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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. Pallet Pete

    Pallet Pete Guest

    I find that a fan pointed at the stove from the end of the house that is cold works far better than a fan on the stove.

    Pete

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  2. corey21

    corey21 Minister of Fire

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    My blower overheats my house i like the radiant heat much better to be honest.

    But i run it on hi speed when the stove is above 650.
  3. evilgriff

    evilgriff Burning Hunk

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    Not using my fan on the Sirocco at all. When I did test it, the stove seemed to burn wood much faster. Not that cold here yet-will try it again when we have a cold snap.
  4. firecracker_77

    firecracker_77 Minister of Fire

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    Great thread all. Really interesting and alot of food for thought.

    My Woodstock gas has a fair amount of cast iron as well as stone. I'm not sure about heat transfer, but the cast runs about 150 - 200 degrees hotter than the soapstone it surrounds. That doesn't tell me about how fast it is passing heat into the room, but that stone is definantly more gentle by the very nature that it is not heating up as much. Not sure which is a better material.
  5. Sprinter

    Sprinter Minister of Fire

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    Oops. There are a few threads here that allude to BK's use of tritanium to explain their "Alien Technology". Tritanium http://en.memory-alpha.org/wiki/Tritanium is a mythical material. The posts were just intended as a joke.
  6. Bluerubi

    Bluerubi Member

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    Your numbers sound valid, but heat transfer is a lot more than a comparison of volume. In the case of forced convection, whether it be from a fan, or draft, the overall heat transfer is the area of the surface in question X the convection heat transfer coefficient X the temperature difference between the surface and the fluid (air in this case). The area inside the firebox is likely far greater than the channel that air is blown through outside the stove, but call it equal. In both cases, the 40 and the 165 cfm, the velocity isn't that great, or that different since the cross sectional area for the space the fan is blowing through is likely larger than the flue. Again, I'll call the area the same to be conservative, which makes the velocity of air 4 times higher for the fan. The flue conditions are known, and is about 2.5 mph at 40 cfm, so call the fan 10 mph at the same cross sectional area as the flue. Both of which are fairly slow, but I'll double the heat transfer coefficient for the fan to account for it. This essentially boils down to the variables outside the temperature difference being twice as high for the fan equation. All that's left now is to look at the surface temperature of the outside of the stove and the surface temperature of the inside of the stove. Since ambient air is the fluid in both cases, all that would have to be true is that the brick temp inside the stove is twice that of the outside surface. Seems easy enough to imagine.

    In the end, I don't disagree that a fan moves a lot of air compared to an EPA stove flue, but we need to remember that the air being moved isn't the same. Perhaps the fan induced energy removal can exceed the flue losses when the air intake is throttled back, but I'd wager that in many situations that isn't true.
  7. Bluerubi

    Bluerubi Member

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    How are you measuring the temperature on the different surfaces? If you feel confident that you know the emissivity of the different materials on the outside, their particular temperatures, and the estimated surface area for each, it isn't too tough to calculate an estimation of the radiation being given off.
  8. HotCoals

    HotCoals Minister of Fire

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    Ok..now if you could give us what you're saying in english a lot of us would appreciate it! lol

    In other words..
    Do you think fans blowing across the stove top have any effect on the fire inside and/or flue temps..sorry to bother you..but I gota know what the heck you're saying..lol.
  9. Bluerubi

    Bluerubi Member

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    Yes, it will impact it, but my only point was that a lot of heat is already heading up the flue, so I don't think you're saving a bunch by not using a fan. Personally I prefer no fan due to the noise, and the radiant portion of overall heat transfer equation has the surface temperature raised to the power of 4, so keeping that outer surface hot makes a big difference in the IR given off. I have a very large insert, and can hear the blower whining away now, so I'd much rather be staring at a nice matte black woodstove.
  10. firecracker_77

    firecracker_77 Minister of Fire

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    Measured with an infrared gun with the laser indicating where the reading is taken.
  11. HotCoals

    HotCoals Minister of Fire

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    Well I don't use my blowers either.

