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Which creates more heat?

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by David Tackett, Nov 29, 2012.

  1. HotCoals

    HotCoals Minister of Fire

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    I was trying mostly to figure out if the rate of wood consumption would increase or decrease with the fans on,or stay the same.
    That's all I wanted to know.
    I already realize you get more heat with the fans on...transfer has to be better with more temp diff between the stove top and inside the firebox.
    That said I hardly run my fans...lol.

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  2. Huntindog1

    Huntindog1 Minister of Fire

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    I think what most people are looking for with these freestanding non-cat burn tube type stoves is to get a long burn time. At bed time they should have been burning the stove so they time it and there is a nice bed of coals at bed time for a reload for an all night burn. This is the test by which everyone I think is going to be satisfied with their stove. At that time you have been burning during the evening and your house should be up to a good temp. At reload your not really needing a blast of heat transfer from the stove. So a blower isnt really needed. As a blower is going to really heat up the room your in, much more than the slower radiant type heat with the blower off. So you load up your stove and get it going up to temps so that you can get the stove shut back down for a all night burn. I might add that good dry wood lets this process happen quickly so the all night load isnt used up just to get temp up in the stove. If your wood has moisture the stove struggles to get up to temp and uses alot of your wood load getting the stove up to temp as it takes so long to get up to temp. Having the fire box hotter as your blower is off not transferring all that heat off the stove top and cooling the flue pipe, this extra heat kept int the stove I think will allow you to get the stove operating at its lowest setting that will maintain a secondary flames up in the top of the stove for the longest burn time. As at that moment the house should be already up to temps , and you hope that you have extended your burn time to have some extra heat at 4am in the morning or a nice big bed of coals giving off heat in the wee hours of the night when your really needing it.

    Now having said all that save your best pieces of high btu wood for your all night loads, pieces that are nicely split so you can load the stove neatly to get maximum performance from the stove as the higher btu wood that is extra dry will let you run your stove at a lower air input slower burn rate but also still have secondary flames still firing burning the smoke that causes creosote build up. When you use high quality dry wood you will notice how much more secondaries flames you get and how much easier it is to operate the stove.
  3. HotCoals

    HotCoals Minister of Fire

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    I don't think anybody is talking about a night load or longest burn times possible.
    I anyways was trying to find a answer to the question I posted above.
    But I believe you are spot on for a overnight load hutindog.
    But it's way easier with a cat..just saying..lol,,don't have to worry about that sweet spot.
    Just get the inside temp of the stove at 500 - 600 with the wood chard pretty good ..shut her down in two or three steps,then go to bed!
  4. Mitch Newton

    Mitch Newton Member

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    Huntindog1, this is the best explanation I've heard. You are dead-on!
  5. MnDave

    MnDave Guest

    Assuming all wood is created equal for sake of this discussion (which of course it is not) and you have settled in on a particular burn rate then

    IMHO, the fan, on or off, makes no difference to the instantaneous rate of wood consumption.

    The rate of wood consumption is controlled by the air to the stove and the draft. The outside temperature of the stove metal will vary significanty with the fan on vs off. The inside temperature of the stove metal will vary only a little.

    The difference is that more of the heat (energy) has found it more attractive to move through the steel than to go up the chimney. The fire does not care which way the heat energy goes.

    MnDave
    jeff_t likes this.
  6. Sprinter

    Sprinter Minister of Fire

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    Seems to be an unfortunate side affect of this thread. I think it needs a dose of KISS. If you like your blower, use it. If not, don't.

    The amount of heat you end up with is going to be virtually the same. The blower will get it there faster until it cools off, and may help the air distribution in the room.

    Too much confusion here between BTU's (energy) and BTU/hr (power).
    Realstone likes this.
  7. HotCoals

    HotCoals Minister of Fire

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    Thank you and I agree!

    I knew if I bugged enough people long enough someone would give in and say what I wanted to hear..lol.

    I don't hardly use my fans..but in playing around i swear you get more heat out of a load with the fans on through thermodynamics..god I love that word! lol

    It's nice to know if we ever get 40 below I could crank the stove and run the fans and keep it toasty in here!

    My hi-jack is over..sorry bout that! cheers!
  8. MnDave

    MnDave Guest

    I am not trying to sound like a know-it-all because I do not know it all.

    I have a Bachelor of Science degree in Mechanical Engineering. My area of study was thermodynamics which includes heat transfer and fluid mechanics.

    They are now teaching in the physics classes in high school, what I learned in the first two years of college . I'm 58. :)

    MnDave
  9. HotCoals

    HotCoals Minister of Fire

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    Well I agree with you and that's all that matters! Cheers!

    I think you're awesome! lol
  10. MnDave

    MnDave Guest

    Thanks HotCoals.

    I think that jharkins and bluerubi have a good handle on it too.

    For a problem/comparison like this, one needs to think in terms of defining a steady state sometimes called equilibruim.

    In this case, steady the burn rate, i.e. no operator adjustment to compensate for anything he/she doesn't want.

    Then look at the two main energy flows instantaneously.

    Based on temperature differentials, the energy flow through the stove into the room is greater with the fan on versus fan off.

    This is what I assume the OP meant by better... more energy flow to room versus up the chimney.

    MnDave
  11. oldspark

    oldspark Guest

    Heat go from here to there, fan make it go faster.
  12. MnDave

    MnDave Guest

    Not to confuse the issue further, if we are doing a complete energy balance comparison, then the electricity to run the fan needs to be factored in. And if you do you find that running a fan in the interior space is essentially 100% efficient.

    But consider that if you get your wood for free and you obviously have to pay for electricity... it may be more cost effective to keep the fan off.

