Bob’s Downdraft Stove

There are a couple of concerns I have about this but they are possible to overcome. First the video concept:

Here are my concerns:

First, he has put a copper coil around the stovepipe. The idea is to circulate water through the tubing to heat it. You probably won’t need a pump either because the hot water would rise to the top of the storage tank, automatically forcing cooler water from the bottom of the tank to replace the water leaving the coil. It all sounds good but the problem is the possibility of the water boiling, turning to steam, and causing an explosion. A pressure release could prevent this but the steam would have to be routed outdoors or somewhere so no one would get burned.

Second, the chimney gasses would naturally be cooled. Cool wood stove gasses will adhere to the walls of the chimney and eventually will catch fire. This is called a chimney fire and it’s a dangerous situation. The solution would be to run the stove in the bypass setting for a half-hour or so every day or two with the draft open to provide an extra hot fire. This could burn the solids off the inside of the chimney before they accumulate to a dangerous level.

Otherwise it looks like a good concept. Does anyone else have any thoughts?

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14 Responses to Bob’s Downdraft Stove

  1. pateriot says:

    I have a pellet stove that I put in copper coils of pipe in the exhaust stove pipe which I run water through to heat up. I do have a bioler pump that pumps the hot water into the basement where the heat is extracted by a hot-water base heater. I made the coils myself (no easy feat) and had to adapt a larger sized stove pipe 5″ instead of the of 4″ pipe to allow for the obstruction of the coils. I have been running it now for a year and have thought of ways to improve it which I may implement next Summer when the stove is not in use. One is, that I would like to put a faucet to draw the heated water for household use. This would require me to add a makeup valve on the pump system to replace the water taken out.

    Right now the water goes through at only a trickle and comes out at about 140 degrees. An improved coil and longer coil run would dramatically improve the output.

    This method utilizes wasted exhaust heat and does not diminish the function of the stove. The only additional energy required is for the water pump to pump the water. As the stove gases are forced out via a fan, theoretically you could extract 100% of the exhaust heat because you don’t need to rely on the heated for draft.

    As it stands right now it delivers the heat of about 1-2 electric space heaters when the stove is running. I have to admit that I have spent about a thousand dollars on it, mostly on the boiler pump system which includes a pressure gauge, expansion tank, emergency hot water release valve, check valve, tubing, connectors and other miscellaneous materials.
    Pretty slick though!!

  2. RJ says:

    Here I thought hot air (and smoke) rises, not sure how you get it to go down and around, but it might work. In my experience anything more than a 90 degree turn in a stove pipe results in smoke not going up the chimney, here he has a 90 and another 90 then a couple feet down and another 90…

    when I was a kid we had a hot water heating system, we incorporated the copper coils across the top of the wood stove worked pretty well, not sure how much ambiant heat effect you could get from non touching copper around a stove pipe.

  3. seagypsy says:

    I used to have a chimney sweep business, so the comment of running the stove without running the water through the coils in order to “burn off the solids” is incorrect. The solids will just remain at whatever state they were, and if they “burn off”, you have the chimney fire you DON’T want. Either have your chimney and stove cleaned when it needs it, or as a self-reliant Revo, get a brush of the proper size for your stovepipe(most likely 6″ or 8″) and enough chimney rods for however tall your stovepipe is. The supplier I used only sells to the profession, but just Google chimney brushes and rods.

  4. notamobster says:

    This system uses gasification by increasing system pressure – allowing the fire to burn more of the off-gases produced by the oxidation of the wood. This process greatly reduces the particulate matter exhausted, as a result.

    RJ – He’s using pressure differential to move the liquids and gases. With the diverter flu closed the pressure (hot expanding gas) builds up. If there were nowhere for the hot gas to go, eventually the fire would smother as it consumed all of the available oxygen. There is, though.

    Without external forces being applied, force (pressure/heat in this case) always tries to equalize. It takes the path of least resistance, which becomes the long way when you close the flu.

