Thoughts/Questions on Stove Efficiency

Thoughts/Questions on Stove Efficiency

PostBy: Ashcat On: Fri Dec 12, 2008 10:00 am

My stove—a Hitzer 983 fireplace insert--has been running continuously for six weeks now. Since it is marketed to heat a 3000 ft.² living area, and I have 4000 ft.² of living space, I knew before I got it that it would only supplement my existing heating system (propane forced air), not replace it. However, due to my enthusiasm for the stove, as temperatures have gotten colder I’ve tried to “push it” to transfer as much heat as possible to my living space. Generally, this involves running stove temperatures of 400 to 500°F. My draft is good—really, too good—and I am generally burning 60 to 70 pounds of coal per day. The firebox is approximately 20" x 20" x 9" deep. I shake and load twice per day and easily get 12 hour burn times. The fireplace is in a vaulted family room, and the warmest spot in the home is the second-floor hallway above, and open to, the family room. The family room itself is comfortable in terms of temperature, but not especially warm—I assume in large part because of the amount of air rushing back to the stove in order to support combustion, and because heat output from the stove flows in an unobstructed fashion to the second floor. This setup works fairly well, and the rooms off the upstairs hallway are comfortable, but not too warm. The coldest areas of the house are on the first floor, where the insert is located, farthest removed from the family room. Still, these rooms are warm enough for reasonable comfort. Only when the outside temperature has been around 20 or below has it been necessary to enlist the aid of the propane forced air system.

With this as my experience so far, I learned something about my stove when temperatures recently went into the upper 50s, and I tried to idle my stove for a total of about 36 hours. At these times, I would load about 12 pounds every 12 hours, and would arrange it in a pile in the center of the firebox. I would shut down the ash pan vent entirely, since with my stove, some air can still get into stove through that vent even though it is entirely shut down. This small amount of air would support combustion of the pile in the center of the firebox and, although I typically reloaded, because of my work schedule, about every 12 hours, I have no doubt that that pile would remain healthy for at least 18 hours. The other thing I did at these times was to slow the convection blower fan speed by about 65%. What I learned from this experience was that, even though when I was pushing my stove and achieving temperatures of 400 to 500°F, and was indeed getting more heat output from the stove into my home, my heat output from my idling strategy was still pretty good. Even while idling, I would get temperatures of 280-300°F, measured at the stove.

This experience has me thinking about issues of efficiency. Although I do not know the precise meaning of this term as it applies to heating/ventilation, what I mean by the term efficiency is: how effectively does the stove release the heat from a given amount of fuel into the living space of the home. I suspect that when I’m pushing the stove, and when stove temperatures are as high as I’m comfortable running and the convection fan is at its maximum speed, my stove efficiency, as defined above, drops pretty substantially. After all, burning a little over 20 pounds of coal per day while idling still produces respectable stove temperatures, while burning three times that amount of coal increases stove temperature only by 33 to 66%. If it is true that my stove efficiency drops, this means that less of the heat content of the given amount of coal is being transferred to my living space, and more is being lost in some way.

If my recollection of physics is correct, there are three ways to transmit heat: radiance, conduction, and convection. A traditional potbelly stove would rely primarily on radiance, plus a small amount of passive convection to the air surrounding the stove. An insert, on the other hand, relies primarily on convection. Given my stove and my installation (the stove rests on top of several fire bricks in order to be high enough to fit into my fireplace correctly), very little heat would be conducted away from the stove. Also, because the vast majority of the exterior surface of my stove consists of jacketing for the convection fan and ductwork, there is very little surface area of firebox wall that is not jacketed, and from which radiant heat transfer could take place.

In a perfectly efficient insert scenario, all of the heat radiating from the firebox wall would be retained by the jacketing, and all of this heat would be transmitted by the forced convection blower fans out the heat vents and into the room. In addition, as much heat as possible would be retained in the stove rather than sent up the chimney.

