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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. Bluerubi

    Bluerubi Member

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  2. HotCoals

    HotCoals Minister of Fire

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    That's what I think happens also, to a slight degree...more that way then the wood being consumed at a faster rate anyhow.

    But yet most say they see their burn time decreased with fans on...and maybe so..I just don't know why.
  3. MnDave

    MnDave Guest

    I did not read everything here but I do believe that running the fan transfers more heat into the room which would otherwise move up the flue as hotter gasses.

    I can see my stovepipe temp gauge drop 50-100 F when I have the fan wide open. My stovetop temp also drops at least 50 F.

    The heat moves through the house better with the fan movement.

    As you can see, I am a big fan of my fan :).

    MnDave
  4. HotCoals

    HotCoals Minister of Fire

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    Cool..or should i say hot..lol.

    Do you notice a change in the rate of wood consumption with the fan on?

    Real world stuff is the best!
  5. MnDave

    MnDave Guest

    I have not used it enough to know if the rate of wood consumption varies with fan usage. I tend to adjust the controls to get the fire to look right for either viewing or for overnight burn. I run the fan at 3/4 when I am in the room and full when I am asleep.

    In our home, the master bedroom is on the main floor above the stove on the lowest level. I put a 12x12 vent above the stove with a duct fan to pump warm air into the bedroom. I run that full when we are not sleeping and back it down to barely audible when we are sleeping.

    One other principle in heat transfer is that conduction through the stove metal requires a temperature difference. So the cooler the outside surface is relative to the inside surface the more temperature gradient there is to drive heat transfer. It is the fan which drives that outer surface down and hence drives conduction through the stove metal.

    MnDave
  6. HotCoals

    HotCoals Minister of Fire

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    Yep..gota love thermodynamics..if nothing else just because it sounds cool..lol.

    Did you know if you put two trays of water in a freezer ..one warm water the other cold..the warmer one will freeze first..yep..it's science!
  7. Sprinter

    Sprinter Minister of Fire

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    I've got to say, I don't really understand this controversy. With a blower, you may (probably will) get the heat into the room faster, but I don't see how that affects the overall appliance's operating efficiency. You're just going to have to feed the stove sooner to make up for it. Am I missing something?

    And it seems to me that if a blower affected the stove's efficiency, the manufacturers would be publishing two figures, one with a blower and one without.
  8. HotCoals

    HotCoals Minister of Fire

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    Why do think you would have to add more wood to the fire just because the fans are on..you're not changing the air supply to the fire.
    That's what we are discussing.
    That and how much efficiency is affected.
    Me..I hardly ever run the fans cause I hate the noise.
    But I am very curious...sometimes to a fauilt! lol
  9. Sprinter

    Sprinter Minister of Fire

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    I'm only saying that the greater the rate of heat exchange, the sooner the stove will cool off. That's all.
  10. MnDave

    MnDave Guest

    This is not intuitive stuff. The only reason that I have a handle on it is because I studied it in college.

    I tend to look at it from a conservation of energy standpoint (1st law of thermodynamics). The energy from the burn is conserved, what does not go into the room, goes up the chimney, and visa versa.

    The math governing the conduction through the metal is Q= -k(T1-T2).
    Q is the heat flow, k is a constant for steel, and T1 and T2 are the two temperatures on either side of the steel. T1 being the higher inside temp and T2 being the outside surface temp.

    Q is negative because the heat is moving from inside to outside.

    The lower the outside surface temperature the greater Q will be.

    The fan will drive the outside surface temp lower and hence have a greater quantity for (T1-T2). This temperature gradient is what drives the heat out of the stove and by the 1st law reduces the energy going up the chimney.

    Fans are good for heat transfer.

    MnDave

    MnDave
  11. jharkin

    jharkin Minister of Fire

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    The controversy is that just like cat v. Non cat, this topic comes up every year. And every time there are folks who are convinced that the blower magically creates more heat from nothing without increasing fuel consumption.

    No amount of math here is going to change preconcevied notions. Maybe somebody should call up a stove mfg and ask what their lab test results were.
    Realstone likes this.
  12. Mitch Newton

    Mitch Newton Member

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    Sorry to get off topic, but MnDave I am curious about your duct vent. What kind of duct fan do you have? Battery operated? Where did you get it? Would a long and narrow vent work just as well? Something like 6" by 25"? I would like to keep it narrow along the baseboard trim.
  13. Realstone

    Realstone Lord of Fire

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    Also, magically drawing too much heat from the firebox, cat or anywhere along the flue will end up with a creosote payoff.

