that creates a greater difference between the inner air and the outer air. The greater the difference, the faster the transfer

Another consideration is that the exhaust and intake are moving opposite directions. The closer the air gets to the stove, the hotter the exhaust is, and the hotter the incoming air is, having gathered heat the whole length of the journey. Seems like this stove pipe design is already an attempt to harvest heat on its way out.

There are many animals who have a similar system. Think of ducks and geese standing on ice. The heat is collected from the blood coming from the warm body by the cold blood coming back out of the feet and headed back into the warm feather covered bird body. It's called a rete mirabile, and here is what wiki pedia says to explain it more succinctly than I could at first try "The rete mirabile utilizes countercurrent blood flow within the net (blood flowing in opposite directions). It exchanges heat, ions, or gases between vessel walls so that the two bloodstreams within the rete maintain a gradient with respect to temperature, or concentration of gases or solutes."

Me again: in the rete mirabile, the vessels are not one inside the other, but are in close proximity. It is the counter current flow that is the same, and I think the stove pipe would also maintain the gradient, in this case hotter near the stove and colder farther away.

I would not be surprised if the RM did not inspire the design of the stove pipe.

I'm so glad you mentioned it.

Anyway this is all conjecture. I have not, as Eric W says, "tried it in my back yard".

And I guess it would be a good idea to test the temperature of the outside of the stove pipe. If it's cool, then lengthening it indoors, putting it incontact with mass to heat is going to accomplish nada.

Now, I wonder, Roger, what it means when you say you plumbed in a woodstove water coil to the elevated SStank . I understand the tank. I don't understand where you put the coil. Seems the place we want to remove heat is the exhaust, so could you put the coil in the exhaust lumen of the concentric counter current stove pipe? Lots of layers of metal to seal up again. I like the concept and think it would work, if I could figure the placement of the tank, but I think it's beyond my skill level.

If you don't have time to explain, if it is technical, it's ok to tell me that, or if there is a place where I can learn all about it, I don't mind being referred to that place.

Thanks
T

Wow, I love these forums, way more fun to use the brain, than say, watching pro sports on TV! The trouble with putting a coil in the pipe is it would create an obstruction, reducing the air flow. The dual direction blood flow in water foul is amazing, I never knew.

My certified woodstove has a horseshoe shaped coil in the fire box, close to the top of the fire box, just below the flue. It is 3/4 inch pipe, maybe copper, not sure, both ends plumbed thru the side of the stove. I got it second hand, so unsure if it had been altered. I read somewhere that certified stoves are not allowed to have a coil in the fire box, as it does cool the combustion, making it less efficient, and adding to air pollution.

I used to commercial salmon fish, on various 58' boats, 6 cylinder diesel engine, nice hot engine room. Our hot water was heated by a copper coil wrapped around the exhaust stack above the engine. The stack was actually wrapped with insulation first, not sure why, but then the home made copper coil was twisted around that. Steaming hot water for the galley sink, worked great. There was also an expansion tank, like your electric hot water heater might have. I knew a guy with a woodstove in a step van, told him about this, and he fashioned a working shower in his step van, wrapping the coil around his stove pipe. Cold water in at the bottom, hot out at the top.

I've not been able to figure a good place to do this on a rocket mass heater. A bath tub in the cob will crack it's finish if empty. Don't want to cool the fire box. Might wrap the barrel, as the exhaust is probably too cool, but maybe the exhaust could work.

Anyway, no, not too technical. The best illustration is the old days wood-fired kitchen cook stove, with the coil in it, and a water tank close by. As long as the hot coil water molecules can travel upwards to the tank, it will work. They had no electricity back then!

Here's another example. If a hot water tank is in your basement, you can have instant hot water (no waiting) at all your taps, as long as EVERY horizontal pipe run goes at an upward grade. The hot molecules want to move upwards, and will, if they are allowed to.

Yes, the concentric pipe is an attempt to harvest heat on it's way out. I still think your easiest harvest point is that hot air blast exhausting outside.

You can put your copper heating coil anywhere. I did one in the middle of my yard, building a little fire in the middle of 8" diameter coil, fed by garden hose, closed valve on the hot top side of the coil. Wait for it to heat up, then open the valve, worked great!

I read that water is actually a better thermal battery than cobb. I could maybe draw you a sketch, but you could probably google thermo-siphon. As long as your coil is downhill from your tank, it'll work. Your coil belongs outside, in that hot blast of waste heat!

