Burn time / BTU output / Magically creating Energy

[Jags]

This subject has been touched on in many places and in bits and pieces throughout the forum. I have noticed some extraordinary claims of stove burn times, heat output and efficiency.

For this thread, I would like to apply some basic math and physics and remove some variables to prove a point.

In lab testing it is basically a well accepted rule of thumb that wood (fire wood) that has been dried to 0% moisture content in a 100% oxygenated environment will produce 8660 BTU per pound of wood.

Lets assume that these "perfect" conditions can be met with our stove.

Lets assume that we are trying to heat a large old farmhouse on the northern boarder of Minnesota, that requires 45,000 BTU per hour to maintain the inside temp during a nasty storm in Feb. (am I painting a picture yet?)

Lets assume we have a large stove that can take loads of fuel of 60 pounds. Thats a pretty large load even in a Blaze King, King. If you don't believe me, load your cold stove up and then remove the wood and weigh it.

8660 BTU x 60 pounds = 519600 BTU
519600 BTU / 45,000 BTU per hour = 11.5 hours.

This is in PERFECT conditions.

Lets add in some variables:
85% efficient stove (pretty darn good cat stove)
519600 BTU is reduced to 441660 BTU
441660 / 45,000 BTU per hour = 9.8 hours

Now wood - lets say 18% on the ol meter. Which will reduce heat output of said wood by roughly the same number. 18%
So we will start again:
519600 BTU output reduced by 18% = 426,072 BTU is available from the wood.
426,072 being burned in an 85% efficient stove = 362161 BTU output of the stove.
362161 BTU / 45,000 BTU per hour = 8 hrs.

This represents the best of the best that can be achieved by one of the most efficient stoves on the market and a very dedicated operator. That is VERY good burn times IMHO for trying to heat an old house on a brutal day.

Whats my point? You can't magically produce more heat than the load of fuel can deliver. It doesn't matter if you stretch the burn times out to 15 hours, you will HAVE to reduce hourly btu output to obtain that. Same holds true if you need more BTU per hour to maintain house temps, you WILL reduce burn time.

This post stems from my knee jerk reaction to some posts I have seen recently. "Yup, I can burn my stove for 20 hours", so what! I can burn a bic lighter for 20 minutes on two table spoons of fuel, but it ain't gonna heat my house.

It comes right on down to how much BTU is available in your fuel load, and how efficiently your stove can burn and deliver that to your living area. Some stove are bigger or smaller. Some stoves can boast a higher efficiency but NONE of them can generate MORE heat than the fuel load that it was feed with. PERIOD.

I know this seems very obvious to many, I simply wanted to clarify the term "Burn time" versus "Heat output"
Sorry for the rant I just felt a need to make this perfectly clear.

 

[Fsappo]

I have to have this conversation many times with customers in my showroom. I am cutting and pasting your example to save to show the more dense customers who tell me "but the stove at the tractor store has a 1.3 foot firebox and says it heats 2000 square feet, why are you showing me a 2.8 foot firebox for more money? Doesnt the tractor guy know as much about stoves as you do?" I've must have said a million times over the past 25 years.."there is only so much energy in a stick of wood"

Thanks, your rant has provided me with a great tool for my customers

 

[TomB]

Fire is magic... heat is just the magical bi-product, dont ya know.

yah, 20 hour burns, yah that's the ticket!

I totally agree with your comments.

Tom

 

[oconnor]

But if you had a magic heat....

Kidding - Excellent logic flow

 

[RustyShackleford]

I kind of agree with you, with two opposing caveats.

One is that for a "large old farmhouse on the northern boarder of Minnesota",
I doubt 45Kbtu/hr is gonna keep the house very warm during a nasty winter storm.

The second is, I don't really care if my stove keeps the house really cozy overnight;
I like to sleep cold anyhow. What I DO care about is that there's a GOOD bed of coals
in the AM, so all I have to do is add some wood, and I'm off to the races.

 

[RustyShackleford]

does the term "85% efficiency" mean how effectively the wood is burned or are u trying to imply that 85% means that the room is gonna gain 85% of the heat burned from the wood?

I'm sure he means the former; the latter makes no sense. The room will gain 100% of the heat generated
by the wood. The room will also lose heat, through the imperfectly insulated envelope of the building. You
might rate a house in terms of BTUs per hour per degree fahrenheit; a typical number might be 1000. So
if the stove is generating 45K btu/hr, it will keep the house 45 degrees warmer than the outside.

 

[Adios Pantalones]

Jags- yup. You load a certain amount of potential heat into a stove and use it quick or slow. That's it.