    But that is not what the OP asked.

    Is this horse dead yet?

    MnDave
  13. Huntindog1

    Huntindog1 Minister of Fire

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    Dave you are right in a way but main engine of the stove is the draft of the flue. The secondaries are always open no adjustment. You can counter cooling the top of the stove thus lowering the draw of the flue by manually adjusting the primary air. But if you leave the primary air fixed and the draft of the flue is reduced due to cooling of the top of the stove and cooling of the stove pipe connected to the top of the stove in most of these free standing stoves, then the fire will cool down. You can go back thru these forums and alot of people say when their stoves go nuclear on them they turn on the fans to the stove on the cooling effect to the stove calms the fire down.
    You cant push air into these stoves it is drawn thru the stove by the draft of the flue and the draft of the flue is based on how hot the flue is.

    Its good to discuss these things to get people thinking even if neither one of us is completely correct. I am just throwing this stuff out there for people to shoot down as thats how we all learn. I enjoy the debate and hope others do also.

    My angle is keeping the stove hotter in the fire box so I can burn at hopefully a lower rate as its the heat in the box that allows the stove to burn with less air going into the stove and at this new higher fire box temps with my best lowest input air setting I hope to keep my secondaries firing for reduced creosote formation at these low over night burn settings.

    I know you other guys are looking for the best heat transfer but I am guessing most people have over sized stoves and best heat transfer isnt an issue so we go back to longer burn times as the main objective. As most people are burning themselves out of the room the stove as its too hot to even sit in there and I think the blower only makes that worse.
  14. jharkin

    jharkin Minister of Fire

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    This is what happens when you put a bunch of MechEs (Mechanical Engineers) in a room.. guilty.

    I think we are all wandering the same ballpark... the point I'm not convinced about is that there is enough to be gained from shaving off the stack losses (without any change to wood consumption) to account for the amount of excess heat transfer a blower can drive. This is all just gut feel based on some ballpark formulas since I dont have a blower stove to test.

    We need to get stoveguy2esw in here and ask him if Englander has done instrumented tests on the blower. Mike might be able to get some figures from their lab and settle this.
  15. Bluerubi

    Bluerubi Member

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    Thanks, but I know just about enough to be dangerous. Bachelors is in chemical engineering, then decided to get my MBA. Most of these comments come from my current job that involves a lot of work with large companies doing thermal modeling. My role is on the business side, so I pull in the practicing engineers to nail down the real numbers. My niche is sniffing out the good opportunities, after which others smarter than I make the numerical magic happen.

    I'm still a closet engineer at heart, so I can't help but really loving this place!
  16. MnDave

    MnDave Guest

    I do get your point.

    Like you, I would like to keep the fire at the optimum state for efficiency. It would seem that we would have to add small amounts of wood continuously to get into that steady state of efficiency.

    This is how a pellet stove operates. I don't know much about them. I can only guess that they are very efficient.

    MnDave
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  17. jharkin

    jharkin Minister of Fire

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    Exactly... and the ballpark convection equations i ran seemed to show that the fan could double or more the rate that you are drawing heat from the stove - a far bigger difference than the stack losses.

    We need a hearth.com test lab ;)
    Huntindog1 likes this.
  18. jharkin

    jharkin Minister of Fire

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    Nothing wrong with that. I'm a frustrated Mechanical Engineer who has been stuck at a software company for 15 years. This gives me an excuse to run some numbers.

    That makes at least 3 of us nerds in here :) :)
  19. MnDave

    MnDave Guest

    Agreed. It is those gut feelings that demand so much conviction. :)

    Even though I get my wood for free, I am all about heat to the room. It still is a lot of work to get wood from the forest up to my doorstep.

    This reminds me of an interesting engineering problem.

    A guy is out on a lake in a rowboat. He drops the anchor out of the boat onto the bottom of the lake. Does the depth of the water in the lake go up/down/ stay the same?

    This is not a trick question. There is one correct answer.

    MnDave
  20. Huntindog1

    Huntindog1 Minister of Fire

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    When I set my stove to its lowest setting to keep the secondaries fireing I dont think I am at my best efficiency of my stove but I am just glad I can keep my secondaries firing at that lowest air setting.

    My objective is to get the longest burn time but yet keep the secondaries firing , a constant battle as there are so many variables involved.

    I did insulate my firebox behind my fire brick with 1/4" ceramic insulation. Its not much but my feel for my stove I feel like I am getting the stove burning at lower rates but yet keeping the secondaries firing, no scientific data just my observations. Stove is easier to operate also. I am still heating my house fine , I am still getting enough heat out of the stove. I think i get most of my heat radiate out the top and front of the stove, what I lost on the sides of the stove is being kept in the fire box and eventually radiated out the top and front.

    Lastly a concept to throw out there is the one reason these stoves are efficient is since you can build the heat in the fire box you can reduce the air flow really low and still keep the secondaries burning and the reduced air input is slowing the amount of heat flushing up the flue.
  21. HotCoals

    HotCoals Minister of Fire

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    I'm guessing stayed the same.
    Reason being the weight of the anchor displaced the same amount of water in the boat.
    Heck I dunno..lol.
  22. HotCoals

    HotCoals Minister of Fire

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    That's where a cat stove shines..and you don't have to have big heat in the box.
  23. oldspark

    oldspark Guest

    Somebody make it stop.:p
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  24. MnDave

    MnDave Guest

    No guessing Hot Coals. lol

    Here is a hint/lead. The anchor is of course made of steel.

    BTW I think that you have the most awesome stove I can imagine. Are there heat shields on that?

    MnDave
  25. MnDave

    MnDave Guest

    Let's talk about beer!!! ;lol
    MnDave

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