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Difference between "Radiant" and "Convection" stoves

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by spirilis, Oct 21, 2009.

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  1. spirilis

    spirilis Feeling the Heat

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    I noticed Morso makes both radiant & convection stoves, and I was wondering what the big difference was. Radiant stoves make sense, the outer castings or steel plates get hot enough to radiate heat. What are convection stoves then? Do they just have a gap between the sealed firebox and the outer panels that allows heat to concentrate, and then a fan (or natural convection) moves hot air through this space?

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  2. Slow1

    Slow1 Minister of Fire

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    Seems that is the difference as far as I can tell - although some may actually have some other design features to better heat the air more than others. Bottom line is that the convection stoves are marketed as heating air and moving it out more than the radiant ones.

    From a practical standpoint those designed/marketed as convective tend to have greater on-stove shielding and generally have reduced clearances. Often times they have fans (or at least are designed to make better use of optional fans).
  3. Fsappo

    Fsappo New Member

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    When determining your heating needs, I use this as a rule of thumb:
    Radiant stoves heat objects in their line of sight
    Convection stoves have some radiant properties, mainly off the front of the stove but they also create hot air which can be moved about the house.
  4. Slow1

    Slow1 Minister of Fire

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    What I have found with both of my stoves (considered radiant models) is that I get significant convective heating along with the radiant. Why? Well - as Franks points out, the stove heats objects in their line of sight. This means the walls, floor (less so) and ceiling get warmed up as well as furniture, people (lucky ones who get to nap in front of the stove). Once all these things warm up, they warm up the air touching them and the air begins to rise and move on.

    In particular the walls and ceiling around the stove warm up and get a good solid convection loop running. I have actually hung bits of paper from the ceiling and seen them flutter as well as measured the temperature of said surfaces (350* stove gets these surfaces up to around 90* enough for good air movement). It is quite impressive the flow of air. Down side of course is that cold air is moving along the floor (hardwood in my case) toward the stove - this brings the dust bunnies. So during 24/7 heating season I have to vacuum this area on a regular basis or they become noticeable. Certainly the amount of this will be a function of your particular install and configuration.

    So there really isn't a purely radiant or convection stove - you will end up having the heat move about in more than one way. (Touch the stove and you will get some conduction in there too :) )
  5. spirilis

    spirilis Feeling the Heat

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    Nice, this is what I thought. My old 1980 VC Defiant works great as a convection heater for the bedrooms on the far side of the house, as it heats the living room ceiling to around 115F when the stovetop is around 500-600... hehe. I put a box fan in the hallway moving the cool air from the north-side (back-side) of the house over to the living room and it gets very comfy in the bedroom. Might be wasting a bit of wood doing that though. Especially since the living room has 2 huge sliding glass doors.

    Sounds like a convection stove would work quite well for folks who don't want their rooms excruciatingly hot, or for a small room where you're trying to heat up that room plus an adjacent room or 2 and want to move air around (and don't want any of those rooms getting too hot...)
  6. Slow1

    Slow1 Minister of Fire

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    That's a really good point - perhaps having a stove designed to pump out more hot air (convection) would be better for a smaller room so that you can them move that air out of the room without having to heat up the walls/objects as much. The corollary then being that larger rooms are better for larger rooms. I'm not sure though that a convective stove would be bad for a large room - it will still heat it up.

    One of my objectives was to avoid any fans both for the noise and for the additional need for power (energy consumption). I happen to be fortunate in that I have a good open location for the stove that is central to most of the home activity.
  7. grommal

    grommal Feeling the Heat

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    Indeed. "Radiant" stoves also heat all the air that's in the line of sight directly, too. That warmer air rises and goes wherever it can, depending on your layout.

    To me, the only real advantages of a "convection" stove would come in two cases only. First is when you require very tight clearances for installation, as the convection air gaps between the stove's "layers" can greatly reduce clearance requiremetns. The second is when the stove is going to be installed at the far end of a room that just isn't designed to help the movement of hot air out and cooler air in, but is not so bad in this regard that the convection feature would be ineffective anyway.

    If you can deal with the clearance requirements, and have a good house layout to allow the air to move around, I don't see any advantage of a convection stove. In fact, there are disadvantages, as the off-season dust does tend to collect in the convection space and is hard to clean out, and the fans that folks tend to use with convection designs do make at least some noise and require electricity to work.

    We are fortunate that the house we bought has a great heat moving design, even though we were not thinking about that when we bought it. The stove hearth is in a room with a vaulted ceiling so the heat has plenty of room to rise away from the stove. The ceiling rises to a peak at the opposite wall, where there's an opening (a balcony of sorts) to the second floor hallway. So, the heat just naturally rises and runs down the upstairs hall to all the bedrooms. Even when we're cranking the stove, there's no more than a 10 degree difference between sitting in the family room with the stove and sitting in the dining room at the opposite end of the first floor. We found out after we bought the house that the builder was a coal stove fanatic, so he obviously designed with heat distribution in mind. For our application, I doubt if a convection stove would improve the heat distribution at all.