My new grill (that my beautiful wife got me) cooks awesome, and hotter with a bigger grill space, but it drinks propane at twice the rate of the old one.

 

[Wet1]

Good post Jags.

Just a couple of observations... a lb of seasoned oak generally weighs about 40 to 60 lbs per cu ft, but lets just use red or white oak which weighs about 45-48 lbs/ cu ft when seasoned. If you have a large stove, such as the 4.3+ cu ft BK King you used in your example, loading over 60 lbs of wood in that stove is very easy to do. In fact 90+ lbs isn't at all difficult to do. So using your numbers and assuming you stick 90 lbs of seasoned oak in the box, that 8 hour burn (which is very impressive given the BTUs being cranked out) is stretched to 12 hours. Now remember, you're talking about that cold Feb. day when 45k btu/hr is required. Indeed we have many of those days during peak heating season, but we also have significantly more where 45k/hr isn't required. IMO, having an 8 to 12 hr burn time during peak heating season is excellent, but even more so, having the ability to get significantly longer burn times for the majority of the heating season (which requires even fewer btu/hr) is even more significant since this represents more of the heating season.

But I completely agree with you, more BTU means short burn times and vice versa. But it's nice to have your cake and be able to eat it too...

And for those that don't believe a wood stove can have 20+ hour burns, look at the numbers, it's obviously possible (it's actually a reality).

 

[Jags]

Pook - the 85% number is the efficiency of the stove not room.

In fact 90+ lbs isn't at all difficult to do.

Even Blaze King only advertises 80 pounds of fuel....

Anyhow, the above was not to dispute burntimes, or fuel loads or weight of firewood or any of the possible variables like room heat loss. Just pointing out that you can't get more heat from the stove, than you put in it. Simple.

Heck, I have had coals in my IR for 24 hours, but that really is misrepresenting the stove.

I like the way All Purpose put it: "You load a certain amount of potential heat into a stove and use it quick or slow"

 

[branchburner]

Now remember, you're talking about that cold Feb. day when 45k btu/hr is required. Indeed we have many of those days during peak heating season, but we also have significantly more where 45k/hr isn't required. IMO, having an 8 to 12 hr burn time during peak heating season is excellent, but even more so, having the ability to get significantly longer burn times for the majority of the heating season (which requires even fewer btu/hr) is even more significant since this represents more of the heating season.

A great point, and why a cat stove makes more sense than a downdraft stove like mine for someone in a moderate climate. My stove design, when burning smoke, cranks it out whether I need the BTUs or not. When run efficiently it has one setting: hot.

 

[tiber]

I am using a BIC lighter to heat my home and I am offended by your post.

Also cold. So very cold...

 

[Wet1]

Pook - the 85% number is the efficiency of the stove not room.

In fact 90+ lbs isn't at all difficult to do.

Even Blaze King only advertises 80 pounds of fuel....

Anyhow, the above was not to dispute burntimes, or fuel loads or weight of firewood or any of the possible variables like room heat loss. Just pointing out that you can't get more heat from the stove, than you put in it. Simple.

Heck, I have had coals in my IR for 24 hours, but that really is misrepresenting the stove.

I like the way All Purpose put it: "You load a certain amount of potential heat into a stove and use it quick or slow"

Look at the specs on page 4, they specify "Oak 90#". In reality, it should not be difficult getting over 90# of oak in a box that size given that it's close to 50 lb/cu ft.
(broken link removed to http://www.blazeking.com/Brochures-En/WoodProductPDF/KE1107TaxCredit.pdf)

Regardless, your point is well taken!

 

[Todd]

45000 BTU's per hour is a pretty high burn rate and I bet once your house is up to temp you would only need less than half that to maintain your comfort range even if it's a drafty old farm house in N Minnesota. It would also be hard to maintain a constant 45000 BTU output with the up and down spikes and other variables associated with wood burning. But yeah, there is only so many BTU's available per load.

 

[Slow1]

Good post. I agree with your key point - there is only so much available energy in the fuel.

I've always wondered about the efficiency numbers on the stoves though since the only real number that matters to me in terms of burn efficiency is "What % of the available heat energy of the wood I put in the stove actually gets into the room". If that number were consistently calculated and published for all stoves that would be the one I'd use for comparison. Just getting it out of the wood is all fine and dandy, but if a lot of it goes up the chimney (as in my old stove with the 450-550* flue temp during most of the burn) then I may as well have left it outside to rot in the woods. I don't think any of the published numbers actually answer this question.

