Size of your firebox, bed of coal

I'm trying to find out an approximate BTU output for a given size of coal fire. I have a huge firebox in my new boiler, and have been using a short 'wall' of firebrick to reduce the size of the area that contains coal.

Right now I'm burning a bed of coal approximately 22"x24"x 4"-8" deep. [the sides of the fire box taper in down to the bottom grate].

So if you can post with what size the area of your burning coal is and the rated BTU output of your stove/furnace/boiler I would find it helpful.

Along the same line, another question. My boiler is a manual feed [shovel it in] I seem to have a variation in the heat output from the bed of coals. What I mean is that when I add an additonal 3"-4" of coal to the exsisting fire, obviously the heat ourput drops for a short while, then there seems to be several hours of peak heat output when the 'fresh' layer of coal is initially burning. The combustion blower only runs maybe 20minutes per hour during this time. But after several hours when the entire bed of coal is glowing red, and has burnt down several inches, the heat output seems to drop quite a bit and the blower runs more like 40 minutes an hour to maintain a given water temp.

Is this what what other manual feed stoves burn like?? or do I need to shake the grate more often, or just leave it alone because this is the way coal burns??

With wood, the 'roaring fire' stage makes the most heat, followed by the bed of coals stage where the heat output drops off. So it appears that maybe coal does the same thing??

I thought that coal would have a more stable heat output, I'm surprised that the same 22"x24" bed of red glowing coal with 1-2" of blue flame is making less heat than the initial pile of black coal with a blue and yellow flame.

Just asking for opinions and your observations. I can get 6-8 hours of high heat from one load of coal, then the fan starts to run more per hour and the water temp eventually drops off as the bed of coal burns down. I can move my 'wall' of firebrick to make a bigger box of burning coal, I could go easily up to 22"x36" but I fear that this may be too much heat output. Maybe I'll try this next week when the temperature is supposed to be in the 'teens.

Thanks in advance for your replies.

Greg L. [LsFarm]

Well i don't know if this is gonna help u or not but on my stoker at max feed
The size of the coal bed is around 8 inches wide 1" thick and around 9" long
It's rated at 85K btu output but don't forget on a stoker the coal is constantly being replace with fresh coal.
According to harmans website 1 pound of coal produces 12,500 btu of heat. So u should be able to tell how many pounds u burn in a given time and figure roughly your getting 70% effeincy. It will atleast give u a ball park figure..

Thanks John,

So with the coal constantly being replaced, the coal on the grate would include the fresh just starting to ignite near the back, in the middle the really hot burning coal and near the front where it is about to fall off the grate into the pan, the 'burnt-out'coal. Is this right??

I'm amazed that the bed of burning coal is so small!! Maybe it is burning at a higher temp because of the shallow depth and constant combustion air feed??

How high and what color are the flames off the coal?? I know this varies, but how long is the coal burning on the grate from when it emerges from the pusher opening till it falls off into the ash pan?

My coal has more yellow flame during the first hour or so of burning, and from then on it has a few inches of blue flame over most of the coal.

Can't believe I'm saying this, but I'm looking forward to the colder weather next week, to see how well my boiler will work with a really big load of coal and wood.

Greg L

u are correct how a stoker works. my flames are blue and it's about 12 inches high.Hits the heat exchanger. But then again when u have air being forces thru it from underneath that helps alot. If i have the air intake restrictor plate fully open and it's really cold outside my flames will actually hit the top of the stove and come down the side of the stove as well. Yes it's a small buring area but it really produces alot of heat.I'm looking forward to the cold as well.Now that the feed motor was replaced i want to c what this stove will do.

Will i never time it as to how long the coal stays on the grates. It would be hard to till as well..The harman uses a unique design.The feed opening is actually located below the burning grates. it actually pushes the coal up a slope about 4 to 5 inches high then as the coal starts to come over the top it ignites..But once it gets cold again and the stove is constantly feeding coal i can c how long it lasts. But that will also vary depending how much heat i want from the stoker

Except when I have a fresh load of coal over a very thin layer of hot coals, I rarely see flames more than 4-5" high. I'm pretty sure the coal and ash block a lot of the slots on the grate which restricts the air flow through the coal somewhat.

I'll have to try to keep the grate 'well shaken' and see if I can get more air through the coal for more heat from the coal.

But of course your combustion blower is only pushing air through about 1" of coal, My bed of coal is at least 6" deep in the center over grate. So there is bound to be more restriction to air flow.

In the photo below I have three 18"x8" oak logs in the back of the firebox and a burnt-down load of coal in the front. The only combustion air is coming up through the coal, the grates are covered in the back 2/3 of the firebox. The hot air from the coal is really burning the oak logs well, there is zero smoke from the chimney.

The bed of coal has been burning for about 8 hours, I will add about 6-8" of coal on top before I call it a night. By morning the coal will have burnt down lower than in this photo, and the combustion blower will be running non-stop. The water temp will be around 140* with the aquastat set for 155*.