In the less efficient scenario when I am pushing the stove to high temperatures, I suspect that the extra heat is lost in several ways. First, the convection fan and ductwork approach may be limited in its ability to retain and transmit heat via convection, so that some of the heat radiating from the firebox is transmitted to the outer wall jacket, and lost via radiance or convection to the masonry of the fireplace and chimney. Also, some of this heat may be lost by going around the insulation surrounding the stove pipe inserted up into the chimney, and lost out the top of the chimney. Second, it may be lost directly from the firebox out the stove pipe and out the chimney, especially since my draft is so high, and airflow supporting draft would be increased with increased rate of combustion. I suspect that the second possibility is leading to more heat lost than the first possibility, although I do not know for sure.

If this is true, it seems to me that there must be an ideal rate of combustion/stove temperature that permits the stove to operate at maximum efficiency. Other than general experience, is there a more precise way to identify temperatures at which the stove would be operating most efficiently? If my other considerations about heat loss in my situation are true, it would seem important to find ways to limit draft further. Is it possible or advisable to use a barometric damper when I have no easy access to the stove pipe, and the stove pipe consists only of a 5 or 6 foot section inserted into the chimney?
Ashcat
 
Hand Fed Coal Stove: Hitzer 983
Coal Size/Type: Nut/Blaschak

Re: Thoughts/Questions on Stove Efficiency

PostBy: grizzly2 On: Fri Dec 12, 2008 7:41 pm

Interesting. I have had some of the same thoughts, though not as well thought out. On the whole, I think you are correct in your conclusions. Measuring the losses would probably require a laboratory set up and engineers. However I think observation, experience and logic, such as you have applied can still suggest to us things we can do to improve overall efficinecy. Another thread has a good discussion about bringing in outsde air for combustion and even to the baro damper, rather than drawing air from the heated room.

An additional observation I think I have made is that the ash in the pan appears more fine and dusty when I have a long, slow, low burn going. The ash appears to be more flaky and chunky when shaking down a hot burn with more coal consumption per hour. This is something I also noticed when burning wood for the past 30 years. When you need high heat output and open the draft to burn quickly, I don't think wood or coal get burned up as thouroghly. We are perhaps throwing away some BTUs in order to get a new load of fuel into the fire, and shake down the ash which may still gave some chunks that hadn't given up the last of thier heat.

If you have a hot fire and a lot of intake air, and no baro damper you can bet you are loosing a lot of stove heat up the chimney. If you have a baro damper you can bet you are loosing a lot of room heat up the chimney. I think I may experiment with an outside air intake for the baro. For my setup, this may be the biggest single thing I can do toward our goals of more efficient heating. :cheers:
grizzly2
 
Hand Fed Coal Stove: Hitzer 30 - 95
Coal Size/Type: pea and nut/ anthracite
Other Heating: Jotul #3 wood stove in garage. Oil backup in house. Electric backup in house.

Re: Thoughts/Questions on Stove Efficiency

PostBy: DOUG On: Sat Dec 13, 2008 12:28 am

Hi, guys. Here is a couple of calculations to play with. But a few definitions first.
COMBUSTION EFFICIENCY:The percentage of heat that is released during burning the fuel. Most units can attain 70-90 percent efficiency.
HEAT TRANSFER EFFICIENCY: The percentage of the heat that is absorbed by the air in the heater. This is the biggest limitation in most units.
OVERALL ENERGY EFFICIENCY: The combination of the previous two. It is the ratio of the heat that is supplied to the home divided by the amount of heat that was available in the fuel.

A measure of the effectiveness of a design that sometimes used is the ratio of heat exchanger area to grate area. Values of 15:1 to 25: are common with the lower ratio better for coal. This ratio is not commonly listed in manufactures' literature but can sometimes be calculated from the information supplied. A ratio that is low would indicate low efficiency because the heat generated by the fire could not be absorbed quickly enough before the gases left the stove. Too large a ratio would be efficient but could indicate possible draft problems. The amount of direct surface area (the fire shines on) is the heat exchange surface area.