    Perhaps a remote thermometer at the top of our flues, maybe a foot or so below the cap would be the ideal place to monitor firebox performance?
  14. Huntindog1

    Huntindog1 Minister of Fire

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    Like I said previously with some other thoughts added:

    Being that a hotter stove burns more efficiently as its all about the heat. If your going to get more btu's out of that smoke gases, you have to burn it hotter that extracts more of the energy out of the smoke. I am going to say by cooling off the stove top the stove doesnt burn as efficiently. Take a look at wood gasifier technology I think their burn chambers are made of ceramic as they burn the smoke gases at like 2000 deg F . Why doe they do that? Because they are extracting more energy from the smoke. They have to use ceramic burn chambers to with stand the 2000 deg F.

    I say all of that as if your cooling your fire box with a fan at a more rapid rate your stove is burning at a lower temp. Equals less efficiency meaning your not getting all the heat out of your smoke.

    I will say this if your trying to get heat over a longer period of time , if your stoves firebox can more easily maintain its temps you can lower your air input lower and have a slower burn rate. So keep your stove blower off for a longer burn. (I know inserts have to have the bower on but free standing stoves usually dont.) Also I think it can be said inserts dont usually have the best burn times as they have to have a blower running. (Cat stoves may be a different story)

    You wont keep the secondaries flaming if the temps inside the fire box drops below the threshold temp needed to have a secondary burn.

    Using dryer wood helps for a longer burn time in these newer type EPA stoves, as the wood more easily out gases at a lower air settings plus eliminates the moisture that kills secondary burning in the top of the stove. As you need to think of these newer type stoves as not wood burners but smoke burners. In the ideal smoke burner mode you get the longest burn times and most heat output. Its the heat in the firebox that is what triggers the ability of the stove to burn smoke. With out the heat to the extra high level needed in these type stoves the stove will not put out much heat. Once secondaries kick in the stove puts out lots of heat.

    I want to ad a thought I had the other day about turning your input air down increments. I think everyone who uses this technique has found its the best way. I said previously you turn down like 1/4 the way then you wait. Like people have said your letting the stove balance out before you turn down another increment like 1/4 or 1/3 some people first increment is 1/2 then smaller increments after that. Letting the stove balance out is basically letting the heat in the fire box raise to a higher level as the air flow thru the box has just been decreased by you shutting down the input air, this means less heat is being flushed up the flue so more heat is building in the stove. Once the heat has raised to a higher level then the fire can deal with another decreased in input air as its the heat in the stove that lets these new type stoves burn at very low air input levels. Its that lower input air level that keeps the heat in the stove and lets it radiate out the stove rather than flushing up the flue. Its all a balancing act as you have to know when the wood is burning good enough and the heat in the stove is high enough to start shutting the air down and experience for the new stove operators will teach them when to make adjustments. So now my new thought is once you turn the air down you will see the secondaries light off. As you turn down more you should see more secondaries. With each incremental decrease in air letting the stove balance out and temps raise my thought is could part of the stabilizing of the fire box be that the smoke gas burning is a much hotter burn and its up around the top of the stove around the burn tubes. So the incremental adjustment technique works well as your easing the stove from a wood burner to a smoke burner mode and the heat zone up around the burn tubes is getting much hotter from the gas burning allowing the air/oxygen coming into thru the tubes to fire off the secondaries. Another way to look at it is if you slam your input air closed all at once there is a rush of cooler air being sucked in thru the secondary tubes, which at that point in time had no secondary flames firing to add the extra heat you get from the secondary flames. So easing the air input closed in small increments helps the stove to heat adjust to the change over from wood burning to more of a smoke burner mode of operation.
  15. firecracker_77

    firecracker_77 Minister of Fire

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    For what I need to know, blow the cold air towards the stove room. This is interesting, but over my head now.
  16. smokeater

    smokeater Member

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    This may be simplifying things a bit but,house cold throw in more wood and turn on blower,house gets warm less wood turn off noisy blower.It works for me anyway.
  17. corey21

    corey21 Minister of Fire

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    This is confusing me.
  18. MnDave

    MnDave Guest

    The thermal conduction equation I listed above is not overly complicated math ... it is like the Ohms Law of heat transfer. :)

    Another important consideration is the shielding on many stoves. The more shielding your stove has the more thermally insulated it is from heat transfer. The shielding is mainly there to reduce clearances to combustables.

    IMO, most people will want to exchange that trapped heated air with cooler air. The cooler air will draw more heat from the stove. The warmer air will help get the chill out of the house faster.

    Guys it's that simple... let's just agree to disagree and let other readers decide if they want to run a fan or not.

