# Stove efficiency calculation

the first thing is to try to optimize the functioning of the stove in possession, actually understand which setting and draft allows for greater performance, the yield, I think that's the right term to use, for me efficiency means more the heat produced in proportion to the wood inside, without considering the quantity that is transferred to the house, yield also considers the heat transferred
For my poor English, if I understand correctly, the term YIELD would be the best one to use and could be easier to understand for all.

At 2000*F the little stove will disappear...lol. In fact, what I wanted to do was to see if many peoples exactly knew/understand the real meaning of the term : EFFICIENCY in the real world or in the house ! I never wanted to frustrate someone on the forum here.
And pretty much everyone has given you the same answer with varying levels of detail. You are just having trouble separating your surface temps from actual analysis of btu outputs losses

And pretty much everyone has given you the same answer with varying levels of detail. You are just having trouble separating your surface temps from actual analysis of btu outputs losses
All that is about when someone see : 400*F on top of the stove and 300*F on the stove pipe, it looks like some heat is going outside. Maybe the translation from French to English doesn't reflect my exact mind ? But I like the idea of prometo : yield.

Some heat is always going outside. That is necessary for the chimney to function and to stay clean.

Even in a case of a stove top temperature LOWER than the flue temp, it can still be putting more heat into the home than up the flue.

the doubt I have is about the baffles, how much heat they absorb, without giving it back, especially when the draft is low

Once the stove is hot (stable temp), the heat can only go two ways. Into your room or up the flue.

This is the practical consequence of the (thermodynamical) law of nature that states that energy does not get lost.

Baffles do keep the (stable) temp of the firebox higher, but they do not "rob heat" from anywhere - i.e. as long as the stove metal and baffle (if not metal) is stable in temp, the heat that is being produced in the firebox is still going either in your room or into the flue (and the ratio is the efficiency).

The baffle does help more completely combust the fuel, meaning less partially combusted compounds (i.e. with contents that could have produced heat but didn't) go up the flue, and thus more heat is produced in the stove.

Prometeo
Some heat is always going outside. That is necessary for the chimney to function and to stay clean.

Even in a case of a stove top temperature LOWER than the flue temp, it can still be putting more heat into the home than up the flue.
Yes always some heat is going to the chimney but when 400*F on top of the stove and 300*F on the stove pipe is not the best yeld. Not much heat is needed to get a draft. I burned anthracite for some yrs and got 400*F on the stove and 100*F on the stove pipe and sometimes less than 100*F without draft issue. In fact , the efficieny advertised for wood stoves is the weighted average from a serie of tests, in the BEST conditions : air damper closed and cat engaged. The other tests results made in different conditions may be a great surprise for many ones...but they are not mentionned on the advertisements.

the doubt I have is about the baffles, how much heat they absorb, without giving it back, especially when the draft is low
What? Baffles are the most basic form of increasing efficiency simply by both slowing the exhaust exiting the stove and lengthening the smoke path. Both of those things allow more time for heat transfer

Prometeo
the doubt I have is about the baffles, how much heat they absorb, without giving it back, especially when the draft is low
Hum, hard to say but the baffles are inside of the stove, so that heat is probably going to heat the home, but the problem is when that hot air quit the cats there is no heat exchange because the exit of the stove is so near of the stove outlet going to the chimney. There is nothing to slow down the gases flue like a longer path or some hear exchanger, The Woodstock PH has a heavy cast iron heat exchanger far from the stove's outlet.

Prometeo
Yes always some heat is going to the chimney but when 400*F on top of the stove and 300*F on the stove pipe is not the best yeld. Not much heat is needed to get a draft. I burned anthracite for some yrs and got 400*F on the stove and 100*F on the stove pipe and sometimes less than 100*F without draft issue. In fact , the efficieny advertised for wood stoves is the weighted average from a serie of tests, in the BEST conditions : air damper closed and cat engaged. The other tests results made in different conditions may be a great surprise for many ones...but they are not mentionned on the advertisements.
No you don't need that much heat to create draft. But you can't go to much lower without dropping below the condensation point causing issues with creosote.

