# comparison of propane vs coal usage in a koker

### Re: comparison of propane vs coal usage in a koker

Rob R. wrote:
lsayre wrote:Also the cost of the extra 6-8 degrees of round the clock warmth was not initially considered,

That much difference in average temperature is significant.

I just tried to fathom this significance using degree days as follows:

So far for February the average daily temperature around here has been 32.86 deg. F.
Therefore the average daily number of degree days has been 65 - 32.86 = 32.14 degree days per day

I'm burning coal at a rate of about 1.6 lbs per degree day, so 17 days x 32.14 degree days/day x 1.6 = 874 lbs. of coal burned (my actual for Feb. is 866 lbs, so this is close)

Now instead of my home being at 62 degrees, it is at 68, so the difference is 6 degrees. If I assume that it was actually 6 degrees colder outside instead of warmer inside (all things being relative), then the above calculations convert to:

Average temp outside at 6 degrees colder = 26.86 degrees
Therefore the average daily degree days would be 65 - 26.86 = 38.14 degree days per day

At 1.6 lbs. of coal burned per degree day for 17 days I now get: 17 days x 38.14 degree days/day * 1.6 = 1,037 lbs of coal burned

Taking the difference as a percentage I get: [(1,037/874 * 100) - 100] = 18.65%

My conclusion is that it requires 18% to 19% more energy to keep my home at a steady 6 degrees warmer (so it takes about 3% more energy for every degree of extra warmth).

I had initially calculated that I would burn 4 tons of coal, and if I multiply that by 1.1865 I get 4.75 tons. An increase of 3/4 of a ton.

If as I assume, I will burn 1 ton more than I had initially calculated, then fully 3/4 of that extra ton can be explained by the warmer house temperature alone.

That leaves only 1/4 of a ton of extra coal to be blamed on various of the other reasons for lower efficiency, and obviously in reality I'm doing much better than 65% to 70% as to my overall system efficiency.
Last edited by lsayre on Fri Feb 17, 2012 9:59 pm, edited 1 time in total.

lsayre
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### Re: comparison of propane vs coal usage in a koker

Your calculations are giving me a headache. Perhaps it is the lack of sleep and Yuengling...

The first week I ran my EFM I did some bar napkin math and discovered that it was about 50% less to heat the house with anthracite. I have no idea what the efficiency of my system is, and frankly I'm not in a big hurry to find out. My family is warm and comfortable, so I just keep hauling coal and taking out the ashes...a small price to pay for a cozy house and almost \$4,000 saved this season.

Rob R.
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### Re: comparison of propane vs coal usage in a koker

Holy *censored*...
You are really over engineering the DD thing relative to effic %...
That extra .25 ton...
500 pounds...
even at 20 cents a pound...
\$400/ton
That is only \$100 over the heating season...
How tight is your house have you done a pressure test...
If that house is not tight then the increased inside temps will increase the heat loss...
The higher the diff in temps will increase the heat loss thru the leaks...
Coal is cheap heat...
The equipment is simple...
KISS is the MO here...
Relax and enjoy the warm house...
CapeCoaler
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### Re: comparison of propane vs coal usage in a koker

One thing I think you need to understand is something I learned the hard way. The Koker is just a big stove with a big blower. Stoves work by radiating heat and convecting heat. You're only capturing a portion of that heat convecting in your ductwork. Putting that Koker in your living room would make a tremendous difference. Think about it this way... when you fire up the Koker in a cold basement, the air temp can be in the 50's but standing in front of it will feel warm due to the radiant heat. It's like walking across a blacktop parking lot on a hot day...the air isn't any hotter (maybe it is on a very localized scale) but it sure feels less comfortable than walking across a field.
Another good example is a camp fire. You certainly aren't heating the woods with your camp fire, but it feels pretty good standing near it.

After you see that difference then think about this one...the next time your propane furnace is running, stand in front of it and see how much heat you feel. It's negligible. That's where your fuel is going. Yeah the simple heat exchanger in the Koker is inefficient at best, but that's not where the real loss is...it's that heat that's being wasted unless you're down there to feel it.

Keystoker makes a traditional furnace style stoker that I believe is the A-120 or something like that. I don't know much about it other than it's actually a furnace rather than a stove with a big blower. Having said that, it probably doesn't get hot on the outside and requires ducting to heat the same room it sits in.
pconn171
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### Re: comparison of propane vs coal usage in a koker

I disagree about the koker being a "big stove".

