Shallow Coal Bed or Fire Pot Designs... Why?

I'd like to know why so many handfired coal stoves have such a shallow depth to the fire box, fire pot, or fire bricks, etc?

For instance, my Jotul 507 holds only about 30LB of coal if filled up beyond the fire brick to the level of the loading door. On a cold night, running hot, it's possible to have it go out overnight or get close to going out. And I'd say to myself, "If I could just fill it a little higher..."

Second example, my Chubby stove (first season with it this coming fall). They say you can get 12+ hour burn times, but I can already tell the fire pot is pretty shallow and I question if I'll make it overnight. There's nothing worse than waking up to the stove out, or unsalvagabley low, and you have to turn on the oil heat and go to work knowing your wasting expensive fuel at home and have to start the hour long start up proces when you get home.

So on and so forth, the stoves I look at online appear to have inadequate height to the burn chamber. Warm morning stoves look like they fill a little higher than the Jotul and the Chubby, but I've never seen one in person to verify. Is it just cost of materials that manufacturers cut the heights down on these stoves? Or is there some metallurgical or physics-related issue with having taller coal beds. Woudn't get enough air flow through the coal bed for instance? It seems to me if my Jotul 507 or Chubby burn chambers were 1 1/2 or 2 times their height, you'd be able to fill the thing with 80 LB or coal, shake it down every 12 hours, and never have to worry about it going out overnight or while at work.

I've seen a modification someone did to a Jotul 507, taking the burn plates out and putting fire brick up to loading door height. That'd help but wouldn't change the capacity of the stove. The Chubby appears to be constructed of two ring-rolled pieces of sheet metal welded together with seem around it. If you could add and weld another ring on the top and extend the height of the stove by 1/3, welding the loading door opening shut and adding a new loading door opening above, you could line the walls with firebrick and refractory cement and get that extra capacity. But all of these types of modifications would be just to compensate for lack of capacity in the stove's design. Why?

Opinions on this?

How shallow? 9-12 inches should be plenty.

Stoves are space heaters and not meant to heat a whole home. An over-sized space heater can sometimes get the job done.
There are different size stoves built to heat different size spaces...one needs only to match up the two in order to get satisfactory results in that space.

Then comes in the need to distribute all that heat effectively or another/new issue will arrive.
No one wants to build a stove to sell and then the owner has his home burn down with it.
It's all in what one wants.

First, to burn any solid fuel with a 12 hour or more fuel supply in the combustion chamber is bad practice in terms of efficiency and clean burning. That it works as well as it does is owing to the particular properties of anthracite coal, which are the relatively small amount of gas in the coal compared to wood or bit coal, and that when burned in an oxygen starved manner it produces carbon monoxide gas rather than smoke. Both smoke and CO indicate incomplete combustion.

The deeper the coal bed the more likely it is that more areas will be deprived of air and more Co produced, yet a minimum depth is needed to maintain adequate heat through the bed. This would be about 5 inches for pea coal and 6 to 8 inches for nut which has larger air spaces between each piece. The shape of the fire pot and how well insulated it is will govern the internal heat and the uniformity of air distribution through it.

A stove has to burn the fuel but it also has to get the heat out which means heat exchange area to transfer that heat to the room. Combustion works best keeping the heat in while heat exchange means getting it out. Using a deeper fire pot means that more of it will function as a heat exchanger with lower burning efficiency and higher stack temperature.

A stove like the French Godin is almost all fire pot. The Warm Morning provides corner chimneys in the fuel bed so that the central coal mass is almost like a hopper with easy egress for the flue gases through the chimneys.

Very high efficiency's are possible with a stove that best combines the conflicting needs of combustion and heat exchange. The design that does that best, in my opinion, is a stove like William's new Crawford number 40. The fire pot is a bit deeper than an Oak design to compensate for it being a bit narrower. It is brick lined and the flue gas circulates down around it to maintain high temperatures while at the same time making full use of all available heat exchange area better than most other designs.

The Chubby will give 12 hour burns at a moderate heat output. If that is not enough then you need a larger stove. Making it hold more coal would make it burn longer but efficiency would slip and managing initial gas release would make for problems.

franco b wrote:First, to burn any solid fuel with a 12 hour or more fuel supply in the combustion chamber is bad practice in terms of efficiency and clean burning. That it works as well as it does is owing to the particular properties of anthracite coal, which are the relatively small amount of gas in the coal compared to wood or bit coal, and that when burned in an oxygen starved manner it produces carbon monoxide gas rather than smoke. Both smoke and CO indicate incomplete combustion.