Regarding combustion efficiency. The shape and volume of the firebox influence the combustion. Too large a surface area cools the fire and reduces the amount of volatile gases that are burned. If the surface area is too small, not enough heat is absorbed by the air heating the home and higher stack temperatures results. You lose 1 percent efficiency for each 25 degrees F. that the stack temperature is above 350 degrees F. To find the average flue gas temperature, take readings at the section of stovepipe before entering the chimney and at the top of the chimney. Add the two readings, divide by two, then subtract the outdoor temperature. That is you average flue gas temperature.

Just a few things to think about. :idea: DOUG
DOUG
 
Stove/Furnace Make: CHUBBY, D.S.MACHINE BOILER
Stove/Furnace Model: CLAYTON 1600

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Re: Thoughts/Questions on Stove Efficiency

PostBy: Ashcat On: Sat Dec 13, 2008 12:47 am

DOUG and Grizzly2--Much to think about there--thanks. It sounds like efficiency as I described it is the Overall Energy Efficiency you describe, DOUG.
Ashcat
 
Hand Fed Coal Stove: Hitzer 983
Coal Size/Type: Nut/Blaschak

Re: Thoughts/Questions on Stove Efficiency

PostBy: grizzly2 On: Mon Dec 15, 2008 6:41 pm

Thanks DOUG for adding some clarity to the subject.
:)
grizzly2
 
Hand Fed Coal Stove: Hitzer 30 - 95
Coal Size/Type: pea and nut/ anthracite
Other Heating: Jotul #3 wood stove in garage. Oil backup in house. Electric backup in house.

Re: Thoughts/Questions on Stove Efficiency

PostBy: VigIIPeaBurner On: Mon Dec 15, 2008 7:40 pm

Wish I could remember where on this great forum I read about this calculation so I could give Kudos to them. You can get a rough estimate of the maximum coal feed rate if you know the maximum BTU output your stove is designed for.

Here’s the example for my radiant only stove. The max rated output is 50,000 BTU/hour. The thread I recall stated that the average heat output from anthracite is ~13,000 BTU/Lb. This example fits with DOUG’s overall energy efficiency post. The manufacture gave the output rating of 50,000 BTU/Hr in the stove literature. Knowing all the variables, if I’m feeding 90 Lb a day I’m near the max of my stove. Eg: (90 Lb x 13,000 BTU/Lb)/24Hr = 48,750 BTU/Hr. After the experiences from more than one heating season, that’s what I end up feeding while keeping the stove near the max design stove top temperature. Plugging in your max feed number of 70 Lb/day would yield (70 x 13,000)/24 = 38,000 BTU/ Hr.

I don’t know what the Hitzer 983 output is. From the size of the firebox and DOUGS comments about firebox design, I’d guess the output is more than my stove. My 2310 is a top loader @ ~ 2.1 cu. ft. , so the entire firebox can be filled and burned. From your measurements, the Hitzer 983 fire box is ~3.2 cu. ft. if it could be fully loaded. That would make it about a third larger in volume. I don’t’ know if you can extrapolate that out to say the Hitzer 983 output is 75,000 BTU/Hr. If so, you could do the math and find out how many Lb/day you could safely feed to stay near the Hitzer design max. Convection blower would need to be adjusted wash the heat off the stove plus the radiant heat from the exposed areas.
VigIIPeaBurner
 
Hot Air Coal Stoker Furnace: Keystoker Koker
Hand Fed Coal Stove: Vermont Casting Vigilant II 2310
Other Heating: #2 Oil Furnace

Re: Thoughts/Questions on Stove Efficiency

PostBy: DOUG On: Mon Dec 15, 2008 8:19 pm

Can the unit burn the fuel fast enough to give the rated output? Combustion rates vary significantly with the type of fuel you are burning. To obtain complete combustion of a pound of coal, 200 cubic feet of air are needed.