Next week I will probably extend the coal bed back another 10" or so to see if it will keep up with demand from my leaky old farmhouse.

Thanks again Greg L

Greg

What size coal are you burning.....also how many cfm is the combustion blower...maybe with that size firebox you would benefit from a larger size coal...or a mix of 2 sizes...

Maybe Coalman will see this and have some input...

your boiler is huge...I seen the pictures of it....

the dealer I get my coal from has an outdoor boiler...and i seen the coal he puts in that thing...they are baseball size...lol..and heats a huge building...

Al

I'm burning a mix of stove coal 2"-4" and nut, 3/4"-2". It is either hard Bituminus or Anthracite. I can wash the coal and it is hard and glossy, and doesn't rub off on your hands. My coal is from an old coal yard that is now selling only rock and sand. They scraped all the coal bins clean and made a huge pile of mixed coal. I have to screen the coal for size. But the price is right for the coal I use.

Anybody want a ton of pea mixed with fines??

I have a two speed blower, but I don't know the CFM of the two settings. If I'm burning wood and use the high setting I have way too much air, it is like a blowtorch in the firebox. I use the high setting to get the coal burning nicely before I close it up for a long burn. Using the high setting shortens the length of burn time for load of fuel.

I'm hoping someone with a batch load stove will chime in with some info about their firebox and coal bed size and depth.

thanks, Greg L

Greg,
What kind of boiler do you have? I have an AHS multi fuel but burn mostly coal and i burn Bituminous coal. I seem to have the same problem as you.
I am having a heck of a time maintaining the temp between 160-180. As a matter of fact my boiler started dumping heat 2 nights ago and it was 80 deg. in the house and my boiler was running at 205deg.!!
Then, on another occasion, it had a very good base of red coals about 6" deep and it would not maintian 150 deg. and the combustion blower ran continiously. I can't seem to get a handle on how to run at a continious rate. I have been using the same coal the whole time so I know it is not the coal.
Sometimes I go out to the garage in the morning and the temp is only 120 or so, and there is a huge red coal in the middle of the firebox with about a half on inch of unburnt coal on top of it. Once I break it apart it starts to burn good. Other times it is running me out of the house when it dumps the heat in the house at 3 in the morning. Any ideas on how to control this? or what are your theories.
I have also wondered about the flame proper flame height . I usually only have 2-5" flames but plenty of red coals.
Since It sounds like our problems are simular and we are one of the few on this site that has hand fired boilers maybe we can figure both our problems if we put our heads together.
BTW, Not sure of the demisions in inches of my firebox, I will measure it tomorrow. the website for AHS says it is 10ft 3. ( that is ten foot cubed) not sure what that would reduce to , I'm no math whizz that's for sure.

LsFarm-
I have a 20yr old Harmon SF 2500 duel fuel furnace.
I believe it's rated at 120,000 BTU.
The fire box measures 16" x 24" x 8". Hard to measure with fire going.
In reality I probably max out with a 7" bed of coal.

You guys looking forward to cold weather, be careful what you wish for. My memory is long and I have not missed the potential frozen water lines, hard starting vehicles and frozen hands and feet.

My fire box is 28 inches long and bout 12" wide at the top tapering to about 6" at the bottom. When it is below 40 degrees out side I burn it the full length filled to the top. The stove is rated for 135,000 BTUs but I never burn it at that output or the house would be 95 degrees or hotter. The temp was in the single digits in Dec. and I had no trouble keeping the house 68-70 using 90# of coal a day. I only see alot of flames right after I load with fresh coal. Once it is going good, the flames are only a few inches tall.

LsFarm wrote:I'm trying to find out an approximate BTU output for a given size of coal fire. I have a huge firebox in my new boiler, and have been using a short 'wall' of firebrick to reduce the size of the area that contains coal.

Right now I'm burning a bed of coal approximately 22"x24"x 4"-8" deep. [the sides of the fire box taper in down to the bottom grate].

So if you can post with what size the area of your burning coal is and the rated BTU output of your stove/furnace/boiler I would find it helpful.

Along the same line, another question. My boiler is a manual feed [shovel it in] I seem to have a variation in the heat output from the bed of coals. What I mean is that when I add an additonal 3"-4" of coal to the exsisting fire, obviously the heat ourput drops for a short while, then there seems to be several hours of peak heat output when the 'fresh' layer of coal is initially burning. The combustion blower only runs maybe 20minutes per hour during this time. But after several hours when the entire bed of coal is glowing red, and has burnt down several inches, the heat output seems to drop quite a bit and the blower runs more like 40 minutes an hour to maintain a given water temp.

Is this what what other manual feed stoves burn like?? or do I need to shake the grate more often, or just leave it alone because this is the way coal burns??

With wood, the 'roaring fire' stage makes the most heat, followed by the bed of coals stage where the heat output drops off. So it appears that maybe coal does the same thing??

I thought that coal would have a more stable heat output, I'm surprised that the same 22"x24" bed of red glowing coal with 1-2" of blue flame is making less heat than the initial pile of black coal with a blue and yellow flame.