    My stove is used so that my furnace doesn't come on. I also like how the house feels when heated with wood. I like the technology of wood burning and how newer stoves are engineered (cats and tubes and air controls etc). Burning wood is fun. I even like hunting for wood, cutting, hauling, and splitting. My wife does the stacking. ;)

    When I am not going for the long burn I like to watch the fire. I like the look of a slow burn.

    If I can look up at the chimney and not see smoke then I am happy with the efficiency of the stove. My EPA stove does this very well.

    If I can get through a cord or two before having to run the brush through then I am not concerned about the rate of creosote formation.

    I burn mostly crap wood because I get all my food free off my land and my neighbors. So a cat stove is not going to work well for me.

    I think that burning wood gives us a feeling of control over at least one aspect of our lives.

    Like a caveman, I can burn this piece of wood to stay warm. :rolleyes:

    Then I will have more money for gas in my snowmobile! ;em

    MnDave
  19. MnDave

    MnDave Guest

    Good stuff! I follow what you are saying. Do you think that just going to a known good long burn air control setting is different than incrementing to that setting or following a certain sequence of control operations to a known good setting?

    I am just asking because I am new to EPA stoves and the one I have tends to operate more like you are describing.

    I used a Kent Tile Fire for 10 years. It is pre-EPA but it has a secondary burn chamber vs tubes. I know that stove (in this house, with this chimney) like the back of my hand. I put a mark on the control and if I set it there I would get the same fire every time. I could load and go and come back and no surprise or I could go up and down and end up there and it would be the same. The good old smoky days :)

    MnDave
  20. MnDave

    MnDave Guest

    It is a 6" AC fan that I bought at Menards. It was too noisy at first until I had the ceiling box made bigger with an 8" outlet and used a reducer down to the fan. I have it set with a duct theremostat (from Menards) but I could not get that set right and decided to run it manually with a speed control switch.

    I thought of trying just a vent at first but I do not think it would have worked very well. That much horizontal will stop the flow. Even with this setup I wish it would move more heated air. I blame my use of a corrugated aluminum duct. I may insulate it. It is only about 5 feet long but the little fins on that currugated (for expansion) aluminum duct makes it behave like a heat sink in reverse. Argh!

    A long narrow vent could work if it does not have to travel horizontally. In fact 2 vents would be better. One "return" for air going down and one "supply" for
    air coming up. Obviously they would need to be in different parts of the room. Avoid putting the supply under a window or patio door.

    MnDave
  21. MnDave

    MnDave Guest

    I reread your post more carefully. That fire will burn as hot as you let it given the air control you provide.

    Radiation is the key heat transfer mode between the fire and the box. Convection and conduction are important but the radiation component is the most significant.

    Either way the radiation, convection, and conduction components all increase because of the increased thermal gradient from running the fan. That is good, good, good.

    It is up to you to keep the fire hot with the air control.

    The higher the thermal gradient the more energy you get out of the fire. That equals higher efficiency. Otherwise why not insulate your stove so the fire burns even hotter?

    Hope this helps.

    MnDave
  22. MnDave

    MnDave Guest

    I have to go work on a snowmobile. We do not have snow so all the fun I get right now is from tinkering with sleds and burning wood.

    I'll get back this evening. I am kind of a late bird.

    MnDave
  23. Huntindog1

    Huntindog1 Minister of Fire

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    If you use good dry wood on a good bed of coals its makes everything very forgiving and very easy to operate the stove.

    If your wood supply is higher in moisture then the stove can get very picky on getting it going and you have to baby the air control.

    Most all of these stoves have been tested to operate fine its just that most people think they have good wood and what they actually have is almost good wood so the stove has to be played with alot to get it going.

    This simplifies it, the whole thing, alot.

    Its all about the heat , its all about the heat in the firebox, you have the heat and it will burn. More heat you have the more efficiently it burns. They insulated the firebox with fire brick to get the heat up in the firebox. Some stoves use insulating firebrick to more easily get the heat up in the fire box and to maintain heat in the fire box at low burn rates.
  24. oldspark

    oldspark Guest

    WOW, what works better in your house, two different wood burners in my house and both of them worked way better with the fan. Big area to heat on stove in middle of house. Over thinking will make your head hurt.
    Huntindog1 and nate379 like this.
  25. nate379

    nate379 Guest

    How about wood furnaces like the Hot Blast that use a fairly high cfm blower to move air through ducting? Following the theory that a stove top fan cools the stove "too much", that type of stove would be able to freeze your beer I guess? ;)

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