    But in theory I always thought the bigger the temp differential the faster the rate of transfer?

    So I would think running the fans and cooling the top surface would be a good thing.
    To go farther with that if the above is true you might cool the inside of the stove down some and the fire would actually slow some..in effect slow the burn and prolong the fuel..but most say they see their burn times decrease with the fans on.
  12. jharkin

    jharkin Minister of Fire

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    I dont disagree that you have valid points... there are a lot of differences but my gut tells me that given the amount of BTUs we can calculate are going up the stack and the amount we know is going into the room and lots of anecdotal evidence that running a blower on high will put out the heat a lot faster - leaves me to figure that the engineers at these stove MFGers sized the blower properly to move some significant BTUs. And we know there is only so much we can reduce the stack losses before you wont have enough temperature differential to keep the draft going.

    As far as the stack losses... Most of the stoves we are talking about are only loosing 25% or less (75% eff) up the flue, 18% if you believe Blaze Kings numbers. I wonder how much lower you can really go before you will need forced draft. I cant recall that Ive ever heard of a natural draft oil or gas furnace over 90% efficiency, so one could guess that wood stoves are getting pretty close to the theoretical limit of natural draft efficiency.

    So lets say by turning on the blower you lowered the stack losses enough to drive the BKK 82% efficiency up to 90% efficiency. You just reduced that 6000btu loss I calculated to around 3200 BTU. So there you gained an extra 2800 BTU to the room on top of the 30,000 BTU we started with. I dont have a blower but based on a lot of anecdotal stories it feels like the typical blower does more that a 10% increase to the heat to the room. But I could certainly be wrong.


    To be honest I'm just guessing/ballparking here... Its been 15 years since I picked up a heat transfer book and I wouldn't remember how to solve the differential equations involved anymore even if I dug mine out ;)
  13. jharkin

    jharkin Minister of Fire

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    Pulling heat from the fire will slow it down as we were discussing above, either cooling it directly or reducing the stack losses as bluerubi calculated, which slows the draft and in turn slows the fire.

    But in the real world I guess most of the time either:

    #1 the operator knows they eant a 500F stovetop and with te blower on they adjust the air intake to keep it that way thus speeding up the burn to compensate for the faster heat removal.

    OR

    # They hav ea thermostatic stove like a BK or VC where the automatic thermostat does the above without intervention.
  14. HotCoals

    HotCoals Minister of Fire

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    I knew the bi-metal t-stat would be brought into this..lol.

    I have the cover off the back of mine so I can watch the t-stat..it never moves when the fans are on during my experiments.

    Mater of fact..at least on my BKK the t-stat is very slow acting and almost useless.
    At the end of a burn it will open some though.
    Maybe the VC t-stat is more sensitive..I don't know.

    I was also thinking that when someone runs the fans they might be adding more air then normal for them.
  15. jharkin

    jharkin Minister of Fire

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  16. HotCoals

    HotCoals Minister of Fire

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    well that didn't help! lol
  17. jharkin

    jharkin Minister of Fire

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    OK, if it will make you guys feel better to see some math, here goes. This is easier than I remembered.

    Convection
    http://www.engineeringtoolbox.com/convective-heat-transfer-d_430.html

    We have to do this in metric. Lets make a guess that the stove has about 1.5 m2 of surface area and its at 300C (just under 600F).

    Using numbers from the ranges for natural and convective heat transfer in air constants they supply their calculator gives me:

    q = hc A dT
    Natural convection (using 15 w/m2K coeff) is 6300 watt or ~ 19,800 BTU
    Forced convection... there is a big range of values, lets try 30 (double the natural coeff) - 39,500 BTU


    Radiation
    http://www.engineeringtoolbox.com/radiation-heat-transfer-d_431.html
    q = ε σ (Th4 - Tc4) Ac (note we have to use absolute temps - Kelvins - in this one)

    At the same temperatures the radiation to the room works out to 6782 watt or 21,300 BTU


