What does stove efficiency mean?

[NoGoodAtScreenNames]

Was looking at the fuel cost comparison tool on the site (pretty cool) and got to thinking about efficiency ratings for stoves.

My mfr lists my stove as 84.7% efficient. Does this mean it burns 84.7% of the fuel? Does it mean that 84.7% of the BTUs burned in the wood are transferred to heat the house vs going up the chimney? I suspect many of you will answer "it's whatever the mfr wants it to mean..."

 

[EatenByLimestone]

Eh, there are lots of variables that factor into how efficient our stoves operate. I wouldn't choose or pass on a stove based on that number. Pick the stove your wife likes.

 

[Lake Girl]

Blog of forum member on actual tested efficiencies http://forgreenheat.blogspot.ca/2016/06/wood-and-pellet-stoves-with-actual.html

This explanation is dated (EPA has changed their requirements on efficiency and particulate emissions) but gives a fairly simple explanation.
http://www.pelletstovefires.com/pellet-stove-efficiency.html

 

[branchburner]

As for the calculator tool on this site:
"Efficiency is defined as the percentage of available heat in the fuel that is actually delivered into the room."

So that is NOT the 84.7 listed for your stove. I would guess that number must include both combustion efficiency AND heat transfer efficiency, the latter which is surely below 85%.

As for manufacturers, this is from woodheat.org:
"Unfortunately, none of the really useful performance ratings shown on stove brochures (like efficiency, heat output, heating capacity and burn time) are standardized and regulated, so they can’t be trusted or compared one to the other.

For example, the EPA does not require stove efficiencies to be reported but for obscure bureaucratic reasons assigns default figures of 63 percent for non-cats and 72 percent for cats. Some manufacturers have paid for independent efficiency tests and show the resulting figures on their promotional brochures. But because efficiency test methods have not been standardized and regulated, you can’t be sure the figures are based on the same tests and calculations. On the other hand it appears that all EPA certified stoves are over 60 percent efficient and some can deliver around 80 percent of the fuel’s potential heat to the house. This is far better than the low-tech uncertified options, many of which are in the 50 percent range and lower. An overall efficiency higher than 80 percent is not desirable because the resulting low exhaust temperature means weak draft and the risk of water vapor condensation which damages the chimney."

 

[Highbeam]

As for the calculator tool on this site:
"Efficiency is defined as the percentage of available heat in the fuel that is actually delivered into the room."

I like this definition and if the manufacturers aren't using it, then they should be.

When looking at fuel cost calculators it is very important to remember that forced air furnaces suffer from very high losses in the duct system. Which can be huge, like 20% or more. This means that the 90% efficient propane furnace that you have in your home is only delivering 70% efficiency for the sake of the fuel cost calculator. This also means that burning an 80% efficient propane "wood"stove would be more efficient.

 

[NoGoodAtScreenNames]

I like this definition and if the manufacturers aren't using it, then they should be.

This is exactly why I started the thread with the question. I couldn't tell and still haven't found a definition of what efficiency means. Or maybe I did and didn't understand it. I'm sure it's fairly complicated.

In my mind if I was going to burn a log in my living room where all the smoke is converted to heat and I lose none of that heat to pay for venting / safety, then that is 100% efficient. Sending smoke and heat up the stack are where the efficiency is lost.

Following some of Lakegirl's links I found the EPA rating on my stove is 75% about in line with other non-cat stoves they have a number for.

A little disconcerting that Enviro (my stove) was called out on a few sites as providing misleading numbers. With the name "Enviro" and the high rating on the website it gives the impression of being efficient and better for the environment than other options, when it's basically the same as the others (smoke dragons notwithstanding).

 

[bholler]

When looking at fuel cost calculators it is very important to remember that forced air furnaces suffer from very high losses in the duct system. Which can be huge, like 20% or more. This means that the 90% efficient propane furnace that you have in your home is only delivering 70% efficiency for the sake of the fuel cost calculator.

I absolutely agree with your premise and your numbers seem right but the furnace manufacturer has no way of knowing what your duct system is like. They can be much better or much worse than that 20% number. So I feel they should be rated the way they are at the furnace but they should also make it clear that you can have massive losses from your ducts.


And yes I agree that is the definition that should be used. But it obviously is not. For example BKVP said the other day on some tests they tests over 100% efficient. I don't doubt that they did but if they did that tells me there is something misleading about the testing procedures because there is no way a wood stove even a bk could possibly put out more btus than the wood you put into it contains. In fact there is no way one could possibly put out 100% of the available btus into the room. And again I am not bashing bk that was just the first example that came to mind.

 

[Highbeam]

I absolutely agree with your premise and your numbers seem right but the furnace manufacturer has no way of knowing what your duct system is like. They can be much better or much worse than that 20% number. So I feel they should be rated the way they are at the furnace but they should also make it clear that you can have massive losses from your ducts.

