A Heat Reclaimer For Your Woodstove; The One Thing It’s Not Is Cool

It’s the middle of winter for those of us who live in the Northern Hemisphere, which naturally turns minds towards heating, or sometimes the lack of it. It’s particularly difficult for those who rely on a wood stove to escape the feeling that perhaps most of that hard-won heat may be whistling up the chimney rather than keeping them warm. It’s a problem [Lou] has addressed with his DIY chimney heat reclaimer.

As can be seen from the video below the break, his stove appears to be in a workshop, and has a long single-wall metal stove pipe. Over the outside of this he’s placed a pair of T pieces joined by a longer length of pipe all of a larger bore, and a mains-powered fan forces air through this air jacket. The result is a continuous flow of hot air that he claims delivers a 45% heat reclamation. We’re curious though whether the reduction in flue temperature might cause extra tar condensation and thus the build-up of flammable material further up the chimney. The stove itself is a double barrel affair with access for smoking, and the video describing it is worth a look in itself.

Whatever the stove, be sure to ensure a constant supply of fuel!

25 thoughts on “A Heat Reclaimer For Your Woodstove; The One Thing It’s Not Is Cool

  1. From research on this topic it seems that there are multiple possible downsides, creosote buildup being one of them. If it gets cold enough (probably won’t with the working fluid being air) and the flue gas drops below it’s acid dew point it will also essentially create acid rain in the chimney and can quickly eat away metal surfaces. Thrre is also the possibility of increased NOx emissions.
    There are a lot of papers on flue gas heat recovery, many from coal and oil perspectives, but also biomass CHP systems. There is both practical heat going up the chimney, and latent heat in the gaseous fluids that would be condensed at room temperature.
    It would be completely unpractical for a single stove, but it appears that a system with multiple technologies allows for the greatest reclamation and emissions reductions, such as electrostatic precipitators, catalytic burners, ammonia filters and a series of composite heat exchangers impervious to the acids.
    There are even fairly (relatively) small sized systems based on organic rankine cycle that can do CHP for a neighborhood or university campus.

    1. yep gotta be real carefull with this, having run my heater into a “condensing” state only to find horrible fluids condensing in the flue and combustion chamber. I now run a fuel temp of ~180c which is still low but as low as i can go without getting nasty deposits. my heater is fired by red diesel. wood without a doubt will be worse.

      1. This. My father was a fire chief in Maine (long ago), and went to countless chimney fires over the years. So called ‘air tight’ stoves seemed disproportionately represented in chimney fire calls. For the unfamiliar, these are stoves that have welded seams and the only way for air to enter is through a draft that you can usually screw down to almost nothing. This lets you turn down the burn rate so the stove can burn ‘all night’. Problem being, it also brings down the flue temperature and builds up creosote at an incredible rate. The town FD advocated everyone with that kind of setup get a chimney cleaning *every* year. It sure is tempting to try to get every last bit of heat, but the flip side is you have to be extra vigilant with anything like that.

          1. fire equals smoke. but you can use coal that glows whole night – to be refired by wood. keeping it hot does the same here as an automotive exhaust system – it “burns itself clean”. still i got an industrial broom as i clean myself extra. the chimneysweepers push it once through and think their job is done. a real cleaning takes me about half a day (ok it was 50 year old house)

          2. furthermore, inspect. take some reflective thing to put into the chimney all they way down to the lowest maintenance hole. push it in to look upwards. if you see fine fluffy stuff thats easily cleaned youre fine. if you have a glossy buildup you should act NOW. the real “pro” brooms should cost around 100€ and have ~1cm diameter on a rather stiff rod. take a size larger diameter brush. demand some beer and have fun… (its no fun work)

    2. What are your thoughts on something like this vs a catalytic wood stove? I don’t know much about them, but my aunt has one in her cabin and it seemed to me amazing at producing huge amounts of heat.

    3. You listed catalytic burners. Are you talking about catalytic combustors in the flue that reduce the combustion point of the flue gases? That’s what I thought of when I read the line about flammable materials. Does a catalytic combustor would burn up the majority of whatever is in the flue gas? I guess it’s probably a matter of temperature?