Here's why.

It has a 1536 CFM blower that washes over the sides and top of the double walled design. I had a 120,000 btu oil furnace before and it did not produce any where near the heat the koker does through my registers.

The residual heat off the sides is just an added bonus.

The heat can't wash off fast enough....the blower can blow heat through my registers at 160 degrees indefinitely.

Stoves either have no blower or no more than 350 CFM blowers.

It has also been tested at 88% effeciency. My oil furnace was at 78%!
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### Re: comparison of propane vs coal usage in a koker

jrn8265 wrote:I disagree about the koker being a "big stove".

Here's why.

It has a 1536 CFM blower that washes over the sides and top of the double walled design. I had a 120,000 btu oil furnace before and it did not produce any where near the heat the koker does through my registers.

The residual heat off the sides is just an added bonus.

The heat can't wash off fast enough....the blower can blow heat through my registers at 160 degrees indefinitely.

Stoves either have no blower or no more than 350 CFM blowers.

It has also been tested at 88% effeciency. My oil furnace was at 78%!

Indeed
I understand from a good source that Keystokers rate their 'furnaces' output at the bonnet, not fuel input.

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### Re: comparison of propane vs coal usage in a koker

I stand somewhat corrected in my knowledge of the Koker. I didn't know that it was double walled, but I still stand behind my theory that the unit is still inefficient. I would like to see the test that determines the Koker's efficiency at 88%. Don't be confused by marketing numbers that don't mean anything. I'm sure the Koker is very efficient at changing the fuel to heat, but what does 88% mean? My intuition would guess that it means the unit can change 100 lbs of coal into 88 "lbs" of actual heat. That automatically implies that your coal is less than 12% ash. Although this is achievable, I seriously doubt that it means the actual heat making it into your plenum is 88%. The bottom line is that if you can't touch the front of the Koker, you're wasting heat to your surroundings.

Also, don't confuse the temperature at the ducts with the amount of heat being produced. Although this is probably a reasonable assumption, but think about it this way...a light bulb is pretty darn hot but you're not going to heat a room with it. The amount of heat being moved in your house is directly related to the velocity of the air as well as the temperature. The bottom line is that if you shove 100 btus through a duct at 100 cfm it will be colder than 100 btus through a duct at 50 cfm, but it's still 100 btus eaither way.

And one last thing about the Keystokers being rated by their heat output rather than input...that's only telling you what the capacity output is of the unit. It really doesn't mean anything other than that Reading might be inflating their numbers by using the fuel input versus Keystoker rating the output. All that really means is the Keystoker will actually put out the heat value it's marketed at and the Reading will not be able to do the same. It doesn't say anything about the fuel consumption or efficiency because it could use 200,000 BTUs of input to create 160,000 BTUs of output. I'm also not so sure that all of the manufacturers aren't doing the same by rating the output. It's my understanding that the heat output of a coal fire is related directly to the size of the grate. My furnace will feed way more than it could burn, so having said that, it's input capacity is probably huge, but the grate size limits the output capacity.
pconn171
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### Re: comparison of propane vs coal usage in a koker

Pconn171

I hear ya on the efficency rating. Not sure how that was determined.

All I know is my experience with the Koker and it is one heck of a heating machine that is perfect for my 2000 SF two story house!

A "stove" can't come any where close to doing what this thing does.
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### Re: comparison of propane vs coal usage in a koker

pconn171 wrote:I stand somewhat corrected in my knowledge of the Koker. I didn't know that it was double walled, but I still stand behind my theory that the unit is still inefficient. I would like to see the test that determines the Koker's efficiency at 88%. Don't be confused by marketing numbers that don't mean anything. I'm sure the Koker is very efficient at changing the fuel to heat, but what does 88% mean? My intuition would guess that it means the unit can change 100 lbs of coal into 88 "lbs" of actual heat. That automatically implies that your coal is less than 12% ash. Although this is achievable, I seriously doubt that it means the actual heat making it into your plenum is 88%. The bottom line is that if you can't touch the front of the Koker, you're wasting heat to your surroundings. ..>8..

I'm not sure I agree with your 100 vs 88 vs 12 relationship. Input shouldn't compare BTU by pounds fed unless you balance the equation with known BTU/Lb values. ~12% ash is irrelevant to the BTU content. One should be aware of this when buying coal. It's simply cost effective to by a ton of higher BTU coal for the same price as low BTU coal. Apples to apples ... combustion efficiency or heat exchange efficiency.

