Burn time / BTU output / Magically creating Energy

[Danno77]

85% of the wood is converted into h2o & co2 which is a perfect burn from smoke analysis

Gotchya, but the energy from converting into h2o and co2 doesn't all go into heating the room. Some of that energy is lost up the chimney. if that were not the case then there would be no heat anywhere in the pipe or chimney, it would all be at the stove. So, efficiency of a stove is merely a comment on pollution, and does make some reference to amount of wood converted to energy, but it does NOT indicate how much of the energy heats the room that the device is located in.

 

[Slow1]

EPA is concerned about emissions, not efficiency. They look at what comes out of the stack - particles and gases. It seems that prior to the recent tax rebate free-for-all manufacturers either used some default efficiency numbers (mostly for Cat stoves) or came up with a number from somewhere else (who the heck knows where?) but still it doesn't seem clear exactly what that number really was or what standard they were going by. Once the tax deal came down and there was an incentive to get a higher number the manufacturers looked close at what was required to get certified, found out whatever definition was needed to get there and got 'tested' where ever needed and what have you to meet that requirement. I still don't see any of them publishing actual comparable efficiency numbers - do you?

Seems like smoke and mirrors. Perhaps someone "in the industry" can actually explain in a simple clear way how they come up with these numbers and IF there is any sort of consistency to them. Knowing how they are calculated may shed light on what they are good for in terms of implications. Likely they are lab values only - too many real-world variables anyway.

I've read so many folks speculate and even make authoritative sounding statements about the meaning of these numbers, but I haven't seen much of any references to published documents to back up these statements. Perhaps there is a standard (ANSI?) out there that folks are using - if so, a reference should be able to be found don't you think?

 

[53flyer]

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.

1) The room will NOT gain 100% of the heat generated (i.e. "up the chimney").

2) There is most certainly TWO distinctly separate ways to think of stove efficiency irt heating:
A) How efficiently the wood is burned.
B) How efficiently the heat generated by burning is transferred to the room.

(A) could be the same for the same model free standing stove and insert but (B) would be better for the free standing due to the extra surface area available for heat transfer.

I truly wish they'd make an industry wide standard that would combined both but it would involve a complicated testing process that would be cost prohibitive so that's a pipe dream of another sort for now...

 

[53flyer]

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".

More in an effort to add another take rather than to dispute Danno I'll say this: Going back to the original post that referred to 1lb of wood having X amount of BTUs- Moisture content has a big impact on BTUs available in that 1lb at 0% would obviously have a lot more potential BTUs than 1lb at 30% because to get to 1lb at 0% requires more wood.

 

[Jags]

Going back to the original post that referred to 1lb of wood having X amount of BTUs- Moisture content has a big impact on BTUs available in that 1lb at 0% would obviously have a lot more potential BTUs than 1lb at 30% because to get to 1lb at 0% requires more wood.

Which is why I added in the 18% value for the calcs.

 

[SolarAndWood]

A stove with a big firebox lets you put a bunch of btus in it. This time of year, junk that is laying around will do while in a few months some nice dense very well seasoned wood is required to achieve long burn times with comfortable BTU output. A catalytic stove buys some flexibility when the heat demand is low and alleviates concerns that it is over sized for your house/climate. Our stove for example, puts out less than 9K but/hr on low burn and has a 4+ cu ft firebox.

As for efficiency, is a cat stove any more efficient than a modern non-cat at anything other than low burn?

I am 12 hours into a burn of poorly seasoned cut offs over a few splits of pine with more than half the firebox full of fuel and the catalytic comfortably active. Perfect for a cloudy day with a high in the 50s, especially in a house with no other source of heat other than solar gain. Below zero temps and high winds are common on the ridge we live on. If we get overnight burns in those conditions, I will be ecstatic.

 

[raven]

dam good post Jags. it wasn't long ago i came across this forum looking for info and facts about burning wood. people come across this forum every day looking for help making decisions. keeping it real is just what they need to make those choices............dam good post

 

[woodgeek]

Jags--agree totally with the post, but I think you need to discount more BTUs for the water vaporization. You do have 18% less combustible wood per pound, but you also need to vaporize the water at 1050 BTU/lb. So that drops you by at least 0.18*1050 = ~200 BTU per lb of (moist) wood.

 

[PunKid8888]

I burn wood because its simple and elegant. If I want more heat I throw more wood in. ahhh warmth!

