Wood burning house heater

Dabbling in alternative heating technology, [Rob Steves] built a wood stove to dispose of his scrap wood while negating his home’s fire insurance at the same time. As the leftover bits from his wood projects started to stack up he wondered how he would dispose of them. Burning the bits for heat means he’s using every last bit of the lumber. The internal tank from an electric water heater was repurposed as a combustion chamber, with exhaust gases escaping through some high-temperature flexible tubing. The glass panes were removed from one of the fireplace doors to give the off-gases a place to go. The result is a rocket stove that burns very hot and does a great job of warming his house.

It’s not the safest way to heat a home, and there may be coding issues with your municipality. But this might go well in a remote location, like that cabin where you have to generate your own electricity.

[Thanks HybridBlue]

Comments

  1. Bergo says:

    haha

    any everyone was complaining the laser hacks were unsafe…

  2. osgeld says:

    love the cardboard … hope the exhaust doesn’t get too hot

  3. NatureTM says:

    Not the type of “coding issues” we normally see around here…

  4. mess_maker says:

    Wow, I misread the title as: “House burning wood heater”

  5. Adrian says:

    Had to doublecheck my RSS reader whether I was on the There I Fixed It blog.

  6. Will says:

    That can not possibly be up to code. Single-wall flex vent, angling down? I sincerely hope his smoke detectors are working. And that he has an escape plan.

  7. fartface says:

    Yup because burning the wood in that FIREPLACE is not an appropriate place to have a fire….

    I get a kick out of people’s projects to replicate the function of another piece of equipment they have already.

    Dave! give me those parts off that car there…. Let’s build a CAR!

  8. Ben says:

    This isn’t as bad as it looks. The whole point of a rocket stove is that the draft is created in the central chimney, not in the flex tubing. By the time the exhaust gets to the flex tubing, it should be have given up most of its heat energy through the water heater shell and thus be relatively cool. Since the draft is created and maintained by the insulated central chimney, the exhaust tube can be oriented level or slightly downward and still work.

  9. Adam says:

    @fartface – except that’s not what this is. Fireplaces are horribly inefficient, sending most of the heat up the chimney, as well as pulling cold air into your house. This would, at least, take care of part 1 of that equation.
    So your burn is like, totally burned

  10. Queeg says:

    I see dead people.

  11. xorpunk says:

    cant wait for the electricity hack

  12. Bhima says:

    Most of these comments are pretty pathetic. So far only two commenters seem to understand what a rocket stove is and I wonder if anyone else even bothered to read the article. If you aren’t going to read the article why comment?

    Having said that, I suspect that the exhaust tubing is slightly too long and the exhaust gases give up too much heat energy before entering the chimney, so they do not create a strong enough updraft. Naturally that would require a different material to replace the glass instead of cardboard (which is poor choice despite the temperatures involved). Alternately it could be that his house is too airtight and that providing a less restricted source of outside air for combustion would help. Then of course making a longer exhaust tube would be beneficial.

  13. medix says:

    @Bhima: You forget one critical point: Looks and first impressions are everything. Yes, it looks clean, and is (possibly) safe, but it still appears shady as hell.

    If it works and the house doesn’t burn down, great.

  14. bilbao bob says:

    In a place where electricity is expensive [nuke+coal+shifty politics] and wood is scarce from the greece effect [too many people, too little land], I have a friend who runs a not-quite legal wood furnace. It started off as an experiment in building a pizza oven in his rural[*] town. He makes pizza for the neighbors from time to time, and bread once in a while.

    It’s very high tech, and when it gets very cold in his concrete house, he fires it up and uses it to circulate heated air in his house with a fan.

    He designed the heat exchanger using cad and it’s pretty cool. CO detectors, thermocouples, and a setup in the heat exchanger where he can pull out the built-up creosote somehow. I guess it’s pretty nasty stuff.