The deeper the coal bed the more likely it is that more areas will be deprived of air and more Co produced, yet a minimum depth is needed to maintain adequate heat through the bed. This would be about 5 inches for pea coal and 6 to 8 inches for nut which has larger air spaces between each piece. The shape of the fire pot and how well insulated it is will govern the internal heat and the uniformity of air distribution through it.

A stove has to burn the fuel but it also has to get the heat out which means heat exchange area to transfer that heat to the room. Combustion works best keeping the heat in while heat exchange means getting it out. Using a deeper fire pot means that more of it will function as a heat exchanger with lower burning efficiency and higher stack temperature.

A stove like the French Godin is almost all fire pot. The Warm Morning provides corner chimneys in the fuel bed so that the central coal mass is almost like a hopper with easy egress for the flue gases through the chimneys.

Very high efficiency's are possible with a stove that best combines the conflicting needs of combustion and heat exchange. The design that does that best, in my opinion, is a stove like William's new Crawford number 40. The fire pot is a bit deeper than an Oak design to compensate for it being a bit narrower. It is brick lined and the flue gas circulates down around it to maintain high temperatures while at the same time making full use of all available heat exchange area better than most other designs.

The Chubby will give 12 hour burns at a moderate heat output. If that is not enough then you need a larger stove. Making it hold more coal would make it burn longer but efficiency would slip and managing initial gas release would make for problems.

Excellent info, thanks. Do you have a picture of a William's #40? I've never heard of it. And, are the corner chimneys in the Warm Morning built into the fire brick? Like holes cast into the brick? Does it allow air from the top of the chamber down to the bottom of the coal bed, like some reverse circulation? Woudn't that just create bypass of draft air going around the coal bed rather than through it?

Here is one link. Okay, I Think This Will Be the Last One, Maybe.

Put Crawford 40 in the search box and also Glenwood 9 which is a similar design.

The Glenwood no. 6 base heater is an Oak stove with the addition of a fire brick lined fire pot as standard equipment and an extended heat exchange passage to better use an area for heat exchange(the ash compartment and under it as well) that normally is much cooler. An Oak stove is one that has the fire pot externals exposed to the outside with sheet metal cylinder above it for heat exchange. The no. 6 also has an internal perforated ring situated above the fire pot which feeds hotter air to better burn initial gasses as well as any CO formed.

An ordinary Oak stove design as well as pot belly or cannon stoves have unlined fire pots so lose more heat from that pot which makes it harder to maintain a low fire and a more uniform burn. The edges always cool faster.

Another design that gets high efficiency at the expense of long burn times is to use a shallow fire pot which in order to maintain output has to burn hotter. Burning hotter in a shallow bed means it is easier to mix air uniformly with less CO going unburned. Stoves like this are the Franco Belge and the Surdiac. Both stoves use extended flue passages for heat exchange and a hopper to slightly extend burn time and to pre condition the coal for quicker recovery time after tending. They also use thermostats to control the air.

The antique base burners used a fire pot enclosed within the stove body (like the Chubby) which also provides a measure of insulation. There are extended flue passages as well as most having a magazine which is what a hopper was called in the antiques. In addition to pre conditioning the coal a magazine in the center of the stove acts like a baffle to force the flue gas closer to the heat exchange surfaces. Many Oak stoves also had a magazine as an option.

The chimneys in the Warm Morning do not extend to the bottom but start above the grate.

Thanks franco for the explaination. I might add that the Warm Morning is designed principally for Bituminous Coal. The four flues are designed to force heated secondary air into contact with the hydrocarbons of burning Bituminous. You can burn Anthracite in these just fine but they weren't really designed for it.

The base heater designs were delevoped to answer the very questions you were thinking about. Base Heaters were expressly made to cater to the exact combustion characteristics of Anthracite Coal.

Is there any modern, still in production stove that matches the base heater in design and function in burning anthracite? Seems like the baseheater is this ancient relic that the average joe can't find. I think I've only seen one or two on Craigs in the last 12 months.