    My neighbor across the street, who used to run a coal stove in his family room, has the opposite circumstance. The room is long and narrow, does not have a high ceiling, and the only opening to the rest of the house is at the opposite end of the length of the room. When he was burning, underwear was way too much clothing in that room, while you needed a coat in the dining room at the other end of the house. He's used fans, he cut vents through the walls and ceiling, and he ran the air handler of his HVAC system, but it didn't seem to help much. His case is so extreme, I'm not sure a convection stove would help much, either. I say "used to run a coal stove" because he simply gave up trying to move the heat around, sold his coal stove, and just relies on his heat pump now.
  8. morgantruce

    morgantruce New Member

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    I have noticed a HUGE difference between radiant and convection stoves in my house.

    My house is: two-storey, well-insulated, with the stove located right in the center of the first floor, and chimney rising straight up through the middle. A staircase rises to the 2nd floor along a side wall. I "cannot" run fans--the house is off-grid and uses photovoltaic solar cells to make electricity. What power I do have is far too valuable to waste on running fans.

    The first stove I had was a radiant-type box stove. At all levels of operation, the first floor was very cozy indeed. The 2nd floor, however remained quite cold. The upstairs bedrooms were uncomfortable to be in unless you were snuggled under a thick down quilt---no fun getting dressed or taking a shower. The heat refused to come up those stairs! I even resorted to cutting a hole in the ceiling directly above the stove--trying to coax some heat via a duct surrounding a section of stove pipe (real pretty...) No heat! You would have to run that stove wide open to get the 2nd floor "slightly warm"---but by then you could bake bread anywhere on the first floor. Needless to say, I did NOT put up with that situation very long!

    The second stove I had (and still own after 20 years) is a Jotul Alpha--a convection-type wood stove. Heating with this stove, house temperatures remain completely even. If it is 75 degrees downstairs, it is 75 degrees upstairs--same thing at 85 degrees. And this is even after removing the lovely hole in the ceiling/duct contraption!

    -------------

    Forget the elaborate theories... radiant heaters tend to work well heating the room in which they are located. In fact, the radiant stove I spoke of earlier, heated a one-room cabin very well. Convection stoves tend to spread their heat evenly to other rooms. Your mileage may vary due to strange things such as serpentine hallways--and other such features that do not belong in any house designed to be heated with wood.

    ---------

    The Side Show: radiant stoves can be used to actually cook a decent meal on; convection stoves are a poor choice for cooking---they are too busy heating and circulating air to have much good effect on pots placed on top of them.
  9. begreen

    begreen Mooderator Staff Member

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    Good examples mt. Our house also has more even temps since switching to the T6. However, the stove, with its swing out trivets, is great for cooking on. Low heat with the trivet in place, high heat direct on the stove top. Works well.
  10. cycloxer

    cycloxer New Member

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    radiation = line of sight heat
    convection = hot air

    They both have their applications and all stoves do some of both.
  11. BrowningBAR

    BrowningBAR Minister of Fire

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  12. morgantruce

    morgantruce New Member

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    I just looked at the PE website. Your T6 does indeed bare a good deal of it's actual top when the trivets are swung out. My Jotul has extendable trivets, but the area of actual stovetop that is exposed is very small. The largest diameter post that can be placed in direct contact with my top is only 6 inches. I do have a small saucepan that fits "in" there---and it is indeed "high heat"---but a tiny saucepan prepares only a tiny un-complicated meal. I've been eye-balling a tall asparagus pot, but it's a losing battle---the Jotul Alpha just ain't much of a cooker. With the trivets "un-extended", I am able to place a very large stockpot on top of the stove for the purpose of heating water---but this pot is sitting atop of the "false top"... it is about two inches above the inside layer of cast iron that is actually exposed to the flame. In the course of the day, this water will get up to near scalding temperature---but never boiling. I routinely use this free hot water for washing dishes.