  15. Luke says:

    You could also look into building a masonry heater, same basic concepts, fast, hot burning, efficient heat usage, they use stone or ceramic to store and radiate heat. They are generally enormous, but smaller single-room heaters are often used.

  16. ejonesss says:

    if you are up to sneakily committing insurance fraud you can rig the stove such that you can remove it when the house is on fire.

    so then when the fire marshal and insurance adjusters come down to investigate they find no evidence.

  17. Hugh says:

    What the hell he has money for a house but not for heating? Typical american idiot.

    I hope he at least plant new trees on the place of those what he cutting out.

    I would so laugh on him if his house would burn down from this :D
    Stupid shit would sell his house and move into a flat nearby he would have enough money for years to pay the bills.

    Not to mention you have to keep putting the woods into this every day. I like to spend my day with making breakfast, showering, watching tv when I’m at home, not putting fucking woods into the heater and worrying about then it might burns the house down lol.

  18. Setatx says:

    @Hugh – Not trying to flame, but it’s not about having the money to pay for electic/gas heaters, it’s about not wanting to give $$ to the oil company’s robbing all of us blind here in america. The same reason people are looking for alternate sources of fuels.

    Indeed he does have enough money for a house, which means he understands how to use his money, which in turn means he knows where HE wants to put his money, not where you want him too.

    At the original topic, I like it! I agree the cardboard needs to maybe be replaced with something a little more dependable but it’s a unique mod and combination of some existing technologies.

    Remember people, it’s not about if it’s worth it for us to it, it’s about being ABLE too =)

  19. ICanRead says:

    I agree with Bhima. Most commenters clearly don’t know what they’re looking at and didn’t read the article. I did so let me summarize: A rocket stove is not like an ordinary wood stove. If it looks “shady” it’s because you’re thinking of regular wood stoves that are just one big combustion chamber. Most of the device shown is actually the heat exchanger. It’s big so it can transfer lots of heat at relatively low (ie safe) temperatures. The small square tube on the front is the combustion chamber and it only burns a handful of wood at a time. That’s the only part that gets hot enough to be a fire hazard and it’s oriented away from anything flammable. If the exhaust temperature is only 60 degrees C as claimed, there’s not much of a danger there. Based on the author’s comments (no visible smoke and no smell) it seems to generate enough draft. I’d take the assessment of the builder/user, rather than speculate after only looking at a picture.

    @Hugh: Apparently you didn’t even read the first sentence in the article on THIS page. He built it to efficiently burn scrap wood. What would you do instead… take the wood to the landfill, or burn it outside, wasting all the energy?

  20. bilbao bob says:

    @Hugh – haters gonna hate.

    I’ve lived in the US, UK, China and France, and apart from the ever-increasing tendency towards becoming a bankrupt banana republic run by a handful of cronies, it’s an OK place. also, we have lots of stupid people here, but we really need stupid people around to keep the folks who want to run everything busy.

    Had you been less interested in bashing, you might have noticed that he has wood scraps – little bits of wood – left over from other things. Rather than throw them away, he is burning them to save money.

    Not everyone is ready or willing to accept the city hamster habitat lifestyle you’ve adopted, but since you are share the same attitudes you assign to others, you cannot understand this.

    I live in a city and prefer the urban life, but now that night clubbing and passive entertainment have lost their charm, I sometimes dream of living in the back country again before I get old. I don’t no how to explain the non-economic benefits of that lifestyle.

    As for risk – he’s 1000 times more likely to die of Carbon Monoxide poisoning than from his house burning down. Fire isn’t that hard to control.

    I’m guessing you’re down on guns, and on people raising their own food as well?

    PS – I just realized that no one has mentioned pot bellied stoves.

  21. Abbott says:

    @bilbao bob:

    You’re right on the money. Pot-bellied stoves have been used for decades before central heating, and are safe if maintained.

    The same applies here. The rocket stove is simply a more advanced design, and every bit as safe _if maintained_

  22. Xb0xguru says:

    Welcome to ‘Kludge-a-day’.