If your stoker can feed more than it can combust, then it seems the designers allowed for lower BTU content coal. You'd have to run more Lbs/hr to get the same amount of heat out of a lower BTU coal vs higher BTU coal.

As stated in this thread, the Koker isn't a true furnace. It's not supposed to be by design. I can heat my basement w/o ducting air from the plenum into the basement space by using the radiant heat from its sides and front. Just what I need. It will warm the basement and the floors with radiant heat and the living space supported by the duct system with the convective heat from the heat exchange system. It is what it is and don't be confused by the intent of the designers.

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### Re: comparison of propane vs coal usage in a koker

VigIIPeaBurner wrote:I'm not sure I agree with your 100 vs 88 vs 12 relationship. Input shouldn't compare BTU by pounds fed unless you balance the equation with known BTU/Lb values. ~12% ash is irrelevant to the BTU content. One should be aware of this when buying coal. It's simply cost effective to by a ton of higher BTU coal for the same price as low BTU coal. Apples to apples ... combustion efficiency or heat exchange efficiency.

If your stoker can feed more than it can combust, then it seems the designers allowed for lower BTU content coal. You'd have to run more Lbs/hr to get the same amount of heat out of a lower BTU coal vs higher BTU coal.

As stated in this thread, the Koker isn't a true furnace. It's not supposed to be by design. I can heat my basement w/o ducting air from the plenum into the basement space by using the radiant heat from its sides and front. Just what I need. It will warm the basement and the floors with radiant heat and the living space supported by the duct system with the convective heat from the heat exchange system. It is what it is and don't be confused by the intent of the designers.

Although I agree with this statement...I have to clarify my position somewhat as I'm trying to say the same thing. My statement was based more on the efficiency rating of 88%. I'd like to understand the calcs that go into this and what it acutally means. If you say a gas furnace has a 95% efficiency, it simply means you'll burn less fuel than a 90% efficient one. If you say that a stoker has a 88% efficiency, it means that you'll burn less fuel than another that has an 80% efficiency. Having said that, it also means that the coal itself has to be equal. This comes back to my point that I seriously doubt that Keystoker can say that out of every 100BTUs burn potential in the coal that 88BTUs of it goes into the plenum to be delivered to the ducts. I could be wrong, but I'm very skeptical of that efficiency and what it actually means.
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### Re: comparison of propane vs coal usage in a koker

pconn171 wrote:Although I agree with this statement...I have to clarify my position somewhat as I'm trying to say the same thing. My statement was based more on the efficiency rating of 88%. I'd like to understand the calcs that go into this and what it acutally means. If you say a gas furnace has a 95% efficiency, it simply means you'll burn less fuel than a 90% efficient one.

This statement if based on fuel usage alone is true if the two units were rated at the same output. This would be an example of (can't think of the term) heating efficiency or heat transfer efficiency. The ability to transfer the heat of combustion to the space one wants to heat. In this example, the 95% unit will use less fuel than the 90% unit will to heat the same space, given all other variables are equal.

pconn171 wrote:If you say that a stoker has a 88% efficiency, it means that you'll burn less fuel than another that has an 80% efficiency. Having said that, it also means that the coal itself has to be equal. This comes back to my point that I seriously doubt that Keystoker can say that out of every 100BTUs burn potential in the coal that 88BTUs of it goes into the plenum to be delivered to the ducts. I could be wrong, but I'm very skeptical of that efficiency and what it actually means.

There are two types of efficiency; combustion efficiency and heat transfer efficiency. It's not clear which one you are stating that Keystoker is presenting in their sales lit.
• Combustion efficiency relates to how completely the ~13,000 BTU are released from a pound of coal. If all of the carbon is completely combusted, then the magical 100% combustion efficiency has been attained.
• Lets just say a stoker is capable of releasing all the available BTU content in that hypothetical pound of coal. Now the ~13,000 BTU are in the stove and must make contact whit the heat exchange surfaces of the stove. If 2,500 BTU goes up the chimney and the remaining 10,500 is transfered to the living space, this hypothetical example would represent a heat transfer efficiency of 84.6%
• A combined calculation that considers both combustion and heating efficiencies would yield the total efficiency of the heating unit. I know how this is done in oil and gas units but it would be difficult to determine in a coal unit.

This has been coved numerous time on the forum. what is our efficiency? one of if not the most recent threads.