So when using one of the many fuel cost calculators and comparing something as common as wood heat to a propane forced air furnace you are asked to input the efficiency of the "system". It is tempting to read the label on the furnace that says 95% and put that in but that would be wrong. You need to know to also consider the duct losses.

I too agree that it only makes sense to label the appliance with the appliance's efficiency. As a user of the fuel cost calculator you need to be careful though. The same line on that calculator could also be used for a propane stove with an efficiency tag of 80%. In that case you have no duct losses so you would use the 80% figure.

 

[tarzan]

I absolutely agree with your premise and your numbers seem right but the furnace manufacturer has no way of knowing what your duct system is like. They can be much better or much worse than that 20% number. So I feel they should be rated the way they are at the furnace but they should also make it clear that you can have massive losses from your ducts.


And yes I agree that is the definition that should be used. But it obviously is not. For example BKVP said the other day on some tests they tests over 100% efficient. I don't doubt that they did but if they did that tells me there is something misleading about the testing procedures because there is no way a wood stove even a bk could possibly put out more btus than the wood you put into it contains. In fact there is no way one could possibly put out 100% of the available btus into the room. And again I am not bashing bk that was just the first example that came to mind.

The reason the BK tested at 101% efficiency was because the tester forgot to account for the match he lit it with

 

[NoGoodAtScreenNames]

The reason the BK tested at 101% efficiency was because the tester forgot to account for the match he lit it with

I know this comment was meant as a joke but I bet you're not far from the truth. There was a recent thread around wood moisture content and someone who does testing said that they use their own preferred wood to get a bed of coals going before they add the EPA defined load. The heat still being pumped out from the coals has to impact the test somehow. Their math probably tries to take account of that but nothing in the lab, much less real life is perfect. You've got to run the tests many many times to balance all those things out and even then you might not be successful.

A homeowner has no way to account for any of the variables specific to their house. So while things like the fuel calculator are great they need to be interpreted very carefully before deciding on which option is best especially when the theoretical math says two options are similar.

 

[Highbeam]

I know this comment was meant as a joke but I bet you're not far from the truth. There was a recent thread around wood moisture content and someone who does testing said that they use their own preferred wood to get a bed of coals going before they add the EPA defined load. The heat still being pumped out from the coals has to impact the test somehow. Their math probably tries to take account of that but nothing in the lab, much less real life is perfect. You've got to run the tests many many times to balance all those things out and even then you might not be successful.

A homeowner has no way to account for any of the variables specific to their house. So while things like the fuel calculator are great they need to be interpreted very carefully before deciding on which option is best especially when the theoretical math says two options are similar.

Yes and it strikes me that today's fossil fuel prices are exceptionally and temporarily low. Be careful with making investments in equipment based on current prices.

 

[branchburner]

With the name "Enviro" and the high rating on the website it gives the impression of being efficient and better for the environment than other options.

As far as concern for the environment goes, to minimize emissions it is probably best to look towards burn technology rather than any efficiency rating ("official" or not). And regardless of the stove's potential, that potential can only be optimized by the user (proper venting, low moisture wood, good operating technique, etc.).

Considering that a loss of efficiency and increased particulate output occurs during startups and reloads, it seems to me the lowest emissions and the most heat extracted from fuel are ideally going to occur during very long burn cycles with the best burn technologies (at lower flue temps), which means cat or cat hybrids with larger fireboxes.

 

[NoGoodAtScreenNames]

As far as concern for the environment goes, to minimize emissions it is probably best to look towards burn technology rather than any efficiency rating ("official" or not). And regardless of the stove's potential, that potential can only be optimized by the user (proper venting, low moisture wood, good operating technique, etc.).

Considering that a loss of efficiency and increased particulate output occurs during startups and reloads, it seems to me the lowest emissions and the most heat extracted from fuel are ideally going to occur during very long burn cycles with the best burn technologies (at lower flue temps), which means cat or cat hybrids with larger fireboxes.

Don't get me wrong, I love my stove and to be honest when I bought it I paid more attention to BTU output than efficiency and given the choice I would have done it again. If I could choose a very efficient stove with no view of the fire, I don't think I'd be a wood burner.

I have a larger house and if I had a cat, sure I'd be more efficient but I'd also be burning more oil to keep up. Maybe that's not true, but I'll never know because I'm never really going to do the hard science on my personal setup and if I did I'm sure I'd do it wrong. That's why I used the fuel calculator to begin with - just see if wood was actually cheaper or if it was just something I like to tell myself. Honestly I think it's a wash, but I get no satisfaction by sitting in the basement watching my oil burner run so wood wins.