    Before you hate, I read the article. I saw no mention on Carbon monoxide detectors.

    Ask yourself why this isn’t commonplace in the home?

    is it:

    a) Big bad oil companies conspire against the common man to make them use their overly-priced fuel.

    or

    b) Fuel into the home is fed into equipment which has passed numerous safety checks, is fitted by qualified engineers (at least in the UK) and can be used by the occupant without the risk of burning the place down (as the perforated sheet metal won’t stop all embers from spitting out) or die from CO poisioning.

    It really wouldn’t surprise me if this story reaches the national press for the wrong reasons.

  23. Drake says:

    I personally would have the burner outside, insulated graciously. Then I would have a largish copper coil wound within. Small electric pump + old car radiator = safer heater.

    Then again some of my engineering is on the redneck-ish side

  24. DeadlyDad says:

    :sigh: Every time I read clueless comments like a lot of these here, I find myself wishing for an incoming asteroid. Seriously, people, is it really so hard to think for yourselves, and spend just 5 minutes to check some facts before opening your mouth and removing all doubt? Hell, I came back up for air after two hours, utterly fascinated by the possibilities. For example, one design made with not much more than a couple of coffee cans and perlite for insulation can boil a litre of water with a single 8″ piece of 1×6. A larger scale model is still only a weekend project. If you nay-sayers had bothered to read the article (with the background of a little bit of research), you’d know that he wasn’t exaggerating about the exhaust line only reaching 60C, and that he expects the scraps in the tub to last an entire week. Plus, because the whole point is to completely consume the wood, there is no creosote buildup to worry about, let alone carbon monoxide or smoke; it emits mostly carbon dioxide and moist air. As for why it isn’t commonplace, how about the fact that it was only invented 18 years ago? Some of you people, I swear… not with both hands and a map.

  25. Osgeld says:

    60C (140F) is still kind of hot, not enough to ignite paper but still hot enough to melt the glue in the cardboard possibly causing some ignition issue (in the absolute worst case over a long period of time with a freak chance of a red hot ember bouncing around)

    cheese n crackers, get some drywall or concrete board scraps, if nothing else the cardboard is a turd on a otherwise ok project (if you want to forgo insurance on your house …)

    heh maybe next he can use some crap yellow walmart 5 min epoxy to link the steering column on his homebrew funnycar

  26. macona says:

    Its not a bad design. Putting the combustor outside would be silly with such a small firebox. You would loose more heat out of the house going inside and out that anything.

    Three things stand out to me. It did not appear to be mounted down in any way. I would worry someone could knock it of the ledge.

    Most code requires 18″ of non flammable material around the unit. Like concrete, steel, or tile.

    And as mentioned install a CO detector in the house. With this system you could easily get a inversion and push the fumes back into the house.

  27. RobSteves says:

    Hi all. I’m the builder of this stove and I’ve read all your comments with interest. I’m glad some of you extended me the same courtesy (ie by reading my article before commenting).

    To address some of your concerns:

    Will I die of carbon monoxide poisoning? Unlikely. The stove cannot operate for more than about 30 minutes unattended. That’s how long it takes to consume all the wood that fits in the combustion chamber. Stop feeding the fire and it goes out… by design. If you don’t enjoy feeding a fire, perhaps this stove is not for you. CO is odorless and colorless; it’s virtually undetectable on its own. However in a wood stove CO is invariably accompanied by smoke, which is neither odorless nor colorless. You can smell its presence at levels much lower than are visible. This stove is no more likely to leak CO than an ordinary fireplace or commercial wood stove, but if it did you would be more likely to notice the smoke immediately since the stove requires tending so frequently. If you weren’t tending the stove, the worst case scenario is you might smoke up your house a bit before the fire extinguished itself. Assuming you have smoke detectors (I do), they would go off long before your carbon monoxide detector. If the exhaust gets blocked, or there is an inversion, the fire will go out immediately since there will be no draft and thus no oxygen supply. That’s one of the benefits of a down-draft design. Basically… it’s safer than an ordinary open fireplace which will continue to burn quite happily even if the chimney is plugged or the flue happens to be closed.