Hi Thekla, I got to thinking, I remember when I first saw the concentric exhaust pipe. I was putting siding on a new building, and read the chrome plate on the outside of the wall. It said caution, hot-and also, do not block, air intake. I was fascinated by that for a really long time, the dual purpose of encircling the exhaust for fire safety, and heating the intake, to make more heat indoors, to make the exhaust hotter again, and on and on. Just hard to get out of your head for a while. "Round - Like a circle ever spinning, in the windmills of my mind" (words from a song from the 60's) Just saying, don't worry, eventually you'll get over it, and life will get back to normal

Roger, I'll tell you one thing your post about the design of the counter current stove pipe has prevented me trying: I had thought I could just extend the tube (maybe bury it outdoors along the edge of the concrete slab that IS the living room floor just 6 inches away from the frigid out side air. Not knowing the intake was there, it's likely I would have tried what ever metal tubing I could find, the cheapest no doubt, a single wall pipe the diameter of the exhaust pipe... I would have really messed things up, because the intake would be coming from the exhaust. What a mess that would have been.

Thanks very much.

Thekla

Thekla McDaniels : You need to go the the top upper right of this page To the Permies Toolbox, find and click on the ''My Profile'' button, and select preferences ,
this will allow you to select auto notification ! While you are there you may want to post a general location, and climate information, this will help with future
questions, and help you find other fellow Permies Members in your Area ! For The Crafts ! Big AL !

Hi Allen, I am sure it is right where you said, but I can't find a tool box anywhere. top of the page on the right I find :

permies
Search | Permaculture Wiki | Recent Topics | Flagged Topics | Hot Topics | Leaderboard | Logout
My Profile | My Purple Mooseages | My Bookmarks and Watches | My Posts

after you tell me where to look, it's going to be there in flashing NEON CAPS, I know, but for now it eludes me. I'd love to find it, and thanks for trying. Maybe you could help me a little more, and send a link to the page where it should show up? I even tried searching for toolbox, but got a mile long list of posts containing the word.

Thanks
T

Hi Thekla, I think your idea could work. If you're careful not to block the cold air intake, your idea of extending the hot outdoor exhaust, down into the slab could work fine.

You're thinking of scabbing cement (with your newly extended hot exhaust pipe in it) onto the outer edge of your concrete slab floor?

Normally getting hot exhaust to flow downhill won't work, but like you said, you have fan forced exhaust. Get some second opinions for sure, I've never tried it.

And make certain you can't ever be breathing the fumes, of course.

Modern slabs are likely on styrofoam insulation. My house is from the 1950's so the slab is right on the earth, makes it damp.

Here's another idea, if you could SPLIT APART the hot and cold pipes, like somehow peel open the outer concentric layer. Then run the hot thru a cobb bench and out the wall like a rocket mass heater. The cold air intake could pull from indoor air, if your house isn't overly "tight" - Maybe that's what you were thinking, originally?

I made a air to air exchanger for my Whitfield pellet stove by modifying a tube type exchanger i got from eBay from a 98% efficient gas furnace.
I had to add a exhaust fan to the outlet to eliminate any back pressure but otherwise it has worked flawlessly , on low setting ( 7/8 bag of pellets every 24 hrs) the exhaust temps at the outlet are 80 deg F or less. on the med setting that i run the most (1 2/3 bag of pellets every 24 hrs) the Exhaust temps rise to around 98 deg F .
It greatly increased the efficiency of the stove, given the outlet temps at the stove itself were 280 deg F

Pellet stoves have a lot of ash that is blown out the flue(mine anyway), i have to blow out the exchanger once a year, i think that's the main reason for direct venting them with the shortest run possible.
Even with the heat they waste they are still more efficient than the average wood stove and they burn so clean that creosote build up is a non issue.
with the addition of the exchanger i added 50 watts to the stoves overall power consumption which was worth it i think for the extra heat i gained ( 2 Fans, 1 surplus center exhaust fan for a pellet stove added to the outlet of the exchanger and the box fan to circulate air over the tubes)
I tried to run it without the secondary exhaust flue fan but had just enough back pressure to cause the pellet stove to shut down.
my stove does not require a double walled pipe that allows the cold air to come in from the outside, it was made to pull the burn air from inside (not a problem in my building with its leaky door and windows)




hope that helps anyone looking to try something similar.

Hi Thekla, the toolbox is that part you pasted into your post. Just click where it says MY PROFILE to change your settings, up at the top of this page.

Once again, I got to thinking... Pellet stoves have that (strong) fan forced exhaust. Air out = Air in. What goes out, must come in. If it couldn't pull outside air, it could deplete your oxygen for breathing.