    As I've said earlier... my convection stove heats the house very well. Here's another thing the convection stove does NOT do well: I've located a 40-gallon water pre-heating tank very close to the woodstove... without good results. I figure that just about any radiant heater would do a much better job of heating something placed adjacent to it. My water preheating project, while amusing, is too long to discuss here. (even for a snowed-in day like this one!)
  13. VCBurner

    VCBurner Minister of Fire

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    "Hey Cycloxer I live in Paxton, MA! Is it just me or have our temperatures benn lower than New Hapshire, RI, Vermont and even Maine in certain cases. Do I live in the center of an ice box?"
  14. rdust

    rdust Minister of Fire

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    My Endeavor is a convection stove. You still get radiant heat from the front and the cook top and convection heat from the convection chamber around the stove. Our hearth room is small 11x16 that is open to an 11x12 kitchen. We had a requirement for a low clearance to combustibles due to the size of the room, we also wanted to limit the foot print of the stove. A convection style stove gave us what we wanted. The cook top can be running at 700* and I can still touch the side which is a big plus in my book. My TV is no more the 2' away from the side of the stove and never gets hot. We have a blower on it but the heat moves surprisingly well without it, we usually run it on low or medium when it gets in the 20's or lower to help move air around. We typically have a temp swing of 6-10* from the main floor to the upstair rooms of the house. Our staircase is L shaped so the heat has trouble making the turn upstairs, I think the convection of the stove helps to pull the cold air down.

    Both stoves have their place and the convection style stove seems to fit our needs best. The wife and I love the way it heats.
  15. cycloxer

    cycloxer New Member

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    Maybe, lol. Well last week was pretty cold. I am up in Harvard. In general, Worcester County gets its fair share of snow and cold. I can tell you that my brother was up in mid-VT this weekend and it was bitter cold - single digits with the wind blowing sideways. I was in S. NH up near Jaffrey and it was about 5 degrees colder than at my house. We are almost always colder than Boston metro and inner suburbia. So...it is all relative.

    I haven't yet had a day where I couldn't keep the inside at 70 with the Castine. The blower helps tremendously even though it is technically a radiant stove. I've found if you blow more air over the hot surfaces you can transfer more heat to the room.
  16. Martin Strand III

    Martin Strand III New Member

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    I’ve read through these posts and seen a lot of gum flapping, opinions, generalizations and few posts based on basic physics explaining the difference between radiation and the other two ways heat reaches equilibrium.

    Here’s an old goat’s interpretation on how part of our world works.

    First, realize “heat” is the result of motion between minute particles of matter and that it is constantly being exchanged in an endless circle wanting and seeking thermal equilibrium.  “Temperature” measures intensity of heat and, indirectly, the amount of motion in the particles.

    CONVECTION

    Gases, like the air we breathe, exchange heat mostly by convection, a means of flow, whereby cold gas particles have less movement and thus are closer together (read more dense) than warm air particles (less dense) accounting for the fact warm air rises and cold air sinks.  When a warm air particle (molecule) contacts a cold air particle, its higher heat energy in the form of molecular vibration is transferred to the cold air particle, which then becomes warmer and rises with its less dense cousins.  This energy transfer, per molecular vibrations, moves like a wave through the cooler particles as they warm up.  There is significant space between air molecules, so this process of molecular collisons passing on energy takes time and causes air currents as warmer molecules rise and cooler ones sink.  In the process, as the wamer particles contact fixed objects in the room, these absorb the vibration energy and begin to warm.  This method of heat transfer (convection) is indirect since the heat transfer goes through air containing much space between molecules. This continual movement of air from a hot stove causing convective air currents can result in drafty indoor conditions with temperature zones (aka “indoor weather").

    Liquids also exchange heat chiefly via convection.  Particles are closer together than in air but the same phenomenon occurs explaining why cooler water is near the bottom of a lake and warmer water on the surface.  But in this example, wind can blow warm surface water away resulting in the temporary surface feeling cold as colder water rises to replace the displaced warmer surface water.

    CONDUCTION

    Solids exchange heat by radiation and, with direct contact, conduction.  In its quest for equilibrium, heat transfers via conduction when two solid objects are in contact.  Vibrations from the warmer object molecules spread by contacting the cooler objects molecules thereby increasing their vibrations and heat which, spread like a wave through the object.  The degree of vibrations, from the amount of heat, can be substantial.  If a very hot object (hot metal stove) with considerable heat energy (lots of vibrating molecules) is touched by a finger, the molecules on the surface of the finger can vibrate so fast that those molecules separate resulting in a burn to the finger.

    Friction is another way to increase particle movement resulting in heat by conduction.  For example, experience the warmth generated by rubbing your hands together.

    RADIATION

    Heat transfer by radiation is a slightly different story.  To understand it, realize that any matter with a temperature above Absolute Zero (0* K, or -273* F) gives off “infrared radiation”, not seen in the visible spectrum of light, but explained by quantum mechanics as a stream of extremely small photons having properties of waves and particles, maybe both, maybe alternating between the two.  So small are these photons that it is debated (when I was in school, please, update me) whether they are pure energy or particles, either travelling at phenomenal speeds.  When these photons collide with other molecules or particles of matter, they cause increased particle movement and more heat.