And of course they are giving you results with the cat engaged and air set to the appropriate level. That is how you properly run the stove. I don't understand your argument there at all

Hum, hard to say but the baffles are inside of the stove, so that heat is probably going to heat the home, but the problem is when that hot air quit the cats there is no heat exchange because the exit of the stove is so near of the stove outlet going to the chimney. There is nothing to slow down the gases flue like a longer path or some hear exchanger, The Woodstock PH has a heavy cast iron heat exchanger far from the stove's outlet.
Maybe that is the case in your stove. But for many others it is not

What? Baffles are the most basic form of increasing efficiency simply by both slowing the exhaust exiting the stove and lengthening the smoke path. Both of those things allow more time for heat transfer
But when hot air continue to get in the cats, that is the hottest and last point of the gases combustion, that is where it would be important to have longer path or heat exchanger before exiting the stove.

But when hot air continue to get in the cats, that is the hottest and last point of the gases combustion, that is where it would be important to have longer path or heat exchanger before exiting the stove.
Yes... and they can't allow the exhaust temp to drop much lower than they have it already on most stoves without having creosote issues right? So how to you propose extracting more heat would work?

What? Baffles are the most basic form of increasing efficiency simply by both slowing the exhaust exiting the stove and lengthening the smoke path. Both of those things allow more time for heat transfer
It is clear that today's stoves are made to work with the baffle, but I wonder if that is the best design, why? Because if I want to use the stove with a low draft, the flame licks the baffle, discharging a lot of heat onto it, and especially a vermiculite baffle, will just absorb that heat, without even giving it to the chimney, I have the impression that the power of the flame was lost that way, while a flame touching the top directly would perhaps give higher temperatures, but the stove should be designed differently.

Yes always some heat is going to the chimney but when 400*F on top of the stove and 300*F on the stove pipe is not the best yeld. Not much heat is needed to get a draft. I burned anthracite for some yrs and got 400*F on the stove and 100*F on the stove pipe and sometimes less than 100*F without draft issue. In fact , the efficieny advertised for wood stoves is the weighted average from a serie of tests, in the BEST conditions : air damper closed and cat engaged. The other tests results made in different conditions may be a great surprise for many ones...but they are not mentionned on the advertisements.
I'll stop after this one as it appears I'm not able to explain it.

NO, you can't conclude that 400 F stove top and 300 F flue is poor efficiency.
The far, far majority of BTUs is going into your room at that point. It is NOT about temperatures, but heat flow. That is determined by temperature differences (stove top vs room air), surface area (because the temp difference gives a heat flow *per square inch*), and interfacial transfer efficiencies.

Because your stove is so much larger (surface area) it puts in a lot more heat into your room than up the flue, even if the temperature is low (400 F). This is because the *energy* content of air (or gases) going up the flue is not high, *even if there temperature may be high*. (In more expert language, the heat capacity of air is rather poor - which is the reason we mostly use fluids to transport heat).

Finally, the efficiency for my cat stove was measured to not be different for running (very) low versus running high - as per bkvp quoting (independent!) test results (where indeed BTU flows were measured rather than only temperatures).

bholler
Maybe that is the case in your stove. But for many others it is not
Maybe but my stove is rated 81% efficiency and that is in specific and best conditions to get that. No relations with the heat staying in the house.I think that it would be better for everybody and easier to understand if the stove advertisements were from : OVERALL EFFICIENCY, wich is different fromcombustion efficiency.

It is clear that today's stoves are made to work with the baffle, but I wonder if that is the best design, why? Because if I want to use the stove with a low draft, the flame licks the baffle, discharging a lot of heat onto it, and especially a vermiculite baffle, will just absorb that heat, without even giving it to the chimney, I have the impression that the power of the flame was lost that way, while a flame touching the top directly would perhaps give higher temperatures, but the stove should be designed differently.
No, once it's been heated up to a stable temperature it won't absorb more heat.