    Will I burn my house down? Unlikely. For reasons given by ICanRead the surface temperature of the body is low and poses little risk of igniting anything. The parts that do get hot are not near anything flammable. Sparks are a greater threat, but having tended many fires in ordinary fireplaces and commercial wood stoves I can say without a doubt that they pose a much greater threat than this stove. To do any harm, a spark must jump about 12″ straight up through a maze of unburned scrap wood (only the bottom of the pile burns)… and then get through the screen. It just doesn’t happen.

    Will the stove fall over? Sure… if you push it hard enough. So will a bookcase or a refrigerator. The stove is quite heavy so it would take a concerted (ie intentional) effort to push it over.

    Does it take a qualified engineer to build and install a wood stove? If it makes anyone happier, I do have two engineering degrees, a B.A.Sc in Mechanical Engineering and M.Eng in Electromechanical Design. Thomas Edison didn’t have any and he got by pretty well. I’m not comparing myself to him… just making a point. People need to get out of the mindset that “only an expert can deal with a problem” and start manipulating their own world to suit themselves. By doing so, they will become experts (the ones who survive, anyway). The less reliably you can predict an outcome, the more possible outcomes you need to be prepared for. Know the dangers. Assess the risks. Exercise an appropriate level of caution. And most of all, be ready to accept the consequences of your actions if you choose not to build to code (if there is one). Follow those guidelines and little is outside the realm of the backyard tinkerer.

    Best comment award goes to Osgeld… “if nothing else the cardboard is a turd on an otherwise ok project”. Agreed… it was a makeshift solution that has to go, not because it poses a risk but because it’s a focal point for people who don’t know what 60 degrees C is (a typical cup of coffee is 75 degrees C and combined with the morning newspaper poses a greater fire hazard than the exhaust pipe on this stove).

  28. Malikaii says:

    It looks to be sitting very precariously on the edge of that ledge. Hope he doesn’t have kids.

  29. Diago says:

    @Xb0xguru. Gas fitters are NOT engineers. In the same way that nurses are not doctors, paralegals are not barristers.

    It’s obvious that both of your reasons contribute to why this stuff isn’t commonplace yet. That the concept is only 18 years old and the majority of consumers opt for convenience (gas or electricity) over price and self sufficiency.

    My uncle is building a rocket stove type barbecue which is giving some good initial results, and I’m sure the principle has many great uses. Anyone know of any commercial uses?

  30. Laurenceb says:

    omg fire its…. DANGEROUYS WERE ALL GOING TO DIE OMG OMG !!!!1111

    seriously hackaday calm the fuck down. If you care to read the article it warns you about the possible risks – anyone over the age of 5 should be able to appreciate that this is perfectly safe if used appropriately.

  31. pod says:

    Wood burning house, heater

  32. fartface says:

    @adam, he is using an open flue fireplace for exhaust.. Losing 90% of the heat he is creating anyways.

    I fully understand what was built, and it’s implementation is retarded because all the gains are lost by opening a giant chimney in the home to act as a heat siphon.

    His efficiency is not much higher than a regular fireplace that was designed right.

  33. Whatnot says:

    Those flexible aluminium tubes are not meant to be used for this, they have a limited ability to handle heat.
    And he should not only have smokedetectors but carbon-monoxide ones too.

  34. ben says:

    How do you get the initial updraft started? I understand that once you have heat flow, you have a constant draft pulling fresh air down through the combustion chamber and then up the inside chimney, but how do you get that cycle started in the first place?

  35. Paul says:

    Very cool, never knew about rocket Stoves before. Took me a while to fully get the ideas involved, a search for images/diagrams of “rocket stove” helped me get it.

