OWB Efficiency Claims

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Eric Johnson

Mod Emeritus
Nov 18, 2005
Central NYS
This is what I get for spending my lunch hour on Ebay. I noticed an auction for a used Taylor Water Stove. The description was a lot of the usual hyperbole about how you can heat a Walmart with one of these, burn “all manner of combustibles,” heat your pool, yada, yada, ya. So I asked the seller (a Taylor dealer) what the “overall efficiency” is on these stoves. Answered within a couple of minutes. To wit:

“I know there are a number of rating systems for the efficiency of a wood burning stove- but publishing a rating without pages of qualifications really isn’t fair to a regular person without ASHTO access and the ability to interpret the results. Taylors are rated between 87 and 93 percent efficient- based on a number of test conditions. That being sais- anyone can understand that the bigger the water capacity of the stove the more efficient it is in wood useage since it’s the water that stores the heat. Taylor and the various knock-offs have the largest water capacity in the industry. We heated a 2600 sf home in NH, had plenty of hot water and provided over 200 gallons of hot water to our horses in the NH winter, using a Taylor 260- so small it’s not made anymore- and 3 cords of mixed green unsplit hardwoods A YEAR.....It’s the water that does it......more water = less wood.”

Amazing! Over 90 percent efficient and somehow you’re getting energy out of water!

Sign me up!

Too bad the model that only burns 3 cords of green, unsplit wood isn’t made anymore. Que lastima!
Sorry for hijacking your thread - at least we were still talking about boiler efficiency....

According to the various older folk that I see, all of the really good stuff isn't made anymore. I suspect that a good percentage falls into the same category as your three-cord wood boiler.
Hard to know what he is talking about about but maybe he means the efficiency of consuming wood during a burn? Normal boiler efficiency is calculated from energy content of fuel delivered against what is extracted via the temperature differential and mass flow rate of water through it.

The loses can be broken down further but basically you have stack loss, the heat loss from the jacket, and the loss from heating and evaporating water in the wood.
No need to apologize, it's a great discussion. A lot better than this one, which is why I gave you guys the space to work it all out. The scientific approach to efficiency is one way to do it, I guess, as evidenced by TMonter's detailed analysis and Slowzuki's thumbnail sketch.

Another approach is to toss out some wild-ass efficiency claim (while suggesting that the data behind it is way too complicated for the average person to understand), and then start spewing nonsense for awhile, finally ending with some outrageous claim that everybody knows is way too good to be true. Sprinkle in a little jargon, a few acronyms and some allusion to the mystical/magical properties of plain water, and you're done with the sales pitch.

The technical, science-oriented fact-type math mumbo-jumbo stuff is OK--as far as it goes. But if you want to sell boilers, better to stick to the latter approach. Apparently people tend to believe what they want to believe.
After a few drinks tonight, I prefer the Jimbo-No-BullS*&% test . . . give me the number of three people in upstate NY that have the damn 'stove', I'll call um and see what they say

I don't give a damn what letters come after your name. . . . if it sounds too good to be true it probably is. If ANYONE can show me a wood fired hydronic that will heat my house with 3 cord of wood - give me any bull s*(% about what a cord of wood is and I'll have the Director of weights and measures shut yer azz down so fast Nascar will be jealous - we'll go into business, not just buy yer damn 'stove'. I hate people that think that someone who needs something is therefore stupid. No coincidence OWB salesman always have brown eyes. . . .

Damn my heads gonna hurt tomorrow
The ONLY reason to have your SIL live nest door :coolmad:
Total efficiency of the whole system is a whole other kettle of fish, any time you are piping heat anywhere you lose heat at a pretty amazing rate. I've seen some accounts of OWB installs losing 15-25% of the heat they produce to the ground during transport. Thats a lot of wood...
When I say "overall efficiency" I'm talking about combustion efficiency and heat transfer efficiency within the boiler itself. What you do with the heat after that is your business. My guess is that OWBs have much lower combustion efficiency than heat transfer efficiency, hence the smoke. But I guess whatever heat going up the stack contributes to the latter, so maybe it's a bad number all around.

But I love how these guys just toss these numbers around like they're true. The 87 to 93 percent is outrageous. And I'm sure the 3-cord nonsense is just that as well. Shoot, he probably lost three cords just getting the heat from the boiler to the house.

EDIT: I stand corrected. I guess that technically, it's not an outright lie. My dad, who tends to be more thoughtful and knowledgeale than me, points out that if you measure the number of btus being released into the firebox and compare that to the amount of heat being transferred into the water through the jacket in an OWB, it would probably be in the range noted above. If your heat input number was a measure of the total btus contained in the wood being tossed in, however, you would come up with a far lower figure. Those numbers don't include a vast amount of unrealized potential heat going up the stack.
Eric Johnson said:
If your heat input number was a measure of the total btus contained in the wood being tossed in, however, you would come up with a far lower figure. Those numbers don't include a vast amount of unrealized potential heat going up the stack.

We need to be careful when throwing around efficiency numbers. TMonter's analysis showed 22% of the total BTUs available in the wood going up the flue in the form of water vapor. Doesn't matter what your boiler design is, there are only two ways to reduce that:

1) Burn drier wood (his analysis was based on 50% moisture)
2) Condense the steam in the flue gas and recover the heat that it took to create the water vapor. Requires additional HX and very low flue temps.

Someone else talked about loss out the sides of the boiler - not an issue for me, but would be if the boiler were outdoors. I do have storage tank losses, though.

I think in general the definition is the percentage of available BTUs in the wood that don't go up the chimney. Primary chimney losses:

1) Water vapor
2) Unburned fuel
3) Heat in the flue gas itself

If you can reduce those losses, you increase your efficiency.

I think that covers most cases.
I suspect their efficiency numbers are based only on unburnt fuel passing out the stack, which has nothing to do with the amount of heat you can capture out of a cord of wood. Most high effic. stoves and boilers have to hit darn near 100% combustion efficiency to get their overall efficiency up. 7-13% of wood going up the chimney as unburned particulate is a mess of smoke.
Having used my OWB for a season- ( no scientific proof, just an observation )

A: I think that the efficenties of converting the (DRY) wood to hot water are only poor to fair in a boiler- but not great because so much of the heat (energy) goes up the stack. I would estimate it at 50% (which means I lose 50% !)
(Next year, I am going to mod mine with firebrick and a stack damper- like the guy on that other web site did).

B: I disagree about the heat loss of the the Pex to and from the boiler (well, in my case anyway) I have a thermomator on my boiler and two on my manifold to the radient pipes in my basement (one on the in and one on the out). The tempature differential between the boiler water and the basement water is only 1-2 degrees. That $1200.00 per 100ft Belgium Imported Insulated Pex Pipe really does work. My pipe is buired 3-4 ft.

C: (As Craig said above) Many (many) OWB users burn...anything. Many use it more as their own privite incinarator. So, if you are burning green wood and tires, you are not going to get maximum performance- not to mention pissing off your neighbors and Mother Nature, and me.

D: I don't consider the losses from the boiler itself to be very great (at least considering that it lives outdoors in a field durring the cold winter)- doesnt seem to lose much heat. Once it is up to temp (185), it pretty much stays there without burning more wood.

E:There is no way in 'He'-double-hockey-sticks, that one could ever get a OWB to 85% effic- (I won't say never, but... well- you know...) Also: OWBs EAT TREES WHOLE ! I put up 12 cords for this year...We will see.
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