Wood stove firebox size compared to heat output

[CaseyForrest]

I think I need this explained to me in terms I can wrap my mind around.

I understand that a larger firebox can have more available BTU's put into it....

But does that always translate into a hotter stove? Why does a larger firebox have the ability to heat more space compared to a smaller firebox WHILE increasing burn times? I get that a larger firebox will hold more wood, but how does it hold more wood, heat a larger space AND have the ability to throw off heat for longer periods of time?

 

[Old Doug]

I wish my stove had a shorter fire boxes. I let my fire go out often and it takes along time to get it hot so the blower comes on. I would like to buy a diffrent stove something with shorter fire box any stove would be shorter.

 

[sunfish]

Larger fire box = larger fire = more mass = more BTU's

 

[066blaster]

I have a blower on each side, and The fire box is big. I have had to push wood/coals to the sides to keep it blowing hot, just a fire in the center doesnt cut it. I wish it was Not so wide. I would say the only benefit is being able to use longer pieces of wood and still turn them sideways a little. That and maybe you don't have to clean out as often

 

[brenndatomu]

Bigger stove means more surface area to exchange heat. And longer burn times because you are loading more wood but controlling the combustion air to roughly the same rate as a smaller stove. Think of it like putting a bigger fuel tank on your truck, uses the same amount of fuel, just more of it on board so it lasts longer

 

[BoBDoG2o02]

Bigger also means more surface area for radiant and convective heat to escape the stove....

 

[BoBDoG2o02]

Damn, beat me to it.

 

[CaseyForrest]

Larger fire box = larger fire = more mass = more BTU's

Now throw in an airtight stove where you can control the heat output.

Lets say you set the air to give a stovetop temp of 500 degrees. I get how the larger firebox will then add to a longer burn since there is more fuel...

But how then does it heat a larger space? Is it that it can put off 500 degrees for a longer period of time?

 

[CaseyForrest]

Bigger stove means more surface area to exchange heat. And longer burn times because you are loading more wood but controlling the combustion air to roughly the same rate as a smaller stove. Think of it like putting a bigger fuel tank on your truck, uses the same amount of fuel, just more of it on board so it lasts longer

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?

 

[brenndatomu]

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?

I suppose because of price, or physically can't fit the stove where it needs to go.
Too large? Probably hard to do with a EPA firebox, could go too big on an old airtight, and make it into a creosote factory by trying to "control" it with less air instead of less wood

 

[CaseyForrest]

I suppose because of price, or physically can't fit the stove where it needs to go.
Too large? Probably hard to do with a EPA firebox, could go too big on an old airtight, and make it into a creosote factory by trying to "control" it with less air instead of less wood

EPA firebox... are you referring to a catalytic stove or one thats got secondary combustion? Our Resolute has a bypass damper than when closed sends the exhaust through a fiber refractory.

Id love to have a catalytic stove but I find all the current offerings to be less than aesthetically pleasing...

EDIT: we also have a unique situation in that all four sides of our stove are visible. So having something large and ugly on what would be the back of the stove is not desirable.

 

[brenndatomu]

EPA firebox... are you referring to a catalytic stove or one thats got secondary combustion? Our Resolute has a bypass damper than when closed sends the exhaust through a fiber refractory.

Yeah, secondary "tube" type stove

 

[Magic_Man]

If both stoves are kept at a 500 degree temp the larger stove will have more surface area to transfer that heat there for more available btu's.

 

[olympyk_999]

don't really know if this is accurate or not...but I would think that with a larger stove, the "bigger" fire wouldn't need to run as "hot" to achieve the same 500 deg. temp. since there is more fire to make the heat...the slower burning "bigger" fire will burn longer than the faster burning smaller fire...
and the same as others said about the heat transfer of the larger surface area of the bigger stove.

 

[CaseyForrest]

Makes sense.


Sent from a field

 

[Iaff113]

Does your big stove have firebrick lining it?


