Wood volume versus heat

[48rob]

Lets say I have a good bed of hot coals, and I place 1 piece of wood on it and set the air intake to 1/2.

The stove will produce X amount of heat, for example lets say 400 degrees stovetop on that setting until the fuel runs out.

What happens if I put two pieces on the coals, instead of one, with the same air setting.

Will the stovetop temperatures be higher than 400, or will the stovetop temps be the same, but be maintained for a longer period due to more fuel

Rob

 

[pgmr]

I'd guess hotter and longer. One piece of wood doesn't burn nearly as well as two or more. Multiple pieces give mutual support and burn more easily and hotter.

 

[woodchip]

Two bits with a small gap between will tend to pull air through the gap and really warm up well. The burn time with additional wood in the box will probably come down to the amount of air allowed in.

I'm still experimenting with my new stove after a couple of months, and suspect I'll enjoy experimenting all winter.

Perfect excuse to stay near the stove when it's cold outside.........

 

[48rob]

Thanks!

I'm trying to wrap my mind around the concept of why larger cubic foot stoves get too hot for smaller spaces.

Rob

 

[fortydegnorth]

Seems to me it's not necessarily the temp of the stove that will overheat the room as much as it is the duration that high temp is maintaned. The larger fire box, or more wood, may not get any hotter but it will burn at that high temp much longer overheating the room. The smaller firebox will lose heat faster as the wood runs out allowing a quicker cycle to cool the room. My 2 cents and best guess.

 

[Slow1]

I expect that the rate of burn will be related to the surface area of the fuel (assuming sufficient air) - thus two pieces will generate more heat than one (even if total mass of the two equals the mass of one in a different identical setup).

So take that to the question at hand about larger stoves - If you have a larger firebox you are more likely to put more pieces in and thus have greater surface area and a corresponding greater rate of burn. Even if you put the same number of pieces (but they are larger) you will again have greater surface area exposed. Now I'll bet that isn't the only factor but it is sure to be a significant one.

Add that to the fact that you will have more fuel in the stove so the burn can (and likely will) maintain higher temperatures longer and will thus more likely end up with a warmer room.

Of course if you just don't put as much wood in the stove then it is less likely - but then you may not get the stove up to temp to burn clean (depending on just how large the stove is and how little wood you put it...).

All speculation of course..

 

[joecool85]

The other reason large stoves overheat a room is more surface area on the stove to conduct heat from the stove to the air. A 12" cube at 400 degrees won't radiate at all as much heat as a 36" cube at 400 degrees.

 

[Battenkiller]

I expect that the rate of burn will be related to the surface area of the fuel (assuming sufficient air) - thus two pieces will generate more heat than one (even if total mass of the two equals the mass of one in a different identical setup).

So take that to the question at hand about larger stoves - If you have a larger firebox you are more likely to put more pieces in and thus have greater surface area and a corresponding greater rate of burn. Even if you put the same number of pieces (but they are larger) you will again have greater surface area exposed. Now I'll bet that isn't the only factor but it is sure to be a significant one.

I agree with what Slow is saying here. Two small splits burn faster than one large one of the same mass, so the rate of heat output will be greater, but for a lesser time period. That can be verified during just about any burn. It is somewhat dependent upon load configuration as well. Those same two splits should burn a lot faster if they are touching each other in a way that the flame path winds thorough them than if they are separate from each other.

Also, more wood means there is more flame closer to the walls of the stove, which should favor the smaller firebox. However, large fireboxes with a full load give off heat at the highest rate. I believe that may be because:


1. There is more burning mass in the stove
2. That mass is closer to the interior walls
3. More wood burning inside will create hotter internal temps, improving burn efficiency
3. What JoeCool said about the larger radiating surfaces of larger fireboxes.

Take home message?

Get a stove that is properly sized for 90% of your burning since you will get the hottest, cleanest, most efficient burns with a fairly full box. IMHO that's a much better option than "building a small fire in a big box" almost all of the time because you deliberately oversized your stove just so you'll have the extra firepower on maybe 10 days of the season when the mercury drops below 0º.