Just asking for opinions and your observations. I can get 6-8 hours of high heat from one load of coal, then the fan starts to run more per hour and the water temp eventually drops off as the bed of coal burns down. I can move my 'wall' of firebrick to make a bigger box of burning coal, I could go easily up to 22"x36" but I fear that this may be too much heat output. Maybe I'll try this next week when the temperature is supposed to be in the 'teens.

Thanks in advance for your replies.

Greg L. [LsFarm]

It's funny that you mentioned about reducing the size of the fire box. I have a Harman Mark II coal stove. I have done the firebrick thing but noticed that I was holding the heat in the stove more than I liked. I have a plan in my head, but haven't had the time to implement it. I will be able to adjust the size of the firebox with out adding firebricks as fillers. As to your question on heat output, I really don't notice a substantial difference in the heat output as long as I keep the firebox filled to the top of the firebrick, and adjust the draft so that all I see is slight blue flame dancing above the coal. I think that once you establish a routine as to how often to shake the grates and when to fill the stove things will settle into a pattern. It takes time to sort things out what with all the variables involved, outside temp, quality of different coal lots, how often you shake the grates, etc. Eventually you will see a pattern and learn from it.

Thanks everyone!!

KTM, my boiler is custom made to fit into my outbuilding, the design is a combination of my old small boiler, several outdoor boiler designs and a LOT of web searches. Look in the 'Pictures of Your Stove' thread, I have two photos there.

I have the same problem with the coal 'bridging' over the base of burning coal. I have to knock holes or gently stir the hard top crust and it falls onto the hot coals and then the fire really takes off. This needs to be done about three hours after loading the coal, but I'm either gone or sleeping at this time.

I think the stoker stoves are the answer, but there is no way I will figure out how to install a stoker/feeder into my boiler. Much less find a source of good graded-sized coal here in SE Michigan. So I'm gonna live with it and try to figure it out.

Although, I could make an adapter to fit in the large loading door, with a feeder on the bottom and a smaller door above the feed..... Naw, not likely.

I'm surprised, like you, that the beautiful glowing-red bed of burnt-down coal doesn't actually produce enough heat to keep the water above 140*. I think the hot-coals stage of a fire from red oak has more heat output, but has a shorter life than coal.

KTM, like you I'm trying to get a consistant pattern with desired heat output, fuel loaded, blower settings, effective burn duration etc etc. I think it is going to be difficult with the warm weather extremes we are having, I think we could figure it out for say 15* nights with 25* days, if it would stay that way for a week or two. But the 35* nights with 45* days are hard to heat without overfiring the boiler. The real aggravation is when I don't load enough fuel and the fire is dead 8-10 hours later and I have to start with kindling and paper to get a fire going again.

I think with the warm nights rather than getting woke up from a 90* house, maybe you could half-load the coal, set the alarm for 0300 and reload for the rest of the night. At least you would not have to try to get back to sleep in a sauna!

I attached a photo of the firebox of my boiler. The grate on the bottom is 7" wide, and 48" long. The firebrick line the 'V' beside grate and the vertical sides of the box. The width from side to side [brick to brick] is 22". Right now I'm using about half the box, the back 24" has a firebrick 'dam' to keep the coal up front and the rear grates are covered to force all air up through the front bed of coal.

Sparky, I agree, I sure don't want to see 0* or below. I remember a week of highs in the 5* range and lows -15*, and it was rough! But that was when I was burning propane and only heating the house to 59*, now with a 70* house temp, I think the pipes in the walls won't freeze as easy. Got to love old farmhouses!!

Thanks again. Greg L

Well i can just tell you waht I know about what I have. The Van Wert is fed by a auger, it pushes up the coal and air from the bottom into a upside down bell shaped "firebox". It's about 2 ft across and 6 inches deep. In all the fire is no bigger than 1 cubic foot. That heats a 16 room house... Smaller rooms they are only 15x20 on average.

The minimum water temp is always 140, we have a hi-lo setting 140 for the lo and 180 for the high. The furnace will not run if it gets above 180 or will run if it hits 140.

When it's really cold and gets fired up the water temp can go as high as 220... even after it has stopped running. This can happen wespecially if there is a big demand fore heat and it hits the 180 mark and there is no more demand...

You said a hard crusty top, that's usually indicative of coal that has impurities in it. They refer to them as clinkers and can be quite aggravating, especially with a hand fired stove. I've some that can be the size of softballs.

NEPA,
You say that your water temp sometimes goes as high as 220* ? WOW !! is that safe as for as your pipes are concerned. I have my boiler out in my un attached garage and have about 50 ft of 1" red pex pipe underground to the house. I'm not sure that the pex would handle that kind of heat. Do you think if i raise my hi limit aquastat to say 205* that my pex would handle the heat. I can not recall the max temp it is designed for.
I think that when my boiler overfires it is right on the verge of not rising any longer right when my heat dump turns on. If I could set the high limit a tad higher i might not have this problem. Any ideas. ?