Sent from my iPhone using Tapatalk

 

[Whitespider]

Why does a larger firebox have the ability to heat more space compared to a smaller firebox WHILE increasing burn times? I get that a larger firebox will hold more wood, but how does it hold more wood, heat a larger space AND have the ability to throw off heat for longer periods of time?

Magic.

It's all relative. If all else remains equal (such as damper settings) a stick of firewood will take x-amount of time to burn. It don't matter squat if there's three more sticks in the box with it, or ten more sticks, burn time will be the same (at least close enough for this subject). However, the box with ten sticks will make more heat over the same time period. Meaning, if all else remains the same except fuel load, the larger fuel load will make more BTUs per hour... but won't give you more hours.

If you don't need the extra BTUs per hour, you can reduce the burn rate in the larger box so it makes the same hourly BTUs as the smaller box... and you get a longer burn. So, if you load both boxes full and set both to a stove top temp of 500°, the larger box is burning at a slower rate... which translates into a longer burn time with same per hour heat rate of the smaller stove.

The problem with a too small stove is the need to run it at a faster burn rate (faster than a larger stove) to keep up with heat demand... efficiency of any stove, whether a new-fangled EPA certified or an old smoke-dragon, is reduced when running it at the higher burn rates. There's an efficiency sweet spot with most any stove, usually somewhere between the medium-low to medium-high settings... the further you deviate from the sweet spot, the further efficiency is reduced.
(Qualifier - I have no experience with CAT stoves.)

The problem often stated about a too large stove is you have to damp them down too much, which also reduces efficiency and they make creosote. That may be true in extreme cases... like if the box is as big as a car trunk. But really, all you need to do is use smaller fuel loads at the same basic settings during times of low heat demand... at the same setting, the smaller fuel load will make less heat per hour, but still burn for the same amount of hours (or at least close enough for this subject).

There ain't no magic... but you'll be very disappointed if you go too small.
*

 

[sunfish]

Now throw in an airtight stove where you can control the heat output.

Lets say you set the air to give a stovetop temp of 500 degrees. I get how the larger firebox will then add to a longer burn since there is more fuel...

But how then does it heat a larger space? Is it that it can put off 500 degrees for a longer period of time?

The larger stove will still radiate more heat, even if both read 500 degrees at the stove top.

I have a stove that's too big for our house and it's very hard to burn it efficiently unless it's below 20 degrees outside. Usually have to build small fire in the morning, let it go out and build another small fire at night. House too hot a lot of the time.

We've also had a stove that was too small. Worked great except for very cold spells.

Next stove we get will be the right size, maybe.

 

[Oldman47]

Sounds like magic to me sunfish. The surface area on a 3 foot and a 4 foot stove are almost the same and almost all heat comes from the surface. You might possibly notice a slight difference with equal surface temperatures but it would be hard to measure in the real world. As WS pointed out the biggest difference is likely to be the fuel efficiency of each stove at similar stove tops. Efficiency is going to be reflected by the varying temperatures of different parts of the stove. You are not heating with a big uniform block of material that holds a constant temperature over the whole surface. You are using a fire that varies a lot from place to place within that stove and that is affecting the outside surface temperatures.

 

[Marine5068]

Sounds like magic to me sunfish. The surface area on a 3 foot and a 4 foot stove are almost the same and almost all heat comes from the surface. You might possibly notice a slight difference with equal surface temperatures but it would be hard to measure in the real world. As WS pointed out the biggest difference is likely to be the fuel efficiency of each stove at similar stove tops. Efficiency is going to be reflected by the varying temperatures of different parts of the stove. You are not heating with a big uniform block of material that holds a constant temperature over the whole surface. You are using a fire that varies a lot from place to place within that stove and that is affecting the outside surface temperatures.

Yes all true and lots of variables that constantly change, but the physics of heat transfer stay the same. 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 (unless you touch the unit and get burnt...ouch!)
Bigger radiant/convection surface area = more heat exchanged to the surrounding air per hour. A larger firebox = more fuel = longer burn times per load.
So to answer your questions, bigger is better.