If you had about a foot of concentric pipe, then peeled open the intake layer, and create your own ductwork just for the intake, so it can still pull in outside air. It would still pre-heat, for that 1 foot.

And your hot exhaust would be isolated, ready for sending thru a cobb bench, then out the wall, in the style of a rocket mass heater. I think this was your original idea, other than the concentric issue

Wow, Randy, that's great you recaptured a lot of waste heat!

In the rocket mass heater (thermal battery) cobb bench, they recommend a minimum amount of turns for the exhaust pipe. Maybe ash could just be vacuumed out when the stove is off?

Thanks for posting, especially the 4 pictures!

Randy, wow, it works! Good for you and thanks for posting the photos and describing your retrofit. I'm inspired to keep up my puzzling. Looks like your set up is in a garage or shop.... my pellet stove is in my living room, an extra challenge.

Roger, I guess I could peel the the intake away from the outgo without too much trouble.... And thanks for the warning on the O2 situation. I am one of those people who does not have an airtight house, but it is important to repeat that over and over again. It is a mistake that could kill me, or anyone else who did not know, and DID live in an air tight house.

And about the modern slab-- despite construction just 6 years ago, mine is not on styrofoam, thanks to the contractor who actually refused to insulate the slab, said he did not know how. I, in my naivety thought my problem was more likely to be overheating than underheating, and so chose other battles, hence the uninsulated and cold slab conducting heat out of the building, and the energy star windows that allow air exchange with the outside around the frame that holds the panes in the sash. I wanted non low e glass, for my passive heat, and the local code does not allow that. I still shake my head in disbelief at what our nation of formerly practical people has become.

I think we here at the permies forums are supporting one another in pioneering new options , and will provide the proof of many new approaches and strategies. Go us!

I did find the toolbox referred to above. My settings already were at "notifications on", so there was a little bug in the system, but it is working fine now, and I did add a signature. I always wondered how to do that. Maybe next I'll figure out the photo posting thing.

Anyway, I'm glad this thread is active, with great info. I think next winter I'll have captured the exhaust heat one way or another.

Thekla, yes it is in a shop were i have plenty of space to work with, it is a lot different inside a home were space can be at a premium and it needs to look good
I was thinking that the running the flue through a RMH heaters barrel might look a little nicer inside a home or perhaps like some one else suggested capturing the heat with a water based system may be built in a more compact fashion as plumbing is easier to rout and hide verses air ducting /flue pipes.
What ever style you try just keep in mind that the system needs to allow the exhaust gasses to flow freely so the stove wont shut down if it detects to much pressure in the flue. i had to add the second fan to make mine work properly.
Have a great day ʘ‿ʘ

Recognizing that the fan was drawing air (and heat) to the left as I faced the stove (Englander insert), and the right side of the stove was only warm, I put a 5" elbow over the burn cup to duct the air and flames to the right. Now, both sides get hot ... grabbing some of the heat that was going up the chimney. Not sure how much I accomplished.

You may want to consider a bell. Here as a half-barrel bell in a long bench configuration: http://donkey32.proboards.com/thread/609/heated-seating-nyc-restaurant

Hi Andrew,
I looked at the donkey thread, but I could not figure out what you mean my a bell. I'm guessing they used barrels inside the tile top bench, but how does it work? Are the barrels instead of stove pipe, making a large chamber with lots of surface area for heat exchange into the mass? And then, when the exhaust is cooled way off, is there an exit lower in the system for the cooler exhaust? Is that why they mention a dead end?

I can imagine the convection keeping the heat high and the coolest "air" low, and an exit low, but 2 problems for me: where does the cooled exhaust go? If is is not warm anymore, will it rise up the pipe, or is it forced out by the incoming warm exhaust, and would one split off the incoming air from the outer layer of the stove pipe, and just run the exhaust from the pellet stove into this half barrel chamber?

It's an intriguing idea....?

And Ed, I guess this is my day of not understanding, because about the 5 inch elbow, is it just inside the fire box, directing some of the hot air towards the cooler part of the stove?

I definitely notice an air current inside my pellet stove, evidenced by a build up of ash in one corner of the stove.

Thanks for the ideas.

Thekla

Thekla, you are correct. I just put a 5" stovepipe elbow on the cup that the pellets fell into and burned, ducting the heat to the right side of the stove. It did not interfere with the pellets falling into the cup and burning on the design of my stove.

I do not know if I could successfully explain what a bell is, but you could look around at donkey's. I think you already have a good idea.

As I understand it, the insulated riser of the RMH and its derivatives does a lot to pump the flue gasses through the system. The fan in your pellet stove would probably work well enough. You still need to be careful that the gasses do not cool too much, especially if you have a tall chimney.