  2. Please tell me that there’s a CO alarm somewhere nearby – we still lose pilots to exhaust heaters that work like this and develop a leak. If that’s been rendered safe, there are catalytic converters for wood stoves that burn off the creosote and other potential chimney contaminants, increasing heat output.

  3. He doesn’t mention it but before doing this exhaust heat reclaimer anyone using fuel to heat a structure needs to address where the combustion air comes from. If it comes from the air inside the structure you are trying to heat FIX THAT. The air for the fuel combustion camber needs to come from outside the structure or else you are actually cooling the building and only heating the air around the furnace and the ceiling.

    WHY? Because all that air going up the chimney causes a vacuum in the structure and pulls in outside air anywhere it can find it. And outside air is very cold and stays low to the ground where us humans like to linger.

    So feed a duct into the combustion chamber from outside and you’ll be surprised how much warmer the entire structure becomes when the stove/furnace/etc is burning.

    And with the outside of the stove/furnace/etc at 400F why not take a 4×8 sheet of the corrugated sheet metal and place it 10′ off the floor directly above the stove. That thing will get nice and hot too and it will radiate heat further than the stove and in all directions. And maybe then consider blowing air over your flue pipe to remove some heat from it. Heck, just 2 fans at 90 degrees blowing on that flue pipe could do just as much as the confined system used in the video. Move the warm air around the room better too.

    1. But that’s kinda irrelevant. Either way, the heat reclaimer makes the situation better.

      And the amount of air the furnace pulls in to the space does not cool it down more than the fuels heats it up. Otherwise the furnace could not put out any heat either way. Think about it.

  4. It’s very common to build fireplaces with multiple up/down channels to trap the heat in a mass of bricks and stone and reduce the heat loss up the chimney. Somehow they don’t get creosote buildup or “acid rain” in the flue problems.This is essentially the same thing.

  5. The way to fix the creosote problem is strangely enough isolating your burning chamber and only extracting the heat from the exhaust. The hotter the burning, the better the conversion gets. Look at a propper rocket stove: they have completely isolated burning chamber and a riser pipe as afterburner and draft generator, ending in a bigger coaxial pipe downwards to extract the heat.

    So he is only half there.

  6. If using this hack as is have the intake be on the floor with a debris filter. Cold air regardless where it comes from needs first attention.
    The best plan in heating a space like that is to do the opposite of a ceiling fan. In the winter having any breeze on you up or down from a ceiling fan is chilling. Put a window fan or furnace blower on the floor aimed up into that hot air zone. Displacement happens up there sure, but above all (pun) the pool of cold on the floor where us humans have to work is plowed up and shot up somewhere else, no breeze on your body. Since the stove is on the floor the intake is mostly cold air near the floor. After using outside air intake the floor might get colder so this will better than help.

    A powerful floor up fan on demand will work wonders in a one or multi-bay shop where people have to work low and there are ins and outs of vehicles. It will spread dust and debris if such gets to it. Even a desk job near a often opened outside door will help will help from cold floor air being moved up instead of a heater.

  7. Big volume of moderately heated air is better than very hot air all sitting at the ceiling. I doubt if the cooling effect so close to the heater would cause too much condensation of tar , creosote etc and increase chance of a chimney fire. He has not added any extra cooling fins on the main pipe to really cool it down. The electric fan would allow periodic “burn offs” every now and then anyway. Overall a good hack I reckon.

  8. This is a good way to cause a serious creosote buildup problem followed by a flu fire. My dad had a flu fire once when I was a teen, and you don’t want to experience one yourself.

    He was running a thermost controlled efficient wood heater with a 6″ flue. Our first warning was a loud roaring sound followed by the single wall part of the flu turning dull red and then bright orangesh red. My dad called the man who inspected our flu one a year and he said we were lucky, much hotter and the flu pipe would have disintegrated. From then on he would run the stove wide open once a week to clear the flu.

  9. My lounge woodburner flue pipe just has some scrap aly sheets bent into a U channel shape, and jubilee-clipped to the back of it where they are out of sight. You have to be careful to shape the aluminium to match the curved profile of the flue to maximise heat transfer. The sheets get pretty hot when the fire is working so they do a good job, and they are entirely passive, so no annoying noises.

    Interesting reading about how to avoid flue fires, I too have a chimney sweeping set, probably time to give it another outing.

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