BTU CALCULATIONS

[Todd]

With all the bloviated manufactures numbers out there and recent threads discussing burn times and btu, I thought I would try to figure out what my average BTU output would be on a 40 lb load of Oak (I weighed it) in a 12 hr burn in my stove and this is what I came up with. My stoves max btu rating is 55,000 and to achieve that I would have to burn hot and reload every 3 hours or so. Not very realistic.

I'm no math wiz so correct me if I'm wrong. I'm figuring 5800 available btu per pound of wood at 20% moisture content according to the book "The Wood Burners Companion". Also figuring a 72% EPA efficiency for my stove and I know it could be more or less real world, but I'm looking at an average. This is what I came up with.

40lb x 5800btu = 232,000 btu

232,000btu divided by 12hrs = 19,333 btu

72% of 19,333btu = 13,919 btu per hr on ave

Sound about right? Am I forgeting anything? I know the btu will be higher at the begining of the fire and drop off well below average at the end.

 

[KeithO]

Your logic sounds fine to me. Sounds like you need about 6 tons of firewood for the season ? I went through about 4 tons of corn last winter.

 

[webbie]

Close, I would use 6,000 BTU at 20%, some online sources say 6200 - so split the diff.

Also, I think real efficiency can vary depending on factors like the amount of exposed stovepipe, etc.

I am going to study up a bit on stove efficiency - from my current understanding, they are calculating this from stack temps and the chemical makeup of the smoke - NOT from actual output (calorimeter rooms, etc.).....this is one reason why inserts are not rated much less than stoves.

Maybe Corie or R&D;man or someone else here can give a briefing on how stoves are tested for efficiency.

I met a guy who was like an "Einstein scientist" at my inventions class (I'm the instructor!) the other night, and I'm going to point him to all the test methods and stuff - and let him cram it into his noggin, and see if I can figure out what the best real world methods are. Apparently, they use different methods in Germany and elsewhere than they do here!

 

[Todd]

Your logic sounds fine to me. Sounds like you need about 6 tons of firewood for the season ? I went through about 4 tons of corn last winter.

I went through 3 cords of Oak last year, so thats about right.

 

[BrotherBart]

Your logic sounds fine to me. Sounds like you need about 6 tons of firewood for the season ? I went through about 4 tons of corn last winter.

I went through 3 cords of Oak last year, so thats about right.

If you had a Blaze King you could get by with twenty four broom handles.

 

[Todd]

People keep poking fun at Blaze King, even I don't even think 82.5% efficiency is possible for a wood stove, but maybe they have a different standard for testing. I have yet to here from anyone who owns one, that their claims aren't so. If you take the Blaze King Princess and fill it with 50lbs of cordwood for a 20hr burn, my calculations come to 11,890btu per hour with a claimed 82% efficiency. Even if you use the EPA 72% efficiency the btu's comes to 10,440 per hour. Once a house is up to temp and you have decent insulation, I don't think you need more than that to keep the temp stable. It would be nice to here from someone as Craig mentioned, about efficiency testing. How about it Blaze King?

 

[BrotherBart]

The charts on their website look pretty believable. Stove owners that look down their nose at the rest of the stoves out in the world are just inviting ribbing.

It gives Elk's magic VC stoves and the PE crowd a break for a little while. The most important question is "Is your house warm?". If the answer is yes then you have the greatest wood stove made no matter what brand it happens to be.

 

[Gunner]

Well, I don't know who's calculations are right but blaze king says on their website that the Princess(2.8cuft) will produce 6400btu/hr "real world" on a low 20 hr burn.

I don't doubt that it can be done....I doubt why it needs to be done.

If it's 50 deg outside and I need to knock the chill off there is no way I'm going to load up 60 pounds of wood into the stove and cook it for 20hrs. The sun may come out and it may warm up, hell the wife may turn on the oven and bake some cookies...once the wood is in I can't take it out.

Let's be realistic here...to work properly the stove still needs to be up to temp and have a good coal bed, then we load 60lbs of wood into the firebox and wait for it to be good and charred, now we can engage the cat and set the "thermostat" on low. Getting your coal bed and charring the 60lb load is going to create a fair bit of heat, probably in the 4-500 stovetop range, so there goes the idea of super low temps.

Fire needs a certain amount of air to burn, cut it to low and to will go out, same with temperature the firebox needs to be hot ie) 60lbs of wood on a small coal bed and not fully charred and the fire will no longer continue to smolder for the cat, it will die completely. With low low air and firebox temps I imagine opening the door to find unburned wood and coals would be common.

As to wether or not 6400btu/hr is enough, well I guess that would depend on the size of your house and how well it's insulated. Seeing as that every single person on this board has a woodstove that puts out more than 6400btu/hr on it's lowest setting I would imagine that it is not enough heat or everybody is being smoked out of their house. My home is 1800sqft 6yrs old and well insulated but lots of windows and I need to burn up 3cuft every 12hr in the heart of winter to stay warm.