    So our stove sitting at 300C (580F) is pumping about 40,000 BTU out with no fan... which is in line with an average stoves rating. then add the fan and we are up to almost 61,000 BTU. A lot more than just shaving a few BTU off the stack losses so I figure it HAS to be having an effect on the fire and these numbers would explain the significantly reduced burn times that members report.
    pen likes this.
  18. jharkin

    jharkin Minister of Fire

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    The other interesting thing - running the numbers above shows me what I rememberd on the first page of this thread about the % split between convection and radiation ranges is wrong:

    The formulas above say at 600F we are getting more radiation. I reran it again with 400C (752F) temp and now I get 26,000 BTU convection and 16,000 BTU radiation. So there is a crossover point where convection heat transfer begins to overtake radiation but its a lot higher than 300F, it might be more like 700F.

    [edit] Then again maybe I remembered the ranges correct but its in C not F.
  19. HotCoals

    HotCoals Minister of Fire

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    Thanks for taking the time!

    I understand what you are saying but I don't see how the fire burns any faster with the fans on.
    You are not adding air to the fire.
    The rate of wood consumption should stay the same but the efficiency should go up as you reported..or at least more btu output with the fans.

    I still think by cooling the top of the stove way more heat transfer is going to the stove top that would have went up the flue.

    I don't know fancy formulas but you're saying with the fans on the output increased by 50%..that sounds suspect and way high .

    I would buy into 10,maybe 20%.
  20. nate379

    nate379 Guest

    Makes sense since the fan is pull more heat from the stove and causing the T Stat to open the draft a bit.

    When it's warm out (20s-30s) I don't NEED to run the fans, the heat will travel through the house fairly well. When it's actually cold though (-20*) I can easily see a 15* temp difference from the front of the house to the back without the fan on. With the fan on, usually only 7-10*
  21. Sprinter

    Sprinter Minister of Fire

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    I've seen those same figures elsewhere..(in F) Can't remember where but it seemed a reputable source.

    I can't get up the ambition to do it myself:)
  22. Bluerubi

    Bluerubi Member

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    Do you have adjustable emissivity? Might be interesting to take a surface temperature probe and measure each type of material to see how accurate the gun is reading. Some of the work I do is with heat flux reducing coatings, and it's always weird to see how on a piece of aluminum the coating will have a lower temp by the surface probe, but the aluminum it's painted onto will read hundreds of degrees lower, even though its close to 350f by the contact probe. I love IR guns, but the results can be deceiving.
  23. jharkin

    jharkin Minister of Fire

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    That's the tricky thing. The formula just gives you the instantaneous heat transfer rate. If we keep the air to the fire fixed the iron is going to start cooling down. It will transfer less heat to the room but pull more from the fire.

    So to keep the stove top temp fixed with the blower on you have to open the air till you reach a new equilibrium.

    Really this is a very complex issue as the heat from the fire is varying based on a lot of factors and there are all kinds of feedbacks. To do it right we would also have to model the combustion mathematically and how its effected by the draft and how the draft is effected by the changes we are making and on and on.


    Edit...sounds like our new friend blue does this for a living. Feel free to check my math, Im way rusty.....
  24. HotCoals

    HotCoals Minister of Fire

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    Well maybe so..but i think we are complicating something we can just do at home and observe.

    Thermodynamics says that heat will transfer faster to a colder object.
    So with the fans on the heat from the fire will transfer to the stove top faster..I think we agree on this.

    Now that said I don't see how the fire can change it's rate of burn with no more or no less air added to the fire.
    If anything I would think the flue would cool just a little and slow the draft down..but I doubt much.
    But I do think less heat will go up the flue because it was used to heat the stove top more then if the top was hotter ,as in no fans on.

    Really though..thanks for your time trying to get to the bottom of this.

    I don't think we need to factor in a whole load over 8 hours or whatever..to me a hour or so in the middle when a stove might be putting out around 30,000 btu would be fine.
  25. Sprinter

    Sprinter Minister of Fire

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    Thinking out loud here, but maybe if the fire is cooled by whatever mechanism, the burning gasses and fuel cool and combustion rate decreases that way.

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