    The pressure increase forces the hot gas down and around to the outlet, by moving to an area with lower pressure. The phrase “Hot air always moves toward cold air” should be changed to “seeks thermal equilibrium” to better explain the phenomenon.

    Likewise, “hot air rises”, but only as a result of the zeroth law of thermodynamics which dictates that higher pressure will seek thermal equilibrium in the presence of lower pressure.

    “If system A & B are in equilibrium with C, then A & B must be in equilibrium with each other.” “Pressure always seeks to equalize.”

    That’s my two-pence worth, anyway. I may be completely full of shit. :-)

  5. pateriot says:

    notamobster: WOW loved your explanations! Very scientific, however it assumes an airtight system. This stove certainly is not as it has to have an intake to bring in the fresh Oxygen that the fire requires to burn.

    What actually draws the air is the draft created by the ventura effect of hot air rising up the chimney flue. In the featured model I doubt that there would be enough draft formed taking the longer route unless a sizeable portion of the exhaust gas is bypassing the draft shut down valve. This could be compensated for by having a long chimney stack which would create more of a draw, assuming the exhaust gas retained much of its heat.

  6. Notamobster says:

    Pat – that venturi effect of the hot air rising results from the physical law of pressure seeking equilibrium, is it not?

  7. pateriot says:

    Nota: True but it is a pull force from the ventura affect caused by the hot exhaust rising in the chimney pipe rather than a push from expanding gases of combustion. But… perhaps this is what you were saying.

  8. Notamobster says:

    I was just throwing in my limited understanding of thermodynamics. I’ve never owned a wood stove.

    If I’m wrong on the cause & effect, I’m wrong.I can admit it, sometimes. :-)

  9. rj says:

    I’ll take ya’lls word for it, I barely passed H.S science class 40 yrs ago…. thus my earlier ignorant blather

  10. BaconNeggs says:

    I love the smell and romance of a roaring wood fire but from my past experiences, first in struggling to get the initial cold heavy winter air to flow up and out the chimney and not back resulting in a smoke filled home, was a common problem.

    Added to that, once the wood fire was burning strongly, 50% plus of the generated heat went straight out the chimney, while the fire feeds itself by drawing the air it has just warmed, out of the room.

    And yep, the build up of flammable tar in the chimney venting flue, if the wood is not completely aged and cured, is a definite fire risk if used often.

    I do like the more efficient design of the Wood stove in the video and if required, a pressure safety vent could be easily fitted to the hot water storage tank to release excessive steam build up.

    However, for me, all things considered other than as a SHTF back up or for occasional use, a burning wood fire is more for the romance than the efficiency, for the average sub-division home.

  11. rj says:

    I always simply closed the damper once the chimney was warmed up on a normal fireplace, left just a crack for smoke to draft up and out,

    When I built our current place I put in a circulating fireplace insert, fan driven air from around the heat box will drive ya outta the living room. I also added outside air for combustion and glass doors.

    when I was a kid our sealed stove drew outside air for combustion. It heated a 16×20 living room and adjacent kitchen on the coldest days and didn;t use much wood.

  12. pateriot says:

    Nota: I thought your explanation was brilliant!! I was and remain VERY impressed! Do you have an engineering degree?

    RJ: Our family had an open fireplace for our primary source of heat when I was growing up. This was not only not very effecient but was also a lot of work! After I joined the Navy at 17, my Father had a freind make him an insert similar to the one you described. It did kick out the heat but still consumed a large amount of wood. Later he bought a fireplace insert which worked well and didn’t burn nearly the amount of wood as the open fireplace did.

    I love the idea of a sealed stove and would like to get one that I could experiment with.

    • notamobster says:

      No. I wish. I went to college for Professional Aviation, but I quit when my active duty enlistment ended. I have a good memory and a habit of reading like the world’s gonna end. That’s where I learn most of the useless information I have.

  13. pateriot says:

    I wish had a mmemory for “worthless” information. That stuff comes in handy sometimes!

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