 

[branchburner]

Get a stove that is properly sized for 90% of your burning since you will get the hottest, cleanest, most efficient burns with a fairly full box. IMHO that's a much better option than "building a small fire in a big box" almost all of the time because you deliberately oversized your stove just so you'll have the extra firepower on maybe 10 days of the season when the mercury drops below 0º.

Maybe more of those cold days than ten, when you factor in the wind. Today, for example, isn't a typical 20F day - the winds of 20MPH make it closer to a 0F day. And my house knows it!

So maybe two smaller stoves are better than one bigger one.

 

[Renovation]

Thanks!

I'm trying to wrap my mind around the concept of why larger cubic foot stoves get too hot for smaller spaces.

Rob

Hi Rob,

This is just the sort of comprehension-challenging question I like, for it requires one to balance a number of factors, and understand basic principles and how they interact.

I think Slow1 is on the right track. Here are some factors:

1. The larger the stove, the larger the radiating surface, and the more heat it radiates at a given stove temperature.

2. An EPA stove has to reach a certain minimum temperature to burn cleanly (e.g. secondary ignition).

3. Cat stoves generally have lower temperature/primaryair/burnrate requirements for clean burning.

1 & 2 Imply that a larger EPA stove's heat output at minimum clean burn will be greater, perhaps too much for a small home, particularly at moderate outdoor temps.

3 Implies that a larger cat EPA stove can have less heat output than a non-cat EPA at minimum clean burn, allowing more versatility and perhaps not overheating a smaller home.

Just for fun and more mind-expansion, my belief is that, if the single stick is already burning all the air at a given primary setting, adding a second split will--to a first order--result in a longer burn at the same temp, since the burn rate is being controlled by the primary air. Yes, there are secondary effects that may raise temps and burn rate somewhat, but this primary effect is the reason some stoves with huge fireboxes can be dialed down for absurdly long burns.

Comments?

I HTH, and that you had fun with this, as I did. Thanks for asking the question, Rob.

 

[48rob]

George and all,

Thank you for trying to help me understand this.

Since stove manufacturers and experienced burners alike recommend small stoves for small spaces...I figure there must be something to it.
However, if a stove is burning a piece of wood, and is x degrees, it just naturally seems that more wood, with the air adjusted accordingly, should produce the same temperatures for a longer period, thus not overheating the space.

If I put a large round in my small sove, it will burn a long time at a set temp, determined by the available air.
If there was room for two or three large rounds...

Just for fun and more mind-expansion, my belief is that, if the single stick is already burning all the air at a given primary setting, adding a second split will—to a first order—result in a longer burn at the same temp, since the burn rate is being controlled by the primary air. Yes, there are secondary effects that may raise temps and burn rate somewhat, but this primary effect is the reason some stoves with huge fireboxes can be dialed down for absurdly long burns.

I can see the extra radiated heat, due to the larger surface area radiating it, that makes sense.

1. The larger the stove, the larger the radiating surface, and the more heat it radiates at a given stove temperature.

2. An EPA stove has to reach a certain minimum temperature to burn cleanly (e.g. secondary ignition).

3. Cat stoves generally have lower temperature/primaryair/burnrate requirements for clean burning.

1 & 2 Imply that a larger EPA stove’s heat output at minimum clean burn will be greater, perhaps too much for a small home, particularly at moderate outdoor temps.

3 Implies that a larger cat EPA stove can have less heat output than a non-cat EPA at minimum clean burn, allowing more versatility and perhaps not overheating a smaller home.

1-2- and 3 all make sense.

I was told that a cat stove would burn longer... and so it must be that lower temps would allow more fuel capacity.

#2 Is pretty important, and make a big difference I suppose, in a 1 cubic foot as opposed to a 4 cubic foot.


I have a much better understanding of the theories/realities now.

Rob