I don't have a way of really directly measuring this efficiency either. Certainly I can't give it a number. However I can compare my two stoves in a number of ways. The most obvious I've already alluded to - that is the flue temperature. Old stove had to be at least 450* or I knew I would likely have smoke. Generally speaking it would be around 500. This is surface temp on single wall pipe. It would sit there for the bulk of the burn - basically until coaling stage. Then I would either re-load and take it there again or let the fire cycle down and out. New stove measuring the same way the flue will run 150-250 during the whole burn once cat is engaged (and no smoke is seen). I have to believe that this 300+* difference in flue temp (or is it doubled to 600* difference inside?) is a lot of heat staying in my house instead of leaving up the pipe. Thus I believe that this stove configuration is more efficient. Over the course of the winter I'll be tracking actual wood burned on a daily basis as I did last winter and compare the fuel used/degree days and see if I am actually using less wood. Again imperfect and somewhat subjective but the best I can do to try and get an objective measure of efficiency. I can't measure the actual BTU output of the stove or know the true BTU needs of the house.

Burn time is a completely different subject to me than efficiency. As posted in other discussions, the definition here is about as clear as mud. Again - some define it as the time from first match light to last ember going cold to the touch. Others would take some stove temperature as "usable output" say 300* (arbitrary example) and say burn time is the time spent above this temp then spend their days arguing the exact temp that is "usable output".

I look at burn time from a practical standpoint of "When I last fed the stove, how long can I go before loading it and still be able to just put splits in the stove and get it back into clean/efficient burn state within 10-15 minutes". In other words - how long can I go between 'normal' loads and not have to do anything special in the loading procedure to get back to efficient burn state. No kindling, no match, no blowing on coals to get them hot enough to burn. I expect my "burn time" to be a variable time period and to vary depending on the amount/type of wood I load and the amount of heat I'm pulling (i.e. air settings) from the stove. Again - look at the marketing material and you won't find a definition of "burn time" yet they all will give something that looks like it should be comparable to other stoves - nice bullet point format etc.

Yes, I'm one of the folks here chatting about amazing burn times on my new stove. It is due to the contrast between what I had before and what I am experiencing now. I am burning low and slow - shoulder season. I put a few pieces of wood in and find hot coals left over 12 hours later I'm still impressed as this would never have happened for me last year. This morning I left the house after getting a fire going with only three splits and kindling - stove putting out 250-300 surface temp and no smoke - old stove wouldn't even have been able to get a coal bed ready to go into clean burn mode with that much wood. My expectations were set rather low by my prior experience. I had high expectations last year due to the marketing material I had read and bought into (before I found this site btw).

I think that manufacturers/sellers/marketing departments have done a disservice in their use of these terms by clouding the meanings and putting "up to" numbers side by side. "Max BTU" "Up to 2000sq ft heating" and "10-12 hours burn time" all next to each other naturally gets folks thinking that they can have all three at the same time. Doesn't make sense of course, but nothing in the literature tells them otherwise. True there are a few honest sellers (seem to be the ones helping folks out here) that will TRY to explain these tradeoffs to folks, but it sure must be hard to break folks out of their fantasy world after some other stove shop has sold someone on the dream of getting all of the best case scenarios at once. Oh well, off my rant

 

[grommal]

Yep, the old "X hr burn time" comments are hard to interpret if you don't know what the person's personal definition of a useful "burn" is!

I can easily get 8-9 hour "burns" with my Oslo, but my definition of the end of a useful burn is that the firebox still contains a nice bed of coals that is enough to efficiently start the next load without the use of kindling or newspaper, and also still has a decent stovetop temperature so that useful heating is still going on (300F or close).

To me, the end of a useful "burn" doesn't need to have active flames and a 500F stovetop, but just having a half cup of pea-sized coals in a 150F stove is not enough either.

 

[Corey]

Preach on brother Jags. Basically comes down to what is interpreted as 'burn time'. I think we hit that subject at least once or twice a year. Is it the point where you have to add more wood to keep warm, point when the flame dies out, point when you just have a coal bed, point when the last coals die out, etc. It's a pretty subjective term. Throwing in different types of wood, moisture content, heat rate output, it doesn't get any better. There is also a big difference in 'keeping XXXX square feet warm' and 'heating XXXX square feet from a cold temperature'

 

[firefighterjake]

I am using a BIC lighter to heat my home and I am offended by your post.

Also cold. So very cold...

 

[RustyShackleford]

does the term "85% efficiency" mean how effectively the wood is burned or are u trying to imply that 85% means that the room is gonna gain 85% of the heat burned from the wood?