Pounds of fuel per square foot of grate per hour:
Anthracite, Pea = 5
Anthracite, Nut = 8
Bituminous = 9.5
Wood = 20

Average heat output of fuel BTU/Pound
Anthracite, 12,500
Bituminous 13,000
Wood 6,500

Efficiency of combustion percentage
Anthracite 65
Bituminous 60
Wood 50

To calculate the amount of heat output based on the number of pounds of fuel that the unit will burn: MULTIPLY:

Grate Area(sq.ft.) x Combustion Percentage x Pounds of Fuel per sq.ft. of Grate X BTU/Pound

EXAMPLE: 1.5 sq.ft. Grate x .65% x 8# x 12,500Btu = 97,500Btu's for Anthracite Coal

EXAMPLE: 1.5 sq.ft. Grate x .50% x 20# x 6,500Btu = 97,500Btu's for Wood
Another formula to play with. :idea: :)
DOUG
 
Stove/Furnace Make: CHUBBY, D.S.MACHINE BOILER
Stove/Furnace Model: CLAYTON 1600

Re: Thoughts/Questions on Stove Efficiency

PostBy: Ashcat On: Mon Dec 15, 2008 9:27 pm

As best I can tell (from stove sites other than Hitzer, which doesn't list BTU output as far as I know), the 983 is rated at 80,000 BTU/hr--close to VigIIPeaBurner's 75 K estimate. The stove itself may be able to generate 80 K/hr, but the real world output may be different because it's jammed halfway up a chimney! That said, I am not at all disappointed with the results, as it heats my house adequately to outside temps as low as 20 degrees. Also pleased with how well thought out the design and its execution are.

DOUG--thanks for you input. Your figure of 200 cu ft of air required per pound of coal is not too different from my estimate of 135 cu ft from this thread ( Air Requirements for Coal Combustion ), which was obtained from a chemical perspective (I'm not saying I was under chemical influence at the time :) ). Do you know how your figure of 200 is arrived at? Just curious. Thanks again.
Ashcat
 
Hand Fed Coal Stove: Hitzer 983
Coal Size/Type: Nut/Blaschak

Re: Thoughts/Questions on Stove Efficiency

PostBy: Cap On: Mon Dec 15, 2008 9:41 pm

Hello Cat,

I have often thought about ways to burn most efficiently. I have concluded that a high simmer is just right for best efficiency on a hand fired..
I have installed thermometers on my hot air box and on my stack. I see greatest heat differential on the positive side when the stove is simmering. See some recent images here: Hand Fired Harman Temps

When I burn hard, throttle open the damper, my stack temp meet my hot air temp and will even exceed it. Thus I am running on the negative side of heat transfer. But having said and concluded this, coal is still cheaper than propane, oil & electric. So, crank her up when necessary.
Cap
 
Hand Fed Coal Boiler: Harman SF 250, domestic hot water loop, heat accumulator

Re: Thoughts/Questions on Stove Efficiency

PostBy: DOUG On: Mon Dec 15, 2008 9:58 pm

The chart I have for Approximate Theoretical Air Requirements to Burn are:

FUEL--------------POUNDS------CUBIC FEET PER POUND

Wood----------------6.5-------------------83
Peat ----------------5.7-------------------73
Lignite---------------6.2-------------------79
Subbituminous------11.2------------------142
Bituminous-----------10.3------------------131
Anthracite------------9.6-------------------122
Coke------------------11.2------------------142

If just the right amount of air needed for combustion were fed into the fire, maximum efficiency would not be reached. This is because the mixing of the air and the combustibles is never complete and some unburnt particles would escape without coming in contact with the oxygen. An additional amount of air is needed to get more complete burning. This usually amounts to 20% to 50%. Although, exess air tends to cool the fire and takes more heat up the chimney. Air is made up of 21% oxygen and 79% nitrogen. Only the oxygen is used for combustion. There is a point where the efficiency of combustion is greatest. But, this balance changes as the fire goes through different stages of the burn cycle.
So you can see that the best we can hope for is an average efficiency that is as high as possible. :) DOUG
DOUG
 
Stove/Furnace Make: CHUBBY, D.S.MACHINE BOILER
Stove/Furnace Model: CLAYTON 1600

Re: Thoughts/Questions on Stove Efficiency

PostBy: coal berner On: Mon Dec 15, 2008 10:08 pm

DOUG wrote:Can the unit burn the fuel fast enough to give the rated output? Combustion rates vary significantly with the type of fuel you are burning. To obtain complete combustion of a pound of coal, 200 cubic feet of air are needed.