    The preheating of the intake air with the exhaust to maintain high fuel burn efficiency, is very similar to what they do in power plants: preheat the intake water, with the lower pressure exhaust steam.

  36. bilbao bob says:

    @Whatnot – The tube is aluminum, and even if it were made of aluminum foil, it can easily get to 475F without deforming. It just looks tacky.

    @fartface – actually, it’s a substantial improvement, even if that doesn’t mean much. The common home fireplaces in north america are really, really inefficient, and even the best ones suck out a substantial portion of the heat they generate. The only way to get useful efficiency is with a radiator/blower mechanism, which never happens.

    @RobSteves – Absolutely correct. However, the desire/ability to actively manipulate and shape the world around you has been bred out of many societies, especially in former colonial powers.
    Many people raised as city-mouse consumers cannot distinguish between “research/design/prototype/fabrication” and “Here, hold my beer for a minute.” activities.

    My philosophy (suitable for everyone) is that I answer only to God, Darwin, and the golden rule. This is a little radical for France, but as Hank Williams Jr. sang, a country boy can survive.

    Having said that,
    >CO is invariably accompanied by smoke
    is a demonstrably false statement. However, given the speed with which air is sucked out by the design, I’m guessing it isn’t a problem unless you have long (24+ hr) temp inversions and your house gets covered in snow.

  37. Velli says:

    My house came with a fireplace insert with a monster heat exchanger that wraps around the entire bottom, back, and top of the thing (all inside the fireplace, so it looks clean). It has an electric fan, and if I turn it all the way up, it sucks so much heat out of the firebox that the FIRE GOES OUT.

    That design could be copied and make a very nice, attractive, “safe” hack. This hack is skeery.

    Also, WHERE IS TEH ARDUINO?! NOT A HACK!

    (Thanks! Keep throwing nods to the hackers out here that don’t run on batteries!)

  38. Will says:

    Ok, having spent a fair amount of time looking at rocket stoves and what not, I will partially retract my earlier comment. It does not look like he’s going to go and burn down the house, and the design of the stove effectively controls the CO risk.

    I’m still leery of the venting, though. More effectively coupling the vent to the chimney flue is a must. I’ve seen some kits for gas fireplaces that run two flex vents (one for exhaust, one for combustion air) down a standard flue, which would seem ideal.

  39. DeadlyDad says:

    Hmmm… Good point, Velli. An Arduino could be used to monitor the combustion chamber and the room, and maintain a set temperature in each by automatically feeding wood pieces in from a ‘magazine’.

  40. bilbao bob says:

    @DeadlyDad

    My god, Man! You’ve rediscovered the lost pellet stove technology of the PNW tribe! Of course, in those days, they did it with a funky chip called the Z80, but that was almost 3 decades ago.

    @will
    We eagerly await your build.

  41. DeadlyDad says:

    @bilbao bob: Heh. Ironically enough, tomorrow I’m going to be talking to the owner of a local business that sells pellet stoves. They’ve taken a real pounding because their clients are feeling pressure from the gov’t to get rid of their wood stoves, and a pellet-fed rocket stove could possibly save their business.

  42. Tachikoma says:

    This place is full of safety experts…

  43. RobSteves says:

    @fartface
    Assuming complete combustion, efficiency is primarily a function of exhaust temperature. Every kg of wood burned produces about 4.5 kWh of heat energy (incidentally, my stove burns about 2 kg of wood per hour so it has about 9 kW of heat output). Some heat goes into the exhaust, some goes into the room. The amount in the exhaust can be calculated from the exhaust temperature. Complete combustion requires about 6 kg of air per kg of wood. The exhaust is still mostly air (or rather nitrogen). Raising the temperature of 6kg of air to 60 degrees C requires around 0.1 kWh of energy. Thus out of 4.5 kWh of energy produced, 4.4 goes into the room and 0.1 goes out with the exhaust. That’s about 98% efficiency. I’m not suggesting I actually achieve that. Truly complete combustion is not achievable even in a rocket stove, but a rocket stove IS much more efficient that typical fireplaces or wood stoves that achieve less complete combustion, have much higher exhaust temperatures (200-300 degrees C) and draw much more air through them than what is needed for combustion (perhaps 4 to 5 times more). Do the calculation above with 4x the air flow and 4x the exhaust temperature rise and you get 1.6kwh lost to the exhaust and only about 65% efficiency which is probably typical of a common wood stove (and still much better than a common fireplace).