 

[Jags]

Jags--agree totally with the post, but I think you need to discount more BTUs for the water vaporization. You do have 18% less combustible wood per pound, but you also need to vaporize the water at 1050 BTU/lb. So that drops you by at least 0.18*1050 = ~200 BTU per lb of (moist) wood.

Yeah woodgeek, that is true, but I was trying to keep it simple. As a matter of fact at the 200 btu per pound that you quoted above x the 60 pound load comes to about 15 min. of burn time. I just didn't want to muddy up the water any more than needed.

 

[woodgeek]

Jags--agree totally with the post, but I think you need to discount more BTUs for the water vaporization. You do have 18% less combustible wood per pound, but you also need to vaporize the water at 1050 BTU/lb. So that drops you by at least 0.18*1050 = ~200 BTU per lb of (moist) wood.

Yeah woodgeek, that is true, but I was trying to keep it simple. As a matter of fact at the 200 btu per pound that you quoted above x the 60 pound load comes to about 15 min. of burn time. I just didn't want to muddy up the water any more than needed.

That's cool. Just figured some anal retentive types (um, I guess that would be me) might care.

With your method, I get 7100 BTU/lb for 18% wood. Mine is 6900 BTU/lb, and doesn't include the energy penalty to heat the H2O vapor to typical combustion/stove temps. I think Craig said the recent EPA eff numbers assume 6400 BTU/lb input for 'air dried firewood', and that this change (assuming a lower theoretical input) was why all the eff numbers just took a step up.

Of course the lower the number, the stronger your original point--that reload frequency is set by demand, not 'burn time'.

 

[Jags]

--that reload frequency is set by demand, not 'burn time'.

Ding - Ding -Ding. I am in no way trying to slight stoves that have the ability to maintain a low and slow fire. I just want it to be perfectly clear that doing so just reduces the heat output of the stove. For many - that is a good thing.

But...I have seen a few claims on burn time and btu that led me to believe that some were squeezing btus out of their stoves that was never loaded through the door. ;-P

 

[Patapsco Mike]

I can see how some might misinterpret long burn times as more BTU's, rather than as the same BTU's over a longer period. It's a good clarification. I bought my BK Princess precisely because I have a LOT more control over BTU output.

In Zone 7 of Maryland, we only have about 1 month worth of days each year when it's darn cold (daytime highs below freezing). That is the only time I need to really crank out some serious BTU's. During those days my cat isn't going to make much difference over my (really nice) old non-cat EPA stove. In fact, I'm sure when it's brutally cold out there will be days I'll wish I had my Olympic and it's insane heat production back. But for the other 4 months of stove use I will be able to get greatly extended burn times and I'll be burning far more efficiently than I was burning small loads in the Olympic that barely get the secondary burn working. I'll be spending a LOT less time fiddling with the stove controls and loading wood- and I'm sure I will burn less wood.

 

[Lumber-Jack]

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.

Obviously what you say about deriving extra BTUs out of so much wood is correct, but sometimes you can heat your house more (efficiently) and comfortably with longer (in-efficient?) burns.
Right now, where I live, with temperatures lows around 5C 41F and highs around 14C 57F I'm just not as concerned with getting all the BTUs out of my wood, at the expense of cooking us out of the house, as I am with taking the chill off the house. A couple small fires a day is all I need and if I can stretch the burns out over a longer period, rather than two quick bursts of heat, I'll have a much more comfortable house.
When the temperature does get colder outside and I need more heat I can, of course, begin to burn more efficiently BTU wise, and when I need those long overnight burns I'll really crank up the stove a while before going to bed and reload, getting it burning good and then shut it down for that long (in-efficient) burn. The combination of heating up the stove room before going to bed, and the long burn helps to insure I have enough heat in the house overnight and I'll still have coals to start a new fire quickly in the morning.
So for me, a guy who burns mostly softwood, The "So What!" about long burn times is not the importance of extracting maximum BTUs, but of maintaining an (active) fire in the stove with the least degree of fiddle factor maintaining that fire, and that has a sort of efficiency (labor wise) in itself.
And by active fire I mean one that does not require the use of a bic lighter to get it going again. ;-P

 

[Jags]

Carbon Lib - I think you are missing the main point of the thread. As I stated above, I have respect for stoves that can cleanly extend burn times for the milder weather. When getting into the coaling stages I will often tune my stove very low and can extend the burn time for many hours.