    As mentioned, gases, like air, have relatively enormous amounts of space between the molecules of the gas.  Liquids have less space between molecules and solids even less (solids being generally more dense than liquids or a gas).  The tighter the molecules are packed together in a piece of matter, the easier it is to absorb any radiant heat photon which may strike it, making it warmer.  Conversely, in air, since the molecules are far apart making the chances of a radiant energy photon hitting the gas particle much smaller.

    Hot objects radiate photons of greater amplitude (like a sine wave) than cool objects.  A photon from a very hot source has a greater chance of colliding with a particle of matter, like an oxygen or nitrogen molecule in room air, in a given distance than a photon from a cooler source. 

    This explains why a hot metal stove at 550* F tends to heat the air around it and induce convection air currents versus a cooler stove at 180* F which will radiate photons with less amplitude, have less chance of colliding with air molecules over a given distance and can heat objects further away from the source than the hot metal stove. 

    That’s the story and I’m stickin’ to it.

    Aye,
    Marty
    Albert Einstein once said, “Nothing happens, until something moves.”
  17. morgantruce

    morgantruce New Member

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    Marty,

    Considering all of that, it's just amazing we manage to stay warm in front of our stoves.
  18. summit

    summit Minister of Fire

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    we ran one as a demo at the shop: neat sswing down side door, big heat exchanger, ripped out plenty of heat... although, the door hinges and latches were shoddy, and the baffle is cheese.
  19. BrowningBAR

    BrowningBAR Minister of Fire

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    Good, cuz I think I'm getting a Mansfield. I will be super excited if this happens as I never thought I'd be able to swing the cost of a stove like that so soon.
  20. brokeburner

    brokeburner New Member

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    man u way lost me with that
  21. NH_Wood

    NH_Wood Minister of Fire

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    I'm in Rindge, NH - next town south of Jaffery! Freakin' chilly lately and lots of snow blowing! Ah, the costs of living in God's Country!
  22. Bigg_Redd

    Bigg_Redd Minister of Fire

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    I turned my "radiant" PE Spectrum Classic (which is working great, BTW) into a "convection" unit by pointing a box fan at it.
  23. morgantruce

    morgantruce New Member

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    An old friend once took a very small fan (no bigger than your fist) and mounted it to a five-foot length of 4" aluminum duct. He then hung the fan at ceiling level--letting the duct hang down and aimed at the center of the top of of his wood stove. The small flow of air was aimed downward.

    The once ice-cold floors of his crude cabin became toasty warm. Actually, the whole cabin became comfortable. No longer did you have to huddle next to the stove to stay warm. As I recall, the tiny fan made almost no noise and you could not feel any air blowing around the room: a rather elegant piece of low-impact engineering.

    -------------

    Years later, this same fellow would go on to become the superintendent of the sewage treatment plant which handled most of the "output" of the three branches of our federal government--just imagine, if you will, what truly nasty stuff must have come from those Senate and House chambers...

    This fellow's frequently-offered advice was: "Don't get any on 'ya!"

    -----

    (apologies for the bi-partisan jabs at our elected officials...)
  24. pen

    pen There are some who call me...mod. Staff Member

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    Sorry, but I missed the "So, as a result of understanding this, I can conclude ........"

    Great physics lesson but you didn't make a point directly about the 2 types of wood stoves.


    pen
  25. jharkin

    jharkin Minister of Fire

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    Marty pretty much nailed it - only thing I would add is that you do get some heat transfer by conduction between gases as well, not only solids. The main difference between conduction and convection is that convection involves the movement of what is being heated (air, water or other fluid).

    Another real world application of this:
    The question of convection vs. radiation also came up over at the wall in regards to steam radiators. The discussion was about the value of adding reflective panels behind a radiator to prevent heat loss through the wall to the outside. One proponent was arguing that since radiators heat by radiation, and the radiation is equal in all directions relative to surface area that a reflective panel should reflect something like 40% more heat into the room. ut folks who had tried them reported minimal differences in the fuel bills.

    So eventually somebody found an actual research study done on them. The findings were that in the real world the panels saved maybe 5% with uninsulated walls and little to nothing with well insulated walls. The study found that for a steam radiator at 212F, over 80% of the heat transfer into the room was actually by convection and only the remaining 20% was direct radiation. (Also found that with good wall insulation the radiant effect of heating the wall contributed to heating the room, which explains why reflectors do more to help when your walls are uninsulated).

    The moral of the story... All stoves heat by radiation, convection and conduction - the difference being that stoves with blowers use forced convection to extract more heat by convection. So theoretically given 2 identical stoves, one with a fan one without- in order to maintain the same stove temperature the stove with the blower has to be burning more fuel (more air, reloads) and thus is putting more BTUs into the room. So I believe you should be able to increase the heat from any stove by adding a fan.

    -Jeremy
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