    @Whatnot
    The flexible aluminum tubing is designed for exactly these temperatures. It is dryer ducting for clothes dryers. Have you ever measured the temperature of the exhaust exiting your clothes dryer? I have. It’s about 70 degrees C. I guess it’s a fire hazard too. LOL

    @bilbao bob,
    I do use a CO detector. I just don’t believe it’s necessary. I have it only because I used to have a natural gas hot water tank which is a much greater CO risk in my estimation since the fuel supply is essentially infinite, operation is unattended, and it runs even when I’m asleep.

  44. RobSteves says:

    @ben
    I light the stove with a propane torch to get down to the bottom of the combustion chamber. Pointing the torch into the stove for a few seconds is all that’s needed to start the draft. Usually there is already a bit of draft anyway. If there is even a slight breeze outside, the venturi effect causes a lower pressure in the fireplace than in the house.

  45. Malikaii says:

    Very professional responses sir.

  46. Eikka says:

    @RobSteves

    One of the points you forgot is, that wood contains water and hydrogen, which will eventually turn into water and steam as the wood is burned. More energy is lost in evaporating that water than is lost in heating the air.

  47. RobSteves says:

    @Eikka
    I didn’t forget it, but didn’t mention it because water vaporization losses are not as significant as you think and because they will be exactly the same whether you burn the wood in a fireplace, wood stove, or rocket stove. The 4.5 kWh per kg I quoted already assumes wood with a 20% moisture content (typical of air dried wood) and is just a ballpark figure (it varies a little depending on the species). 1kg of wood at 20% moisture contains about 0.17 kg of water. The energy required to raise 0.17kg of water from room temperature to 100 degrees C and vaporize it is about another 0.1 kWh. Including that figure in the calculation only drops the efficiencies by a couple percent… to around 96% for the rocket stove and 63% for an ordinary wood stove (actually the wood stove will be a little lower since more energy is lost raising the 0.17kg of water vapor from 100 degrees C to the 200 or 300 degrees C exhaust temperature).

    In any case, the biggest source of error in my “back of napkin” calculation is more likely the invalid assumption of complete combustion. Unlike exhaust temperature and moisture content, completeness of combustion is not easily measured (though it will certainly be higher in a rocket stove).

    You said this was “one of the points” I forgot. Anything else?

  48. Mr Foo says:

    @RobStevens

    That’s an impressive device you’ve built there, despite a few relatively obvious failings that you’ve noticed or have had pointed out to you on the comments for your blog (and not the ones here, which seem mainly clueless). Well done.

    A few (more or less random) questions…

    You’d suggest your stove is ~9kW, with a hypothetical efficiency of ~95%. That would place it in the same ballpark as a 14kW insert with an efficiency of ~65%, right? I ask this because a 14kW insert is roughly what I’m about to need, and I have access to a lot of steel (including, oddly enough, old hot water cylinders) for nothing. Yeah, you can see where I’m going, I’m sure.

    I assume it has a pretty much on/off heating profile, would you consider that could be improved with some form of thermal mass whilst keeping the form factor similar? I’m thinking along the lines of “form a load of concrete inside the cylinder” as a potential solution.

    Have you considered running the exhaust through a heat exchanger to eke a few more watts out?