The main point I was trying to make (with my simplistic math analogy) was that BTU output (in total) has nothing to do with burn time. It has to do with fuel load and efficiency. Can't get more out of it, then you put in.

 

[Slow1]

Put another way:

You can get 45K btu/hr from your stove.
You might get some heat and coals in your stove for 16 hours on a single load.
BUT you are NOT going to get 45K BTU/hr for 16 hours on a single load.

How's that Jags? Good enough summary of your point?

 

[Jags]

Put another way:

You can get 45K btu/hr from your stove.
You might get some heat and coals in your stove for 16 hours on a single load.
BUT you are NOT going to get 45K BTU/hr for 16 hours on a single load.

How's that Jags? Good enough summary of your point?

I likey!

 

[grommal]

You can get 45K btu/hr from your stove.
You might get some heat and coals in your stove for 16 hours on a single load.
BUT you are NOT going to get 45K BTU/hr for 16 hours on a single load.

That about sums it up.

 

[tickbitty]

Similar argument could also be made for human body regarding calories, exercise, and weight loss (or gain!)
But just cause it's logical doesn't mean it's gonna sink in for everyone!

 

[Slow1]

Put another way:

You can get 45K btu/hr from your stove.
You might get some heat and coals in your stove for 16 hours on a single load.
BUT you are NOT going to get 45K BTU/hr for 16 hours on a single load.

How's that Jags? Good enough summary of your point?

Now, with that said it doesn't mean that all stoves are equal at their ability to deliver the BTUs in a given load of wood to the room where the stove lives vs up the flue. That, however, is a very different argument and clearly requires a different language. If only we could figure out how to measure this objectively so that folks could tell if the way they are operating their stoves really is optimal - for those anal types like myself who like to optimize things to the n-th degree.

Clearly the 'big wins' have been identified - things such as well dried/seasoned wood and proper stove maintenance (gaskets sealing well, keep cat dusted, whatever).

However, I'd like to know for example does adding a flue damper actually increase efficiency (defined here as amount of heat delivered into a room per load of wood) or simply spread those BTUs out over a longer period of time? Or another one - it seems that there should be a 'sweet spot' operationally of running a stove the right temperature to maximize efficiency. I would postulate that this temperature is a range and that in fact it will vary not only by stove, but by particular installation of even the same stove model. Ok - if you accept that, then how can someone objectively determine if they are operating their stove within that sweet spot?

I'm not looking for emotional arguments (or any other arguments really) rather discussion on how, if it is even possible, to make said determinations. Would it require lab type equipment that is outside the reasonable realm of home folks? Could it be done based on temperatures only or would one need to monitor CO levels in the flue as well? Perhaps this deserves it's own thread...

 

[branchburner]

Another comparison is gas milege and fuel tank size and speed/distance traveled. But, hey doesn't a Magic Heat turn your stove into a woodburning version of a Prius? I thought that was the "magic" part - it comes loaded with free phantom BTUs!

 

[Stephen in SoKY]

How about my bi-metallic thermostsat? It seems to allow me to use only the BTU's necesary to maintain room temp thereby spreading those BTU's over a longer time.

 

[Jags]

Would it require lab type equipment that is outside the reasonable realm of home folks?

I think to get verifiable data, that this is the case.

 

[SolarAndWood]

If we accept that, at a decent burn. efficiency differences between a cat and a non-cat are in the noise, what is there other than the amount of fuel you put in and the amount of heat you need to keep the house warm? These things are space heaters and need to put out whatever we feel we need to be comfortable at any given moment regardless of their sweet spot. 45k/hr for an entire 90 lb load in the stove would probably take the paint off the walls in my house.

 

[Lumber-Jack]

Nope Jags, I don't think I missed your main point at all. Go back and re-read my post, in my first sentence I agreed with your main point, but what I was commenting on was why you said you made the post in the first place, and that was your knee jerk reaction to recent post like “Yup, I can burn my stove for 20 hours”, and your statement "So What".
I then went on to say why long, though in-efficient, burns are important to some people, why they might make posts about their long burn times, and that they CAN actually heat their house (under certain conditions), unlike your bic lighter hyperbole.
So, now if I make a post that I can sometimes get 8 hour burns with my softwood your knee won't jerk so bad. However if I make the statement that I can heat my house on one load for 20 hours, when it is -10 F -23C outside, be careful that you don't knee yourself in the chin.