Wood stove firebox size compared to heat output

[Oldman47]

I will not disagree with what you say except from one point. The convection starts with conduction. Without conduction the outside surface of the stove never gets above room temperature. That heat transfer must happen before you get any convection currents, unless you use a fan. The stove surface temperature is after both convection and conduction have been in play. You got convection to carry the heat from the fire to the box surface and then conduction through the metal itself.

Physics aside, yes I understand it quite well, until you have a significant temperature difference the practical heat transfer is negligible. A 90 degree stove in a 70 degree room basically provides no real heat to the room air. It takes a serious temperature difference to drive the heat transfer that we use for heating and that is where the external temperature at local spots on the stove becomes very significant. With the heat transfer of radiation varying with the 4th power of distance, the small temperature differences we are able to create in a wood stove do not get much from radiative heat transfer. Yes, you can feel the radiation at a fair distance but you can feel a 2 degrees air temperature difference too. That bit you feel from radiation is because it is raising your skin temperature a trivial amount.

Heat transfer is driven, as you suggested, by convection but that relies on first heating the air right next to the stove surface and then providing an efficient air flow path to carry that heat to where it is wanted.

 

[TimberWolf530]

This is where I get lost....

If you set the stove at the 500 I mentioned in the previous post... and a small stove gives off the same 500 degrees but for a shorter duration, wont they heat the same space the same if I keep the smaller stove fed to maintain that 500 for as long as a larger stove would burn a load of wood?

What Im really wondering is why one would buy a small stove and and sacrifice needing to feed it constantly when they can buy a larger stove and maintain the same temp for a longer period of time? Can you buy to large of a stove if you can control the heat output?

Here is an experiment you can try at home to illustrate why this works. Next night its below 20 degrees, start 2 fires in your back yard: 1 with just one 4" split, and another with 20 of them. Let them get going then strip down naked. Stand next to the 1 log fire, then by the 20 log fire. I think you will see a tremendous amount of difference in the heat each puts out. The heat they produce is the same temp, but with more wood, more of it is produced. OK, so the naked part is optional, and that's an extreme example, but you get the picture, right?

 

[Whitespider]

With a wood space heater, we are heating the air around the stove/insert and we only use two of the three methods of heat transfer. Radiant and Convection. Conduction isn't used...

That's incorrect... radiant heat does not warm air, it passes through it (mostly) until it strikes a solid surface which absorbs the heat. The warmed solid object than warms the air via conduction, and the warmed air rises via convection (and is replaced with cooler air to be warmed via contact/conduction). A wood stove only warms air directly via conduction from the steel to the air molecules that come in direct contact with it... not radiation (not enough to be noticeable anyway).

A space heater wood stove uses multiple instances of all three mechanisms to warm a room... radiation, convection, and conduction.

You got convection to carry the heat from the fire to the box surface...

Not really. Convection may move heated gasses to the top of the box where it may conduct a little heat to the steel before it passes out the flue... but only the gas molecules that actually come in contact with the steel. Radiation is the main heat transfer mechanism from the fire to the inner box surface. From there conduction carries it to the outer box surface. The outer box surface warms air via conduction, and also radiates heat to solid objects, which in turn warm the air via conduction. The warmed air then rises do to convection, which brings more cool air in contact with the box or the solid object warmed by radiation.

And yes... I stayed at a Holiday Inn Express once
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[Oldman47]

A space heater wood stove uses multiple instances of all three mechanisms to warm a room... radiation, convection, and conduction.


Not really. Convection may move heated gasses to the top of the box where it may conduct a little heat to the steel before it passes out the flue... but only the gas molecules that actually come in contact with the steel. Radiation is the main heat transfer mechanism from the fire to the inner box surface. From there conduction carries it to the outer box surface. The outer box surface warms air via conduction, and also radiates heat to solid objects, which in turn warm the air via conduction. The warmed air then rises do to convection, which brings more cool air in contact with the box or the solid object warmed by radiation.

And yes... I stayed at a Holiday Inn Express once
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WS, your statement is mostly true of a typical smoke dragon from many years ago, but the convoluted passages in a modern stove are that way intentionally to promote better heat transfer by routing the hot gasses across the stove surfaces. In my own secondary combustion type stove there is fire brick between the combustion chamber and the stove surfaces on top, bottom and 3 sides. The only side where radiation can play a significant part is the front where they have left it open for those of us that like to watch the fire, not for better efficiency. The hot gas passages are between those pieces of fire brick and the metal surfaces.

No, I have never stayed at a Holiday Inn Express.

 

[Whitespider]

...convoluted passages in a modern stove are that way intentionally to promote better heat transfer by routing the hot gasses across the stove surfaces. In my own secondary combustion type stove...

I find this curious... my secondary combustion stove has no convoluted passages for combustion gasses... what stove do you have??
In mine the brick sits directly in contact with the sides and back, there's a baffle in the top that supplies combustion air to the firebox. As the smoke passes by the baffle it ignites and burns as it travels between the baffle and stove top on its way to the flue. The main mechanism for heat transfer to the stove top is radiation from the burning smoke and gasses, it has little to do with conduction. Conduction requires the gas molecules to come directly in contact with the steel... gas molecules are spread far apart, the hotter it is the further apart they are. Meaning, not a lot of those molecules actually contact the steel at one time, and for any length of time (required for conduction). Any hot molecules that travel through the center of the passage (not contacting the steel) cannot conduct heat to the steel... however, they can radiate heat to the steel.
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