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### Re: comparison of propane vs coal usage in a koker

I don't think I'm confusing the creators intent. They call it a furnace. Right from their website below.

"Keystoker Koker Furnace

The Keystoker 160,000 BTU double wall constructed Koker furnace has a 9" wide flat grate stoker and patented thermal heat exchanger. This popular furnace can be used as an add-on or used freestanding to heat a small home. It comes with an opening on the front (3 X 25) and an opening on the top (11X19) to distribute warm clean air into your home.

Tested at 88 to 90% Efficiency

The Keystoker A-120 Add-On Furnace is made from heavy gauge American steel and has an insulated metal jacket. It burns anthracite rice and buckwheat coal. All surfaces that are in contact with hot gases are wiped with air from convection blower to exhaust maximum amount of heat from furnace.

Furnace is heated with our patented flat grate stoker. Comes complete with necessary controls and thermostat. Optional water coil available."
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### Re: comparison of propane vs coal usage in a koker

pconn171 wrote:...8<...After you see that difference then think about this one...the next time your propane furnace is running, stand in front of it and see how much heat you feel. It's negligible. That's where your fuel is going. Yeah the simple heat exchanger in the Koker is inefficient at best, but that's not where the real loss is...it's that heat that's being wasted unless you're down there to feel it.

Keystoker makes a traditional furnace style stoker that I believe is the A-120 or something like that. I don't know much about it other than it's actually a furnace rather than a stove with a big blower. Having said that, it probably doesn't get hot on the outside and requires ducting to heat the same room it sits in.

jrn8265 wrote:I don't think I'm confusing the creators intent. They call it a furnace. Right from their website below. ...8<...

As described, the Koker is uninsulated. That doesn't make it inefficient in technical terms. The unit uses both radiant and convective delivery. It doesn't meet the requirements as a traditional insulated furnace that is designed to minimize heat loss and put all the heat to the duct system. It isn't designed to but it is sized to do the same job. Heat isn't wasted if it meets your needs the intended installation requirements.

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### Re: comparison of propane vs coal usage in a koker

Much of my position on this unit is related to the fact that the creator of the thread seems to be interested in why his Koker seems to be burning more coal than he expected. Part of it seems to be related to the fact that his temperature is set higher, but I'll argue that you can't compare the numbers based on a heating potential of the fuel because the Koker will loose much of it's heat to the surroundings. I also think that Keystoker is being a little misleading in their literature to say that it's 88% efficient because when a normal person thinks of a 88% efficient furnace, they think it'll deliver 88% of the heat into the ducts and I don't think that's true with the Koker. I believe that their literature means quite simply that 12% of the heat is lost up the flue. How much actually gets transferred into the plenum is a whole other issue. I personally believe that the flat plate heat exchanger design is inefficient at best and that the manufacturers really need to consider changing their designs out to incorporate more surface area for convected heat exchange. A simple thing they could do is add heat sinks. If they want to market the Koker as a furnace, they should make it get more heat into the ducts and market it to that actual number rather than touting it's ability to not lose heat up the chimney. We can discuss the errors in my wording over and over, but it still doesn't change the fact that I think their numbers are misleading to the average consumer that didn't spend every night on this forum researching.

This issue is sensitive to me because I'm a mechanical engineer so I should be well aware of the math it takes to come up with these values, but yet I was sold on the similar unit from Reading based on the marketing literature. I have a 170,000 BTU "furnace" in my garage that heats my house. The garage gets used about once a week and it's probably in the mid to upper 70's in there but the rest of the house is 68-71. I had the unit custom fitted with an insulated jacket around the sides and reconfigured the blower to wash the heat away and into my ducts, but the radiated heat off the front is still an incredible loss - enough to heat the uninsulated garage to higher temps than the rest of the house. If the unit was marketed properly (i.e. a stove with big circulation blower) I would've probably searched for an alternative like the Keystoker A-150. I bought mine in '08 though when there was a shortage of the units and I didn't have the luxury of seeing them before buying so I was left with only the literature on the websites.
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### Re: comparison of propane vs coal usage in a koker

The Koker has more than just a flat plate heat exchanger. Inside the back heat exchange space of the unit is an inverted "Y" exhaust tube. This manifold greatly increases the heat exchange surface area. The tips of the Y collect the exhaust gasses from each lower rear corner of the combustion chamber. The tips merg to form the 6" top exhaust. Convective heat is collected from the enclosed Y, the flat plate of the back and top.

We only intrept what we read, normal or otherwise.

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