Another thing that I found curious on the "Scientific Burn" they did on the king 1107, notice how the flue temp stayed right about 225deg not matter if the cat probe was 1400deg or 400deg
http://www.blazeking.com/King1107.pdf
I call BS on the in house scientific testing.

 

[stoveguy2esw]

actually the "efficiency" question is this , does "combustion efficiency" translate to "heat transfer efficiency" listed efficiencies per EPA are combustion efficiencies this is not directly translated to "heat transfer efficiencies" but it does factor in.

essentially , combustion efficiency is basically , how much of the wood is completely consumed v/s how much escapes in the exhaust or lies in the firebox as ash. thermal efficiency(or heat transfer efficiency) is essentially how much heat is delivered to the space to be heated , v/s how much is sent up the stack.

pair that with most of the industry taking "default" for combustion efficiency which means they must exceed a certain percentage, but only the test lab knows by how much. a stove rated at 63% may in fact burn at close to 80% or better. also , when dealing with wood, the amount of BTU's the wood contains in imperical anyway, one ten pound chunk of oak at 20 % moisture may not contain the same amount of btu's as the next one. testing is done wit a measured charge that is figured by weight at 20% moisture, but one charge may contain a larger volume of wood at a lesser density, which would burn quicker possibly not allowing the secondary system (cat or noncat) to be able to reburn as much of the fuel. the douglas fir at 20% is used because its the most consistant wood available based on weight v/s density

all that said , here is my take when shopping for a woodstove: look at the data provided, square footage rating, GPH (dont beat on me web i still say if it tested cleaner in the lab it burns cleaner in the real world (comparatively speaking)) how many cfm does the available blower give you (if applicable) and above all, is the look and the features pleasing to you.

i build stoves , as do a few of my fellow members in this forum , i feel that for the most part , stoves are very similar in performance based simply on firebox size, virtually all of the EPA stoves on the market perform with similar results in their size classes, buying a non-EPA stove when an epa unit is available and a viable option in my opinion is a mistake, they are simply better , breaking it down by brand though among epa units is splitting hairs, be it PE , ESW ,VC,JOTUL, etc none are head and shoulders better. so buy what looks best to you and has the features and burn times you like, you will get a good stove and will not have "missed out" on a better unit.

 

[Todd]

Thanks for the post Mike,
I thought stack temp had something to do with efficiency. I noticed my stack temps on my current stove is much lower than my previous non cat stoves and seems to burn less wood. As far as splitting hairs of EPA brands, I like doing that as do others. It's just interesting to me I guess. Maybe some day those hidden numbers will come out of the lab or Craig will get his lab setup and we will see some interesting results?

 

[webbie]

I won't beat on you Mike.....

But let me try to simplify the efficiency thing....and please correct me if I am wrong.

1. The vast majority of stoves are not tested for efficiency with cord wood.
2. Most manufacturers do not test for "actual" (total) efficiency using a calorimeter room, but rather use a simpler method that calculates the wood input, and the amount of CO in the stack, stack temp and perhaps other stack chemistry....

Another thing which seems to be the case in my reading is that actual real world efficiencies would tend to be LOWER (not higher as you suggest) in many or most installations and uses....than lab tested. This info comes from researchers when the EPA stuff started out (Shelton Research, etc.), so it may be dated. Executive summary - I think 70% is a good number for a freestanding stove with some pipe!

As I suspected, one article said that a calorimeter room is the "best" way of measuring actual home performance efficiency, but that very few manufacturers use them. I remember seeing one at Earth Stove in Oregon when I visited there (I gave a speech to their dealers in 1996 about the net!).

Here is an OLD article about some of that stuff, but I think a lot of the info still stands. One interesting point that it makes is that the current (stack) method of efficiency testing could vary as much as 15% from just one assumption (the amount of hydrogen that is assumed to be in wood).....

from: http://www.motherearthnews.com/DIY/1984-09-01/Wood-Stove-Efficiency-and-Emissions-1984.aspx

"When you're judging appliances according to their overall efficiencies, then, it's important to know what testing method was used. Calorimeter measurements appear to be pretty dependable; but if the appliance has been tested by stack loss, you should take the results with a grain of salt. Recent studies sponsored by the Wood Heating Alliance show that the overall efficiency of a given appliance measured by calorimetry is usually lower than the rating provided by stack loss measurement. Further, stack-loss efficiencies from two labs aren't always comparable. If you're comparing numbers generated by the same testing facility, though, you're on more solid ground"

------------------------------------------------
A good point which the same article makes (and I agree) is that stoves tested by the same people in the same facilities (say we all sent them to Omni and had the same techs there do the testing) would likely be somewhat of an accurate comparison. By inference, the opposite is not true.....
(Note - yes, article is from 1984, but read it - it is quite accurate)

 

[Todd]

Another thing that I found curious on the "Scientific Burn" they did on the king 1107, notice how the flue temp stayed right about 225deg not matter if the cat probe was 1 or 400deg
http://www.blazeking.com/King1107.pdf
I call BS on the in house scientific testing.

