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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. David Tackett

    David Tackett Member

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    Letting a freestanding stove radiate the heat or turning on the blower?
    I know turning on the blower moves more heat in the immediate vicinity, but I also know it cools off the stove. So which is better?

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  2. sblat

    sblat Member

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    I am really curious of the responses of this post as well. I run my T5 with the fan on auto all the time. This is my second season with it. I have never really tried burning with out the fan on. My stovetop usually get up to 600 then settles in around 450-500 for a couple hours then slowly drops. Not sure how it would perform. I guess I should try this for the evening. Would keeping the stove hotter longer help with coal build up? I have a lot of coals left, almost too much at the point when I need to reload.
  3. jharkin

    jharkin Minister of Fire

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    The short & simple answer is that the amount of heat you get from a given load of fuel is fixed. The fan only affects how fast you transfer that heat from the mass of the stove to the room.

    ...

    The scientific answer is a lot more complicated. Given 2 identical rooms with 2 identical stoves with identical fuel load, identical fire conditions, temperatures, draft and air control settings. One has a blower and the other does not.

    The stove with the blower will move heat to the room faster. So the room temp rises faster but the fire cools off. This could reduce flue temps meaning that less % of your heat is going up the chimney (more heat from the same fuel), but at the same time the lower fire temps could mean less complete combustion meaning more smoke - unburned fuel - up the chimney and less heat from the load.

    Another twist. You turn on the blower and then adjust the air controls to maintain the same stove top temp on the blower stove as the non blower stove. Result is you are heating the room faster but the fire is also burning the fuel down faster. So you get a higher rate of heat transfer to the room over a shorter length of time. Resulting in roughly the same quantity of heat over the entire burn cycle, but compressed into a shorter time. But again not exactly because the faster burning fire may be more or less efficient than the slow fire in the non-blower stove sending more or less heat up the flue.

    We would need to do a lot of complex math or some instrumented test to say exactly.
    Sprinter, Joful, corey21 and 2 others like this.
  4. begreen

    begreen Mooderator Staff Member

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    Neither is "better". It's a question of which is better for the task at hand. If you are in a big old room and you want to feel the strong heat of a fire 15' away, go radiant. If you are trying to warm up a large area quickly, say with more than one room, go convective. Got a grand room? Got with a stove that is radiant and has a convection blower.
    corey21 and tfdchief like this.
  5. Huntindog1

    Huntindog1 Minister of Fire

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    Several people on here have said when their stoves are starting to get too hot and the fire is burning at too high of level they can turn the blower on and on alot of these stoves the blower blows air up the back of the stove and then the air is directed over the stove top usually by having a deflecting angled piece as part of the back heat shield of the stove. This cools the stove top takes heat away from the stove and slows down the over heating scenario of a somewhat out of control stove. The natural laws of the burning and heat of the stove are affected by the blower cooling the stove as its the draft of the flue driving the air being sucked thru the stove. The stoves secondary air tubes or manifold is always open not controllable. As the stove heats to hotter levels the flue heats hotter increasing the draw of the flue pulling in more air into the stove thru those always open secondary inlets. This feeds the fire even more and lets it burn hotter and the cycle just keeps repeating as it all feeds on its self. These stove are supposed to be tested to with stand situations like this so when most people think their stove is running away on them in actuality the stove can handle temps higher than 800-900 stove top temp that most people get nervous about. But you dont want to get in those situations very often.

    But not having the blower on lets the stove be a little hotter and thats what its all about the heat. The efficiency of the stove is some what based on the fact the firebox is insulated with firebrick and can maintain heat levels that allow you to turn the stove air input down to low levels and get longer burns but still have the secondaries firing for you burning up the smoke gases that cause creosote and low efficiency ratings. I think my stove operates better with my blower off making it easier for the stove to maintain its operational heat levels at low air input settings for long over night burns. The stove will still radiate the heat. Using the blower also seems to over heat the room the stove is in and makes it uncomfortable to be in that room. Radiant heat seems to be more comfortable as it slowly radiates thru out the house with out overheating one room.
    corey21 likes this.
  6. jharkin

    jharkin Minister of Fire

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    One thing to remember that the term "radiant heat" applied to our stoves is a bit of a misnomer. All wood stoves transfer heat via the 3 standard mechanisms - conduction, convection, and radiation. At the temperatures we operate our stoves at (300-700F) convection is the predominate means of heat transfer, blower or not. Using the blower just replaces natural convection with forced convection and speeds things up.

