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  • OT: Wood Stove Design

    Hi everyone.

    We heat our home using a wood stove during winter. There is plenty of wood that falls around the house and I am able to chop it up during summer to burn during winter. We have a fairly large Jotul and it can build some serious heat.

    However...

    I can't help but think that this traditional wood burning design is just horribly inefficient. It takes a lot of wood to heat the house and keep it warm. Most of the heat blows right out the chimey, so the heat going into the room is by way of natural convection and radiation.

    One simple improvement could be to blow a fan at the stove to increase the temperature gradient and improve heat transfer efficiency to the room.

    But it has got me thinking... could I design my own ultra-efficient heat exchanger wood stove? Essentially, some thick steel plate with heatsink fins, ducting, and a built-in fan. The indoor chimney pipe could have a corrugated shape to increase surface area. This way, the outside of the fireplace itself would be just warm to the touch because the fan was constantly forcing air across the surfaces.

    I actually used to own a fireplace with a built-in fan and catalytic converter in the exhaust. It put out so much heat we had to turn off the fan because the house would get too hot.

    Why isnt this more common? Cost? Complexity? Are there some safety concerns or something I am missing?

  • #2
    A cold stove (i.e. one that has cool walls and pipe) is a real bytch to keep a fire in. Unless it is of forced draft type like a fire tube boiler.
    A cold flue is a creosote monster. If you pull the heat out , the build up can clog things in a hurry.

    If you want to maximize the fuel energy in your wood, look to Tarme down draft gasifiers or the like. But those units make ugly parlor stoves!

    If you want to get the most out of your deadfalls and clearing. look at a small stove that you can run full tilt and not cook yourself out.

    If you want the wood to burn cleanly and efficiently, HIGH stove and pipe surface temps is what you WANT. Having an open ,tall center chimney is a real boon for keeping the heat in the living space and not being uncomfortable. Radiant heat that warms the OBJECTS in the room (you and the furniture etc) and NOT THE AIR is the goal. Air just slips away through the cracks. Euro masonry heaters are a model as well, but they take an hour of attention several times per day to heat anything. Great for a "domestic kitchen" situation.
    Think "radiant floor".

    I've heated with wood exclusively here in Vermont for the past 35 years (and don't see changing anytime soon)
    We have a brick lined Fischer log wood stove down stairs that used to heat everything (boy was it warm down there on cold days ) There is a Soapstone glass door parlor stove about 10 feet from me now. Central in the house, has a top baffle and secondary air. Brick lined as well. Nice combination.

    We burn about 5 cord + . This winter has been good , warmer than some ;-)

    Look to the highly rated commercial stoves. Those guys are no dummies. You might learn to appreciate just what works with wood burners.

    But hey! Try anything you can think will work. If it does, and you build a better mouse trap. Ride the glory!

    Oh, A wood burner (or pellet stove) that needs electricity to operate correctly, INCLUDING INDUCED DRAFT, is a royal PIA when the power goes out.

    Ask me how I know, I've got a couple neighbors that have told me about it and are looking at battery back up inverters ;-)

    eta

    This house is "open concept" in a way. Open stairs both up and down, Always open french doors to the kitchen dining area etc. The ceiling fan runs pretty much all winter to keep the upper level from getting uncomfortably warm.

    eta 2nd

    radiant heat exchange is to the FOURTH POWER of the temperature difference. Suggesting that heat extraction is better if the wood burner is hot. PLUS, radiant heat travels across any open space regardless of distance, and carries no draft (or dust) .

    It's the little details that make this topic interesting :-)
    Last edited by CalM; 01-09-2022, 10:39 PM.

    Comment


    • #3
      You need a high quality airtight stove. Problem solved.
      Cheers,
      Jon

      Comment


      • #4
        Originally posted by CalM View Post
        A cold stove (i.e. one that has cool walls and pipe) is a real bytch to keep a fire in. Unless it is of forced draft type like a fire tube boiler.
        A cold flue is a creosote monster. If you pull the heat out , the build up can clog things in a hurry.

        If you want to maximize the fuel energy in your wood, look to Tarme down draft gasifiers or the like. But those units make ugly parlor stoves!

        If you want to get the most out of your deadfalls and clearing. look at a small stove that you can run full tilt and not cook yourself out.

        If you want the wood to burn cleanly and efficiently, HIGH stove and pipe surface temps is what you WANT. Having an open ,tall center chimney is a real boon for keeping the heat in the living space and not being uncomfortable. Radiant heat that warms the OBJECTS in the room (you and the furniture etc) and NOT THE AIR is the goal. Air just slips away through the cracks. Euro masonry heaters are a model as well, but they take an hour of attention several times per day to heat anything. Great for a "domestic kitchen" situation.
        Think "radiant floor".