I would say that the Vigilant ll from Vermont Castings is not very far from the base burner in some points.
Instead of going down to the base like a base burner, the gases are splitted in 2 parts at the top of the stove, one part to right and one to the left of the stove. Then the gases have to go down in the side chambers to make a U turn to the back/rear of the stove, there both flues join and go up in a Z shaped chamber from where they escape in the flue pipe.
The long gases path send the heat in the house instead of directly in the chimney, about like a base burner. But a real base burner, I mean an antique one is very hard to beat, I tried both and can easily vote for the antique one.
That said, a Vigll is a good modern and nice stove.

Should've bought a DS. My DS 1600 holds 120+ pounds of coal in the bed. I never have to worry about it going out running it over 500*.

I'm On Fire wrote:Should've bought a DS. My DS 1600 holds 120+ pounds of coal in the bed. I never have to worry about it going out running it over 500*.

How many square feet of living space are you heating (upstairs and down if floor vents or relying on it to heat other areas than the room it's in)? And, how many pounds a day are you averaging running at 500 degrees? And, for curiosity sake, how much does a DS 1600 cost? I paid $350 for the Jotul and $400 for the Chubby. I sunk some repair costs into the Chubby (new fire pot and grate, gaskets, mica, sandblasting, paint, etc) so it's cost me about $700 when all is said and done I think.

franco b wrote:First, to burn any solid fuel with a 12 hour or more fuel supply in the combustion chamber is bad practice in terms of efficiency and clean burning. That it works as well as it does is owing to the particular properties of anthracite coal, which are the relatively small amount of gas in the coal compared to wood or bit coal, and that when burned in an oxygen starved manner it produces carbon monoxide gas rather than smoke. Both smoke and CO indicate incomplete combustion.

The deeper the coal bed the more likely it is that more areas will be deprived of air and more Co produced, yet a minimum depth is needed to maintain adequate heat through the bed. This would be about 5 inches for pea coal and 6 to 8 inches for nut which has larger air spaces between each piece. The shape of the fire pot and how well insulated it is will govern the internal heat and the uniformity of air distribution through it.

A stove has to burn the fuel but it also has to get the heat out which means heat exchange area to transfer that heat to the room. Combustion works best keeping the heat in while heat exchange means getting it out. Using a deeper fire pot means that more of it will function as a heat exchanger with lower burning efficiency and higher stack temperature.

A stove like the French Godin is almost all fire pot. The Warm Morning provides corner chimneys in the fuel bed so that the central coal mass is almost like a hopper with easy egress for the flue gases through the chimneys.

Very high efficiency's are possible with a stove that best combines the conflicting needs of combustion and heat exchange. The design that does that best, in my opinion, is a stove like William's new Crawford number 40. The fire pot is a bit deeper than an Oak design to compensate for it being a bit narrower. It is brick lined and the flue gas circulates down around it to maintain high temperatures while at the same time making full use of all available heat exchange area better than most other designs.

The Chubby will give 12 hour burns at a moderate heat output. If that is not enough then you need a larger stove. Making it hold more coal would make it burn longer but efficiency would slip and managing initial gas release would make for problems.

This was pretty damb spot on info here^^^^^ , it is indeed physics of coal burning that dictates the "why" manufactures do what they do (except for those idiots that make big, square football fields with a 4 inch depth (those are just plain stupid manufactures.... but rich so maybe they not so stupid). anyways... their is a definitive perfect burning coal bed and dimensions are kind of important for this (Franco hits it pretty good above 6-8 inches for nut) this can be expanded slightly or reduced slightly determined by the width of the coal bed and how efficient it is (IE: secondary air, suspended pot, etc.)

JRLearned wrote:

I'm On Fire wrote:Should've bought a DS. My DS 1600 holds 120+ pounds of coal in the bed. I never have to worry about it going out running it over 500*.

How many square feet of living space are you heating (upstairs and down if floor vents or relying on it to heat other areas than the room it's in)? And, how many pounds a day are you averaging running at 500 degrees? And, for curiosity sake, how much does a DS 1600 cost? I paid $350 for the Jotul and $400 for the Chubby. I sunk some repair costs into the Chubby (new fire pot and grate, gaskets, mica, sandblasting, paint, etc) so it's cost me about $700 when all is said and done I think.

The stove ran me $1750 and some change. It his 120# in the bed, another 40-60# in the hopper and at 500° it will go through about 60#s. The stove sits in my living room and heats it to a cozy 75°+. My house is only one floor and about 1200 sq. ft. Last season I ran it above 500°s a few times.