    I cant find the reference, but I recall that math of heat transfer says something like this for a dark colored steel/cast iron surface:
    • Below 150F - most heat transfer is via radiation
    • 150F-300F - roughly equal heat transfer via radiation and convection,
    • Above 300F - most heat transfer via convection
    (in all cases conduction contributes a negligible %)

    You can increase the heat distribution by radiation by increasing thermal mass - like a heavy soapstone stove or a masonry heater or a large brick/stone hearth behind the stove. By heating a lot of mass to a lower temp you put out a greater % of the heat via radiation. This is probably where the soft heat reputation of soapstone stoves comes from.

    Also, radiant heat transfer only effects solid objects with a line of sight to the stove. Heat moving to other rooms of your house is all by convective currents in the air.

    Note that all of the above reflects heat transferred from the iron and stone of the stove to the room - the actual fire itself puts out most of its heat by radiation, but when its inside a stove that radiation is heating up the stove mass not the room directly (notice that when your stove is cranking you feel more heat facing the window than the side or top? thats radiation :ZZZ ). The radiant nature of the open fire is is why the Rumford fireplace design with angled sides made such a big improvement over square box fireplaces - the angles reflect the heat radiating in every direction back into the room.
    MnDave, n3pro, Sprinter and 2 others like this.
  7. corey21

    corey21 Minister of Fire

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    Great thread i was wondering the same thing.
  8. Jacklake2003

    Jacklake2003 Member

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    Does your car's radiator work better with the cooling fan on or off?

    Not quite that simple but close. You're trying to "exchange" heat from one place to another.
  9. David Tackett

    David Tackett Member

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    Well, with my car I am blowing the air through the radiator to rid it of heat into the atmosphere. Where as the blower on the stove is moving the warm air into the space I want heated. Also, with the car I am cooling off something I want cooled, where as with the blower on my stove, am I cooling off something I want hot? I want to use the radiator as a cooling heat exchanger, I am trying to heat an area with my stove, so am I doing it less efficiently using it as a heat exchanger or as a conduction unit?
  10. HotCoals

    HotCoals Minister of Fire

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    I agree with most of what you say.I just don't agree that the fire inside changes any with the blowers on.
    I do think you have better heat transfer from the fire to the stove when the blowers are on..though I don't like to run them.
    Heat will move to a cooler object faster then to a hotter object.
    Therefore less heat up the flue is my thinking.
    If anything the fire may slow some if the fans are on high because of lower temps around the fire..as where the fire lives..but like I said the heat that is in there will transfer faster to a cooler surface.
  11. corey21

    corey21 Minister of Fire

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  12. jharkin

    jharkin Minister of Fire

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    The transfer of heat from your car radiator and a woodstove are both governed by the same laws of physics.

    Bottom line is that there is a fixed amount of BTUs available in the mass of fuel (wood). If the blower moves that heat to the room at a faster rate its taking said heat from the fuel faster - which is going to cool down the interior of the stove. Either that means the temperature of fire cools down, slowing the rate of reaction, or the temperature of the flue gasses cools down, which reduces the draft strength, pulls less air into the fire and.... slows the rate of reaction. Way too many variables here and I just dont know what the exact result would be (im not a combustion engineer - but I did take thermodynamics and heat transfer in engineering school many years ago) so I wrote all those "could" statements. My best guess is that you get an initial rush of heat to the room but the fire would later slow down if the air control was not opened up (either by the user or a thermostat) to compensate.
  13. My Oslo heats my home

    My Oslo heats my home Minister of Fire

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    I spent one burning season in a T&E process in moving warm air from the stove. I no longer use the fan to move the stove heat. I found that the fan did more harm than good in wood consumption and burn times. Even at a low fan speed it made such a small difference with the setup in my home. The stove is centralized in the center of the house and radiates fairly evenly to the outward rooms and the second floor. The fan moved the heat quicker but no for longer.
    Realstone and Trilifter7 like this.
  14. DMbekus