        I've heated with wood exclusively here in Vermont for the past 35 years (and don't see changing anytime soon)
        We have a brick lined Fischer log wood stove down stairs that used to heat everything (boy was it warm down there on cold days ) There is a Soapstone glass door parlor stove about 10 feet from me now. Central in the house, has a top baffle and secondary air. Brick lined as well. Nice combination.

        We burn about 5 cord + . This winter has been good , warmer than some ;-)

        Look to the highly rated commercial stoves. Those guys are no dummies. You might learn to appreciate just what works with wood burners.

        But hey! Try anything you can think will work. If it does, and you build a better mouse trap. Ride the glory!

        Oh, A wood burner (or pellet stove) that needs electricity to operate correctly, INCLUDING INDUCED DRAFT, is a royal PIA when the power goes out.

        Ask me how I know, I've got a couple neighbors that have told me about it and are looking at battery back up inverters ;-)

        eta

        This house is "open concept" in a way. Open stairs both up and down, Always open french doors to the kitchen dining area etc. The ceiling fan runs pretty much all winter to keep the upper level from getting uncomfortably warm.

        eta 2nd

        radiant heat exchange is to the FOURTH POWER of the temperature difference. Suggesting that heat extraction is better if the wood burner is hot. PLUS, radiant heat travels across any open space regardless of distance, and carries no draft (or dust) .

        It's the little details that make this topic interesting :-)
        Fantastic experience you have! You make a very good point about burning cleanly. We burn exclusively Oak and Madrone which are fantastic, energy dense and clean burning woods. Every so often I will need to burn Fur and my chimney's spark arrestor gets clogged rather quickly.

        Comment


        • #5
          Originally posted by Jon Heron View Post
          You need a high quality airtight stove. Problem solved.
          Cheers,
          Jon
          I am actually thinking of adding a fresh air inlet so that the fuel air to the stove is not being sucked through the cracks in the house. Probably should have added one long ago.

          Comment


          • #6
            You and I have opposite goals my friend. I would like to insulate what's left of my 40 foot chimney to keep the heat in. The later half is double or triple wall stainless. Still a creosote monster. I run it as hot as I feel comfortable with. I'd say very little heat comes out of my chimney, it's never that hot unless the fire is roaring.
            21" Royersford Excelsior CamelBack Drillpress Restoration
            1943 Sidney 16x54 Lathe Restoration

            Comment


            • #7
              Originally posted by The Metal Butcher View Post
              You and I have opposite goals my friend. I would like to insulate what's left of my 40 foot chimney to keep the heat in. The later half is double or triple wall stainless. Still a creosote monster. I run it as hot as I feel comfortable with. I'd say very little heat comes out of my chimney, it's never that hot unless the fire is roaring.
              Maybe my goals are completely misguided! Glad you guys are posting so I can learn how to have the most efficient fire setup.

              Comment


              • #8
                Originally posted by kuksul08 View Post

                I am actually thinking of adding a fresh air inlet so that the fuel air to the stove is not being sucked through the cracks in the house. Probably should have added one long ago.
                PLEASE RUN THE NUMBERS!

                In order to have a Healthy House, it takes a certain number of air exchanges PER HOUR.
                A wood stove's combustion needs are a minuscule fraction of a typical house volume. (tiny houses exempted)

                Drafts are undesirable through a living space, But it's NOT the stove's combustion needs that are causing an issue. Convection is more likely.

                PLUS, Cold intake air to the fire box is COUNTER to efficient burn. Run the intake air though a heat box attached to the stove. Then you have done something positive towards efficient burn.

                Like "the metal butcher", I have a hard time keeping a warm flue. In fact, I need to brush the flue some time this month ("again", was November that long ago?)
                This warm weather SUCKS in that regard, and it's all ice up there now!

                Here is one opinion on "fresh air"
                https://www.woodheat.org/the-outdoor...h-exposed.html

                Plus, just about any "great new ideas" regarding wood stoves has been run through the mill on this forum ;-)
                Last edited by CalM; 01-09-2022, 11:40 PM.

                Comment


                • #9
                  Originally posted by The Metal Butcher View Post

                  snip

                  I'd say very little heat comes out of my chimney, it's never that hot unless the fire is roaring.
                  Nothing a good chimney fire won't cure! ;-)

                  Comment


                  • #10
                    Originally posted by CalM View Post

                    PLEASE RUN THE NUMBERS!

                    In order to have a Healthy House, it takes a certain number of air exchanges PER HOUR.
                    A wood stove's combustion needs are a minuscule fraction of a typical house volume. (tiny houses exempted)

                    Drafts are undesirable through a living space, But it's NOT the stove's combustion needs that are causing an issue. Convection is more likely.

