EPA stoves have around a 75-80 percent efficiency ratings. Which I gather means that this percentage of the heat enters the room, not the outside via the chimney. Could this efficiency rating be improved on? Or, does the need to vent exhaust, and create the draft in the chimney with warm air, result in a ceiling for efficiency around this 80 percent mark? This question has no practical application for me, I'm just curious if anyone has thoughts on it. Thanks
A theoretical question about stove efficiency
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Actually it is combustion efficiency. They don't measure thermal transfer. But your conclusion is right. Ya got give up some heat to that pipe the to make the thing work.
Highbeam
Minister of Fire
I believe it means that 20% of the btu's from those available as determined by the amount of consumed fuel are measured escaping through the flue. The rest has to go somewhere. it has to go into the room I would assume.
From the EPA, "Net (or overall) efficiency is the product of combustion efficiency multiplied by heat transfer efficiency."
So a stove with 80% combustion efficiency that has 90% heat transfer efficiency would be 72% efficient. With a typical stove, trying to get better combustion efficiency can come at the expense of heat transfer efficiency, and vica versa. A hotter burn means more complete and cleaner combustion (and a cleaner flue), but perhaps more heat lost up the flue.
The EPA is not concerned with how much heat you keep, but only with how clean you burn. And only with how clean you burn in theory, not in practice. Some stoves seem to be designed along those lines. I believe that in real-world use, properly operated cat stoves must have greater net efficiency than most other EPA stoves. But masonry heaters must be tops, I would think.
So a stove with 80% combustion efficiency that has 90% heat transfer efficiency would be 72% efficient. With a typical stove, trying to get better combustion efficiency can come at the expense of heat transfer efficiency, and vica versa. A hotter burn means more complete and cleaner combustion (and a cleaner flue), but perhaps more heat lost up the flue.
The EPA is not concerned with how much heat you keep, but only with how clean you burn. And only with how clean you burn in theory, not in practice. Some stoves seem to be designed along those lines. I believe that in real-world use, properly operated cat stoves must have greater net efficiency than most other EPA stoves. But masonry heaters must be tops, I would think.
ericj said:
That's the $64,000 question.
What is should mean is, "The percentage of the wood fiber that was actually converted into carbon dioxide and water". That has no practical meaning in a home heating situation, however. It assumes that the latent heat contain in the evolving water vapor is available to heat your interior, but it's not available since it goes up the flue. That heat is then released outside when it condenses again. It's not a trivial amount of heat, either. On a weight basis, it's exactly the same amount of heat (970 BTU/pound of water) which is lost through the evaporation of the water in your firewood (which, unfortunately, is not recoverable either).
Adding to the disappointment is the fact that burning wood creates much more water than could ever be contained in burnable wood. For every pound of 0% dry wood fiber, about 0.54 pounds of water are produced as one of the two products of complete combustion (CO2 and H2O). That's a lot of water, about 79 pounds, or close to 10 gallons of water going up and out your flue for every 100 pounds of seasoned wood you burn! And as long as all that water vapor in your flue gases leaves the top of the stack at higher than 212ºF (and it should), every bit of that latent heat contained in it is lost to the outside air. Bummer.
This leads to the concept of "low heat value", which is the total amount of potential heat in the wood (0% MC wood fiber) minus the heat lost by exhausting water vapor into the air before it has a chance to condense again. This is about 7800 BTU/pound for wood that is 20% water by weight vs. the 8600 BTU/pound used on most BTU tables.
But, heck, that's only the beginning of the problem. If we assume that low flue gas temps are the result of high thermal transfer into the living space, we ignore excess air being pulled through the stove and lowering the stack temp. As well, that air has to come from the outside, which is always colder than the inside. A stove could theoretically burn at near 100% efficiency, but draw so much excess air in to accomplish this that the net efficiency might be more like 60%.
Then there is the temperature differential between inside and outside. Heat always flows from a area of high heat to one of lower heat. The higher the temperature differential, the faster it flows. Stoves that have high peak outputs are probably getting their highest efficiency numbers at these output, but they are also raising the room temp up considerably higher, causing the heat to leave the building at a faster rate. So, it really pays to have great insulation and a stove that perfectly matches the living space. Most folks go larger than that out of fear of not having enough heat on the coldest days. The penalty for that is deceased combustion efficiency at lower burn rates and increased thermal loss during high burn rates.
I could go on with even more conundrums, but I don't want to be perceived as verbose. %-P It's all in good fun, but to me, it doesn't really matter. You do what you have to to stay warm. Obsessing over heat loss in a sure way to invite a trip to the ER, or at least cause you to develop an ulcer. I do have a problem with the way most of these efficiency specs are presented, however. I think they are sometimes grossly exaggerated and deceitfully presented, but since I cannot get the independent test data, I can't site any specific examples. I believe many, if not all, of them are using peak efficiency high heat values and then comparing them with the total net efficiencies determined 40 years ago on the original airtight stoves in calorimeter rooms. If they used their quoted figures and then multiplied them by all of the real world factors I mentioned above, then subtracted the heat lost up the flue, they'd surely arrive at a much less marketable number.
In the end, I think you would find that cat stoves all have about the same real world efficiencies, and that non-cat stoves all have about the same numbers, and non-EPA airtight stoves like mine fall in line below them both. That's why the EPA gives an arbitrarily derived efficiency number for both types. With the new hybrid stoves being developed, there may be a need for a third number... which will be just as meaningless as the other two. The only number that really matters is the number your room thermometer displays.
midwestcoast
Minister of Fire
Shoot BK, you should add the title "Senior Wood Burning Scientist" to your sig.
To the original question, since we're talking theoretically. I'd say it may be theoretically possible to solve the combustion efficiency vs transfer efficiency problem by adding a system than allows the water vapor (and creosote & other gunk) to condense out of the exhaust before going up the chimney, releasing much of the latent heat which could then be transferred to the room. The much cooler exhaust gases would then have to be power vented up a chimney or out a side-wall. The list of problems to be solved to make all that happen would be pretty daunting & I doubt I could even conceive of most of them. Even if they were solved, the economics would be ridiculous. I wouldn't expect to see an exhaust condensing woodstove ever come on the market. Maybe if we ran out of all other fuels, no other heating tech was available & wood became super valuable, so the 20% or whatever possible efficiency gain meant big money...
It's kinda fun talking out ones @$$.
To the original question, since we're talking theoretically. I'd say it may be theoretically possible to solve the combustion efficiency vs transfer efficiency problem by adding a system than allows the water vapor (and creosote & other gunk) to condense out of the exhaust before going up the chimney, releasing much of the latent heat which could then be transferred to the room. The much cooler exhaust gases would then have to be power vented up a chimney or out a side-wall. The list of problems to be solved to make all that happen would be pretty daunting & I doubt I could even conceive of most of them. Even if they were solved, the economics would be ridiculous. I wouldn't expect to see an exhaust condensing woodstove ever come on the market. Maybe if we ran out of all other fuels, no other heating tech was available & wood became super valuable, so the 20% or whatever possible efficiency gain meant big money...
It's kinda fun talking out ones @$$.
midwestcoast said:
I'm fluent in several languages originating from that orifice.
I was gonna say "several tongues", but that just sounded so very wrong.
midwestcoast
Minister of Fire
Todd
Minister of Fire
midwestcoast said:
Ya, leave it to me to spoil yet another lunch. Then usually it's, "Hey... are ya gonna finish that?" :roll:
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