Well i'm not too sure about that test either, but can tell you my stack temp rarely goes over 300 with the cat engaged, and it does drop slowly as the fire dies. What kind of stack temps is your Summit running? My old Hearthstone ran 350-450 with the same stove top temps as my current stove. I say that's wasted heat up the chimney.

Oops, maybe not after reading that artical that Craig found. It would be interesting to see the results of every stove going through the calorimeter test.

 

[GVA]

I wonder something here.....................
Iff one were to burn a wood at say 10,000 btu per pound and they only sent 7,000 btu (per pound)to the home would this not be a 70% efficient stove?????or unseasoned wood???
So if the stack temp is higher with the same wood, then that stove is less efficient.....Right???

 

[Gunner]

Todd, I find my stack temps (double wall with a probe thermo) to be consistantly 200 deg lower than my stovetop. 500 stovetop is 300 in the pipe.

Now when charring a new load stack temps are always higher, but settle in once you adjust the desired air setting. That's what I found odd about the BK chart, a 1400 deg cat and a 400 deg cat yielding the SAME stack temp....????

 

[webbie]

I wonder something here.....................
Iff one were to burn a wood at say 10,000 btu per pound and they only sent 7,000 btu (per pound)to the home would this not be a 70% efficient stove?????or unseasoned wood???
So if the stack temp is higher with the same wood, then that stove is less efficient.....Right???

I assume you mean per hour, because wood only contains about 8500 oven dry BTU per pound (all species)...

But, yes, if two stoves were burned at the same rate - say 1 kilogram per hour of wood, and they both had similar CO stack measurements but one had a 100 degree hotter stack than the other, then (all things being equal) the one with the hotter stack temp is less efficient (less heat transfer to the room=more up the chimney)

Cat stoves tend to have lower stack temps, and therefore higher efficiencies.

There are so many things to take into account - a fireplace insert way back in a masonry fireplace COULD be only 40% efficiency in total, when everything is taken into account, even if the tag on it says 68%.

That is why actual "rooms" and a bunch of custom assemblies (such as an exterior masonry chimney on said room) would be needed to actually create accurate numbers. Given EVERY test I have seen over the decades, it is always safe to assume that you will get LESS, not more, than published numbers. Example: The first pellet stoves in the 90's claimed 80-85%+ efficiencies. Actual field testing by Omni showed a low of 42% (that honor went to Earth Stove, as I remember) to a high of 70-something percent, with an average in the 60's. Certainly they are better now, but my point is that the fine art of sales and marketing generally does not underestimate benefits to the customer.

The enclosed chart shows the actual and reported (to and by the EPA) efficiencies of in-home use of various stove types as of the mid to late 1990's. I don't think a lot of tests have been done since, but as discussed before not too much has changed since then in terms of actual or reported efficiencies.

 

[elkimmeg]

My magic stove is working BB It might just be the operation is in concert knowing how to extract every Btu out of the damn thing
really who ha a wood supply 3 or more year out? and rotates the piles to move the bottom up to the top.

The real reason I posted here ,is not about my stove, But why did the EPA stop publishing efficiency numbers, instead of the default pass numbers 63 and 72%?

In the past I remember seeing 76-78% and I did see some posted in the 80% range.. I believe the PE summit was quoted around 70 % possibly 72 %

what about efficiencies burning bio Bricks? that would take out some variables.

 

[DiscoInferno]

So in general would we expect that inserts and ZC fireplaces are less efficient, because they have little if any exposed stove pipe? My Ultima has a short stub exposed behind the top louver, but nothing like a 5-6' run up to a ceiling that a freestanding stove might have. I do have blowers to help reclaim some heat from the stack, but I still expect I'm losing more heat up the chimney than is needed for draft (which I have plenty of). Ah well, keeps the air-cooled chimney clean anyway.

 

[elkimmeg]

calorimeter rooms, if these calculations are from heat dissipated from exposed stove pipe, that two is a double sword issue. Would the stove work better holding in heat and draft better? then doublewall pipe would screw up the situation. Do we want to extract so much heat from exposed single wall pipe that we risk increased creosote formations?

If we are extracting too much heat from the stove pipe, then it has to reduce stove performance. To a point heat gain from a pipe is not bad but exceed that point then there exist other problems and factors. To me I would rather have a better draft path in the stove, than to have to gain heat from the vent pipe.

Problem is the age of the studies mentioned none were applied to EPA stoves where the drafts were reduced as well as draft temps. 1984 in those beast fishers we had to put inline damper to slow then down not really valid in a modern PE Summit with EBT. Back then vent heat gain was a greater than todays stoves.