    What are you doing about the inevitable corrosive effects of the lower-temperature exhaust – a “standard” fire will generate less nitric oxide, and its exhaust should be hot enough to push the nitric acid “enhanced” exhaust out into the atmosphere instead of letting it condense on the walls of your chimney

  49. RobSteves says:

    @Mr Foo
    Great questions. 95% (ish) is the theoretical efficiency assuming complete combustion and no additional air passing through the system than necessary to support combustion. I haven’t thought of any simple way to actually measure completeness of combustion or the amount of excess air consumed, but I would guess the stove has an actual efficiency in the ballpark of 80% while an inexpensive wood stove might be around 50% (the 65% I mentioned above also assumed complete combustion).

    The heating profile is on/off as you say and could be improved by adding thermal mass. Concrete inside the cylinder is an interesting idea. It would work well for permanent installation but would add considerable weight to a portable design.

    One option I’ve considered is to use a gas hot water tank instead of electric and simply fill it up with water for thermal mass. Then the water can be drained to move it. If you do this, be sure to leave the tank open to atmospheric pressure or it could explode when heated. I would drill a hole in the top of the tank for this purpose… you could just leave one of the inlets unplugged but that’s just asking for someone to come along and plug it later. The tube through the center of the gas hot water tank would need to be capped at the top end (could be done with a removable cap for inspection). The rocket stove chimney would need to go inside this tube (usually a little over 4″ internal diameter) from the bottom. The rocket stove chimney pipe would need to be about 2.5″ in diameter. Adding 0.5″ of insulation would bring it to 3.5″ diameter, leaving a little over 0.25″ gap for the exhaust gases to go back down. This may seem small but the small gap will improve heat transfer to the tank and its about the right size for matching the area of the gap to that of the chimney. However, such a narrow chimney will not develop good draft so an exhaust fan would be required. It should be placed at the end of the exhaust pipe where temperatures are lowest and where it will develop low pressure throughout the system drawing air into any leaks instead of blowing exhaust out. I’ve had success in experiments with a 12V computer power supply fan. Despite being plastic it works fine due to the low exhaust temperature. I don’t know about lifetime, but they are pretty cheap to replace.

    I’ve found that the exhaust temperature drops about half way to ambient for every 20cm of travel through the foil exhaust pipe. The simplest way to build a heat exchanger is to make the exhaust pipe longer (though this may not be pretty). For example, doubling my exhaust pipe length from about 80cm currently to 160cm should reduce the exhaust temperature at exit from 40 degrees above ambient (60 degrees C) down to only a few degrees above ambient. However, doing so would also reduce the draft, likely requiring an exhaust fan. This is why I was experimenting with the fan, but in the end I decided to go with the simplicity of a shorter exhaust pipe.

    For me, this project is about disposing of some scrap wood as efficiently as possible. I don’t have enough wood to burn that I’m very worried about acidic condensation in the chimney. If I wanted a more permanent installation I would simply pipe the exhaust all the way outside (either up the chimney or out the side of the house through something like a dryer vent). The exhaust temperature is well below the melting point of PVC which should not corrode. A good design would allow any condensation in the system to be collected. Again, a longer exhaust pipe will likely require an exhaust fan.

    If you do go to the trouble of piping the exhaust all the way outside, it would not be much additional trouble to pipe in fresh air to the combustion chamber, making for an even more efficient system. For example, if I piped my exhaust all the way up my chimney through a PVC pipe, I could draw fresh air for combustion directly from the fireplace.

  50. bilbao bob says:

    @Rob Steves

    “my invalid assumption of complete combustion.”

    Sir, I have developed the most profound respect for you based on that paragraph. I wish you many BTUs of happiness and warmth with this project.

    I also want to suggest that using low velocity air as your circulating medium rather than water will save you endless trouble. Boilers (intentional or not) are a bitch, in general.

    Lastly, just because it’s interesting, I suggest you consider adding thermopiles to power the blower fan. Easier to get working than a stirling engine.

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