Not having a baffle leads to hot gases go directly up the flue hole. The much shorter path of that leads to much less energy transferred to the room. (See masonry stoves with their long flue paths- a baffle in a stove is the same concept).

It is clear that today's stoves are made to work with the baffle, but I wonder if that is the best design, why? Because if I want to use the stove with a low draft, the flame licks the baffle, discharging a lot of heat onto it, and especially a vermiculite baffle, will just absorb that heat, without even giving it to the chimney, I have the impression that the power of the flame was lost that way, while a flame touching the top directly would perhaps give higher temperatures, but the stove should be designed differently.
That is how stoves used to.be designed and it was found that a baffle greatly increased efficiency which is why most stoves now have them

Prometeo
Maybe but my stove is rated 81% efficiency and that is in specific and best conditions to get that. No relations with the heat staying in the house.I think that it would be better for everybody and easier to understand if the stove advertisements were from : OVERALL EFFICIENCY, wich is different fromcombustion efficiency.
Most people don't seem to have an issue understanding it

I'll stop after this one as it appears I'm not able to explain it.

NO, you can't conclude that 400 F stove top and 300 F flue is poor efficiency.
The far, far majority of BTUs is going into your room at that point. It is NOT about temperatures, but heat flow. That is determined by temperature differences (stove top vs room air), surface area (because the temp difference gives a heat flow *per square inch*), and interfacial transfer efficiencies.

Because your stove is so much larger (surface area) it puts in a lot more heat into your room than up the flue, even if the temperature is low (400 F). This is because the *energy* content of air (or gases) going up the flue is not high, *even if there temperature may be high*. (In more expert language, the heat capacity of air is rather poor - which is the reason we mostly use fluids to transport heat).

Finally, the efficiency for my cat stove was measured to not be different for running (very) low versus running high - as per bkvp quoting (independent!) test results (where indeed BTU flows were measured rather than only temperatures).
The tests are quoted from labs and serving for the advertisements from companies are the BEST ones in the best conditions for a cat stove so : air damper completly closed and cat engages. They never show the other and far from the best ones. And there are the ** combustion efficiency tests ** and the : ** overall efficiency tests ** the last one would be a lot better for everyone.

Most people don't seem to have an issue understanding it
NO, I did ask to a lot of peoples, even wood stove stores peoples, and even here and not many can explain simply and really what efficiency from stove advertisemts really mean !

The tests are quoted from labs and serving for the advertisements from companies are the BEST ones in the best conditions for a cat stove so : air damper completly closed and cat engages. They never show the other and far from the best ones. And there are the ** combustion efficiency tests ** and the : ** overall efficiency tests ** the last one would be a lot better for everyone.
You do know they test those things right? And it's readily available to anyone who cares to look.

Now of course the stove companies are going to pick the test results that make them look the best to include in their marketing material. That is just common sense

NO, I did ask to a lot of peoples, even wood stove stores peoples, and even here and not many can explain simply and really what efficiency from stove advertisemts really mean !
Is it that they can't explain it? Or that you don't understand the explanation? Because it has been explained here quite a few times and you still aren't understanding.

@bholler @stoveliker @nortcan

I always think of a full pot, to boil that water, direct flame works, but if someone invents some kind of baffle, that water won't boil, only with the heat produced by the flame, this is why I wonder if baffle is really the optimal solution, I agree that heat goes a long way but the power of the flame that licks the metal directly is lost

You do know they test those things right? And it's readily available to anyone who cares to look.

Now of course the stove companies are going to pick the test results that make them look the best to include in their marketing material. That is just common sense
Do you know many peoples looking to buy a wood stove that read all the lab tests for different stoves then compare the lab results from all the series of different tests before buying a stove ?? If they were doing so , they all would come on the forum to ask : what is the best stove to heat my house LOL.