    DMbekus New Member

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    I was going to ask this myself. intersting, but I'm still not sure........
  15. nate379

    nate379 Guest

    With the fans off on my stove I don't get as well of heat distribution though the house. Don't really care if I only get 12hrs burn vs 20 if the bedrooms are 50* vs 65*
  16. tfdchief

    tfdchief Minister of Fire

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    Interesting. My old Buck is a heating fool with the fan on. Of course we are talking about an insert. The Hampton, I run the fan on auto, low, all the time. I want the heat from my wood stoves in the space I am trying to heat. So, I turn the blower on, heat exchange, from the stove to the room. Modern heating appliances all use a heat exchanger. They quit relying on radiant heat quite some time ago.
  17. HotCoals

    HotCoals Minister of Fire

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    I agree with all that now that you worded it differently...for sure the bold.
    That's inline with my sig.
  18. Trilifter7

    Trilifter7 Feeling the Heat

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    A little off topic here but I also do some experimenting every year with different ways of moving the air around the house and every time the radiant method wins. I do not have a blower on my stove but I do have a ceiling fan directly over the stove. I also have my stove in a room that is about 3-4 feet lower than the rest of the house. In every scenario, whether it be running the fan on my furnace, putting a box fan in different locations throughout the house or running the sealing fan... I gain the most heat in the house and get the longest burn times from having nothing on and letting the heat transfer on its own. The shortest burn times I get are when the ceiling fan is on.
  19. Huntindog1

    Huntindog1 Minister of Fire

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    If your looking to learn about how to move heat thru you house there are some terms you can search on Google that brings up alot of info.

    Stack Ventilation
    Cross Stack Ventilation
    Natural Ventilation

    I was looking up info for my new divider wall in my basement. As it has changed my air flows in my house as to getting heat upstairs.
    There is a term for vents in a single wall, single-sided natural ventilation.
  20. Huntindog1

    Huntindog1 Minister of Fire

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  21. Bluerubi

    Bluerubi Member

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    In my opinion the more air you can move the better, since heat transfer is driven by temperature differential. If you can get cool air blowing across a hot surface by forced convection, that increase in velocity will help to pull more heat form the surface. I also worry very little about this impacting the state of the fire since air has a relatively low heat capacity, so compared to the inferno inside the brick lined box very little heat is being pulled away due to outside air, especially compared to what is already heading up the chimney.

    With regards to radiation, assuming your stove is flat black(black body radiation), driving up the surface temperature makes a massive difference in radiant heat output. The equation for energy output due to radiation has the surface temperature raised to the power of 4, so even a 50 degree difference will yield a big increase in radiat heat output.

    All this assumes that the surface has a very high emissivity, so if you really care about heat output stick to the matte black stoves.
  22. HotCoals

    HotCoals Minister of Fire

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    Wow...unlike us you sound like you know what you're saying! lol
    Now would a soapstone clad stove slow the radiating of the heat considerably?
    Also it's been rumored that part of the magic of a BK stove is that maybe the top is titanium ..would that be a big plus..I would think so.

    It's great to hear I have the fastest color!
  23. BrotherBart

    BrotherBart Hearth.com LLC Mid-Atlantic Division Staff Member

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    Relate that to a stove, non-cat, that requires a high temp in the firebox to produce the desired secondary combustion to get the maximum heat value from the fuel load. Below that temp efficiency drops like a rock.

    And that that blower is blowing right over the top of the firebox where that combustion needs to occur.
    Realstone likes this.
  24. jharkin

    jharkin Minister of Fire

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    There is no titanium in a blaze king. No reason to use it, the thermal conductivity is very poor at around 12 but/hrFft vs around 30 for plate steel and almost 45 for cast iron. Not to mention that making one panel out of the stuff would probably cost more than an entire stove.

    If you want to make a stove out of some magic metal a better choice would be Aluminum (120) or copper (200+), which is why those are popular for high end cookware. But they wouldnt stand up to the temperatures of the direct fire.
    HotCoals likes this.
  25. jharkin

    jharkin Minister of Fire

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    Not quite. Its been measured that the average epa stove has a flow rate of around 30-40cfm up the stack. Let's use a BKK as an example. 82% efficiency at medium burn, which is prob around 30000 but/hr and the optional fan is 165 cfm.

    At 82%, 30k btu going to the room is sending 6k btu up the stack at 40cfm. The blower to the room can move 4 times as much air as the stack. Do the math.

    There is a reason those blowers have thermostatic shutoffs. They can draw a LOT of heat from the stove.
    HotCoals likes this.

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