In the bell, heat rises and pools at the top, while cooled gasses continue unimpeded to the exhaust, whose opening is positioned as low as possible. The bench bell is a little different than most applications because it is not very tall, but it still seems to work fine. Matthew explains in another thread that he adjusts the height of the exhaust opening to fine-tune the system. His earlier half-barrel bench designs had the exhaust near the stove, but for the long benches he put the exhaust at the end of the bench.

Other bells have been made from stacked barrels or made with masonry. I used the bench bell reference because you had mentioned a bench.

My pellet stove was throwing a lot of heat overboard!  I would bug me to come home and watch hot gasses rippling out of the chimney.  So I made my own heat exchanger out of 3 inch single wall vent pipe.  The pipes are folded side to side behind the pellet stove.  12 feet of straight tubing plus another 5 feet or so for elbows, etc.  I joined and sealed all connections with hi temp rtv.  The parts nearest the stove are steel, about 8 feet, then switches to aluminum single wall for the remaining portions.  I found that they don't radiate well when all shiny so I painted them flat black, with a silicone based high temp spray paint. All tubes come out annually for cleaning the ashes out.  You can feel a lot of heat radiating from the tubing, and there is a big temperature drop in the tubing surface from the stove end of the tubes to the colder blower end.
I added a couple computer type fans blowing room air onto the hot tubes,  controlled by the thermal switch on the exhaust port so the fans are on whenever the exhaust is hot.
My stove has an air pressure switch that will shut down the stove if the burn chamber is not held in vacuum by the exhaust blower.  I removed the exhaust blower from the stove and placed it after all the heat exchange tubing so it still draws the burn chamber down into vacuum, but it also draws down the heat exchange tubing so never an exhaust leak!

This cut my fuel consumption by nearly half!!  Now the stove is set on the lowest low-low setting and that is too much unless it is in low 40s outside or colder.  If it is high 40's or 50s outside the stove cycles on/off by the wall mounted thermostat.  If it is in the 20's outside It cycles up/down from 1 to 2.
Only rarely, like after a power failure, I'll run it at 3 but no matter how cold it is outside it only takes a half hour to take a cold house to 72 degrees. Never run it at 4 cause it's too hot and will damage the burn pot.

I think it would be worth adding another 4 aluminum sections 2ft each between the cold end and the blower.  That would make a total of about 20 to 23 feet overall of the 3 inch tubing.  So now you know how much tubing it takes to capture all that waste heat!

The biggest factors for thermal energy exchange are temperature differential and time.  So I have tried to slow the passage of the exhaust gasses by limiting incoming combustion air, to give it more dwelling time inside the tubing.  Also, having the least fresh air leaking into the burn chamber or exchanger tubes to maximize exhaust gas temperature.. I wanted only enough air to support clean combustion, but when I slow the air that much the fire pot holes get blocked every other day, so have to clean more often, but on the good side less ash is carried into the exchanger tubes with the slower incoming airspeed.  So I get the most heat by reducing incoming air leaks and slowing combustion air to minimum for clean combustion.  Another thing I tried was to block 1/2 of the holes in the burn pot and reduce the burn pot volume to 1/2 by fabricating a stainless steel insert  that fits inside the burn pot.  Now with half the holes blocked the air speed in the remaining half is doubled which keeps that half of the pot cleaner longer, cleaner hotter yet smaller fire. The firepot insert must be a snug airtight fit, but it must be removed if the stove is run above 2.

Well, that's about as far as I have taken it...

Hi. I have 2 pellet stoves and both require an external air intake. I've never seen dual walled 3" pipe used as the air intake. I'd be wary of that information. If you inspect where the exhaust connects to the back of your pellet stove you'll see the difficulty in implementing that in the design. It would take some pretty ingenious engineering to make preheating the incoming air in the small space between pipes a reality. Besides,there are obstructions and fireproof packing where each pipe section twists together that prevent such airflow.
 To capture more of the heat you can run as much pipe as possible inside the house before it exits. You lose some floorspace, but... Be aware that most pellet stove manufacturers dictate the length of a horizontal pipe used, and the number of permissible bends, as the exhaust is powered and the fans can only push so much.
  You could look to simpler means to harvest some of that lost heat. I might try encasing the 5' of my interior horizontal vent pipe with 6" pipe, connected to a small fan at one end. I've also turned a 20" box fan into an air filter that I put on top of the hopper, drawing air from direction of the vent , on my Comfortbilt HP22 N pellet stove.

Maybe a useful product for this topic https://youtu.be/zlEMADUz0Ak