                    PLUS, Cold intake air to the fire box is COUNTER to efficient burn. Run the intake air though a heat box attached to the stove. Then you have done something positive towards efficient burn.

                    Like "the metal butcher", I have a hard time keeping a warm flue. In fact, I need to brush the flue some time this month ("again", was November that long ago?)
                    This warm weather SUCKS in that regard, and it's all ice up there now!

                    Here is one opinion on "fresh air"
                    https://www.woodheat.org/the-outdoor...h-exposed.html

                    Plus, just about any "great new ideas" regarding wood stoves has been run through the mill on this forum ;-)
                    Huh. Grandpa put in an air inlet by the stove. I partially shut it off with a rag, maybe I need to fully shut it. Otherwise he went to great lengths to insulate and seal the house, at least in his early days.

                    Now why is hot air better? That goes against the conventional car engine advise.

                    Originally posted by CalM View Post

                    Nothing a good chimney fire won't cure! ;-)
                    Oh yeah! I brushed my vertical section but not the uninsulated horizontal section. I didn't think it was that bad. Well I get a hot fire started and the pipe is really ticking. Then you start to hear the characteristic creosote crinkling sound, and the pipe was getting mighty warm. I fully choked the stove, let it cool for a minute and just kept giving it minuscule amounts of air to keep the stove fire alive. The pipe cooled off. I had a bucket of water and rag ready too. Spooked me good though. I swept it shortly after that.

                    P.S. hearing is very important to burning. Thermal expansion ticks, the crinkle of creosote, the roar of air when it's going. All signs that say a lot. So do chimney thermometers. I should get another. Lost my last one like 4 years ago, which is pretty irresponsible on my part to have not replaced it.
                    21" Royersford Excelsior CamelBack Drillpress Restoration
                    1943 Sidney 16x54 Lathe Restoration

                    Comment


                    • #11
                      Originally posted by The Metal Butcher View Post

                      Huh. Grandpa put in an air inlet by the stove. I partially shut it off with a rag, maybe I need to fully shut it. Otherwise he went to great lengths to insulate and seal the house, at least in his early days.

                      Now why is hot air better? That goes against the conventional car engine advise.
                      Hmm

                      Remember those wrapped spring things on the old intake manifolds? Heat risers they were called. (still are) To heat up the intake manifold and the incoming fuel charge.

                      FOR BETTER EFFICIENCY AND OPERATION.

                      Of course, if all you are looking for is HORSEPOWER, Cold air induction is the buzz word. Cold is dense, dense is more O2, blah, blah blah.

                      Nothing a little NOx wouldn't do better.

                      But a wood stove is NOT an IC engine. HOT , RED HOT inlet air is what is wanted. Other wise, the heat of the burning log must heat the inlet air to combustion temperatures BEFORE additional combustion can happen. A HOT fire box is a healthy fire box.

                      And on the other note, A tight house STINKS! at least to my senses. Fresh air is a good thing, but keep the floor warm!

                      Comment


                      • #12
                        "The indoor chimney pipe could have a corrugated shape to increase surface area."

                        Good luck with trying to rod that clean!

                        Comment


                        • #13
                          The Russians have known for a long time how to best use wood heat. Build a roaring fire with some excess air that burns cleanly and transfers that heat to a mass of
                          masonry. No smoky fires, no creosote buildup, no chimney fires. Passes EPA standards. Neighbors not complaining about smoke. Fire only burns for a short period each day.

                          Comment


                          • #14
                            The only thing I can add is, if there is ever any sort of fire that happens with your homemade stove I’m pretty sure your insurance company is going to tell you to pound sand on any claims.

                            I also can’t believe that the state of California will be too happy with homemade wood stoves. I can’t believe that state hasn’t just outlawed them all together years ago.

                            Comment


                            • #15
                              Originally posted by oxford View Post
                              The only thing I can add is, if there is ever any sort of fire that happens with your homemade stove I’m pretty sure your insurance company is going to tell you to pound sand on any claims.

                              I also can’t believe that the state of California will be too happy with homemade wood stoves. I can’t believe that state hasn’t just outlawed them all together years ago.
                              Here in So. California they do not let you put a fireplace in a new home is what my neighbor had told me.

                              Ed
                              Agua Dulce, So.California
                              1950 F1 street rod
                              1949 F1 stock V8 flathead
                              1948 F6 350 chevy/rest stock, no dump bed
                              1953 chevy 3100 AD for 85 S10 frame have a 4BT cummins motor, NV4500
                              1968 Baha Bug with 2.2 ecotec motor, king coil-overs,P/S

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