Pounds of fuel per square foot of grate per hour:
Anthracite, Pea = 5
Anthracite, Nut = 8
Bituminous = 9.5
Wood = 20

Average heat output of fuel BTU/Pound
Anthracite, 12,500
Bituminous 13,000
Wood 6,500

Efficiency of combustion percentage
Anthracite 65
Bituminous 60
Wood 50

To calculate the amount of heat output based on the number of pounds of fuel that the unit will burn: MULTIPLY:

Grate Area(sq.ft.) x Combustion Percentage x Pounds of Fuel per sq.ft. of Grate X BTU/Pound

EXAMPLE: 1.5 sq.ft. Grate x .65% x 8# x 12,500Btu = 97,500Btu's for Anthracite Coal

EXAMPLE: 1.5 sq.ft. Grate x .50% x 20# x 6,500Btu = 97,500Btu's for Wood
Another formula to play with. :idea: :)

Where did you get 12.500 BTU Per lb For Anthracite coal That is very low grade of Anthracite at that Number it would be more like 13.000 to 14.000 BTU Per lb around here in Anthracite Country . I have Read Numbers up to 15.000 Btu Per lb
Also Read Average for Anthracite coal is around 14.200 btu per lb Have not found any that high Yet.
The Highest I Found is 13.800 btu per lb

http://books.google.com/books?id=crNPAA ... &ct=result

http://www.rocksandminerals.com/coal.htm
coal berner
 
Stoker Coal Boiler: 1986 Electric Furnace Man 520 DF
Stove/Furnace Make: Electric Furnace Man
Stove/Furnace Model: DF520

Re: Thoughts/Questions on Stove Efficiency

PostBy: DOUG On: Mon Dec 15, 2008 11:57 pm

coal berner: Not to get smart with you or for you to take me the wrong way, but did you read your thread from books.google.com all the way through. From what I understand from reading your thread, the stove design, radiation and improper firing rarely gives the theoretical heat value average of anthracite coal 14,200Btu's per pound. More like 80% of the theoretical heat energy of the fuel. Which brings us back to my earlier post that the combustion efficiency of most units can attain between 70%-90% efficiency. So if we use the two post's information, we can infer that 14,200Btu avg. x .88% efficient stove = 12,496Btu's or 80% efficient stove x 15,000Btu's = 12,000Btu's. Yes? How about the different Btu's of Wood? Isn't it somewhere between 5,000-10,000.
Anyway, It's fun to try to figure it out without being in the "controlled laboratory." All these formulas give us a pretty good Idea :idea: of what is happening with our own unit in our custom installations. :idea: :) Just more stuff to play with. :) DOUG
DOUG
 
Stove/Furnace Make: CHUBBY, D.S.MACHINE BOILER
Stove/Furnace Model: CLAYTON 1600

Re: Thoughts/Questions on Stove Efficiency

PostBy: Yanche On: Tue Dec 16, 2008 1:10 am

coal berner wrote:Where did you get 12.500 BTU Per lb For Anthracite coal That is very low grade of Anthracite at that Number it would be more like 13.000 to 14.000 BTU Per lb around here in Anthracite Country . I have Read Numbers up to 15.000 Btu Per lb
Also Read Average for Anthracite coal is around 14.200 btu per lb Have not found any that high Yet.
The Highest I Found is 13.800 btu per lb
The highest Anthracite BTU rating I've seen in data reported to the EPA is 14,376 BTU/lb
Most of the data is in the 11,500 to 11,900 range, excluding refuse Anthracite which is lower.
Yanche
 
Stoker Coal Boiler: Alternate Heating Systems S-130
Coal Size/Type: Anthracite Pea

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