I'm sure he means the former; the latter makes no sense. The room will gain 100% of the heat generated
by the wood. The room will also lose heat, through the imperfectly insulated envelope of the building. You
might rate a house in terms of BTUs per hour per degree fahrenheit; a typical number might be 1000. So
if the stove is generating 45K btu/hr, it will keep the house 45 degrees warmer than the outside.

ok now figure in stove fluegas temps & associated heatloss ...

Sorry, I see what you meant. I THINK when BK or somebody claims 85%, they are allowing
for the heat loss through the flue gasses - but I'm not sure ..

 

[Jags]

I am using a BIC lighter to heat my home and I am offended by your post.

Also cold. So very cold...

Sorry tiber - I was not trying to slight the BIC lighter heating units or those that use them. (you may want to step up to the Zippo world, they are at least twice the btu output). Just trying to help a brother out.

And Todd - you probably are correct that a constant 45000 btu output is a bunch of heat for a given house, but I wasn't gonna try and work out the math. As a side note to that, I have calculated the heat requirements of my house on those crappy winter days, and it was surprising how much I needed. Just more proof of the improvements I need to make.

But the numbers and the math was done for nothing more than to back up my point. Completely arbitrary, yet true. A "visualization" if you will.

Arguments can be made for long burn, short burn, soap stone, cat, non-cat, etc. But you can't generate more energy (heat) than what was applied or available.

 

[Highbeam]

“You load a certain amount of potential heat into a stove and use it quick or slow”

This is not true in terms of heat delivered to the room. Well, it needs to be clarified as potential heat is a long ways from delivered heat. The efficiency of any stove is not constant at all output settings.

You can't use the energy too slowly or your efficiency drops and you blow it out the chimney unburned, you can't use it too fast or it cleanly burns up and flies up the flue as wasted flue heat. There is a sweet spot with a non-cat that is smaller than the spot on the cat stove where you get the most of that potential heat converted into room heat.

This is fine if you want to operate in the sweet spot but when we are trying to get max burn times or max heat output we are operating outside of the sweet spot. I strive for max burn times at the expense of lost efficiency and lower heat output, since the large majority of burns are to maintain the heat level in my home.

 

[Adios Pantalones]

“You load a certain amount of potential heat into a stove and use it quick or slow”

This is not true in terms of heat delivered to the room. Well, it needs to be clarified as potential heat is a long ways from delivered heat. The efficiency of any stove is not constant at all output settings.

You can't use the energy too slowly or your efficiency drops and you blow it out the chimney unburned, you can't use it too fast or it cleanly burns up and flies up the flue as wasted flue heat. There is a sweet spot with a non-cat that is smaller than the spot on the cat stove where you get the most of that potential heat converted into room heat.

My point was that out of some amount of wood you'll only get a certain amount of max total heat. Of course there are more and less efficient stoves and conditions.

 

[PapaDave]

I am using a BIC lighter to heat my home and I am offended by your post.

Also cold. So very cold...

Just out of curiosity, how many bic cord do you use/month? :cheese:

Dave

 

[Danno77]

lots of reading, but good original post with decent follow up.

with regards to the 85%. This means that 85% of the energy created is used to actually heat the room by the stove. maybe you gain another 1 or 2% from stovepipe, etc.

Anyway, to follow up with the post of HighBeam, the energy content of some wood is constant no matter how quickly you burn it. This is clarification, it does not conflict with his explanation, but it's important to note that if you have 100,000BTUs in some wood then it doesn't matter if it burns the entire load by simply smoldering, or if it explodes in one huge bang.

Don't tell me that moisture content will make 100,000BTUs actually 80,000BTUs. the number of BTUs in any given load should be considered an absolute. Potential BTUs should be used in place when saying "Oak has X potential BTUs...." and then considering moisture level to be at "M%" and saying that a particular load is "approximately X BTUs".

SOOO, back to 85% efficiency. If you have an 85% efficient stove, then that means that 15% of the BTUs went up the chimney without warming the room.

If you light wood in a stove with no chimney pipe, then you have a 100% efficient heating device. You'd be dead, but you would get a really efficient use of the energy stored within the wood.

 

[Danno77]

wrong = 85% of the wood is combusted per EPA pollution standards for smoke exhausted

Fair enough. 100% of the wood isn't converted to energy with 15% of said energy wasted. It's more like 85% is converted to energy and 15% is carried away with some more wasted energy.

So, an 85% efficient stove only uses X% of 85% of the wood's BTUs.

 

[daveswoodhauler]

Yeah, but if the wood is in rounds vs. splits, will it take up more room in the stove?