Outdoor Wood Furnace Info
All-Purpose OWF Discussions => General Outdoor Furnace Discussion => Topic started by: intensedrive on January 27, 2015, 09:13:14 PM
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You never see Efficiency numbers on OWB. I understand that some are more efficient than others. Would it be a safe bet to assume most around around 50%?
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Depending on the brand and style, 50% is being generous.
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I'm not trying to pick on any company but seems Portage and Main have some damn good stoves and keeping on top of the industry as far quality and efficiency. So I'm going to throw it out there... Efficiency of the Optimizer 250?
40%, 50%, 65%... or even higher... Just bugs me how much heat we are loosing on most stoves.
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I know that the EPA stopped posting efficiency ratings on tested units because a few companies were manipulating the tests and coming up with ridiculous numbers, I believe the numbers on the 250 were 84% I think that would be a fairly accurate number in perfect operating conditions.
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Intensedrive:
Are you unhappy with the efficiency performance of your Ridgewood? Measuring efficiency I think would be difficult as virtually all installations and conditions are different. My Ridgewood (finally up and going after a couple of months of installation - thank you SlimJim ) is heating a 4,000 sq. foot log home with tall ceilings plus my hot water and I am not using any more wood than I was with two fireplaces ( with inserts ). I can't say whether it is efficient or not, but I LOVE IT. Should have installed a unit years ago.
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I'm glad it worked out for you VP! I think efficiency is over rated in most cases, there is something that outweighs it and that is the Liberty to be off the glut and independent, pretty much any OWB does that for you!
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I calculate my Optimizer 250 to be around 70% under normal operating conditions. You could probably get this up to 75%+ under ideal conditions, but we don't live in a perfect world. I calculated this using multiple methods and they all seem to support this number. My old non-gasser was around 40%.
This is using the Lower Heating Value (LHV), not the Higher Heating Value (HHV) as these are not condensing boilers! Manufacturers like to use the HHV number for efficiency as it is higher, but I consider that practice a scam. >:(
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Never worried about efficiency. I get railroad tie ends with my own dump truck and heat the house for less than $50 per yr! No cutting or splitting just throwing them onto a big pile after I lay skids down. I am curious though just how efficient mine really is. How do you even determine that?
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Hi,
No I'm not unhappy with my stove. My question was in general..
Intensedrive:
Are you unhappy with the efficiency performance of your Ridgewood? Measuring efficiency I think would be difficult as virtually all installations and conditions are different. My Ridgewood (finally up and going after a couple of months of installation - thank you SlimJim ) is heating a 4,000 sq. foot log home with tall ceilings plus my hot water and I am not using any more wood than I was with two fireplaces ( with inserts ). I can't say whether it is efficient or not, but I LOVE IT. Should have installed a unit years ago.
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Wouldn't the efficiency of the furnace be determined more by the type of wood you burn ? I would think how well the home or building is insulated would matter a great deal too on how effiencient it would be?
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There is a specific amount of BTU's per pound of wood. You need to know the BTU's required to heat for a specific time and weigh how much wood was used to provide those BTU's. Divide BTU's needed to heat by total potential BTU's consumed and you will know your boiler effeciency.
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This is a quick test you can do with a boiler that contains a fair amount of water such as the Optimizer 250. Ok, I wouldn't say it's particularly quick, but no good (accurate) experiments are. :)
With the fire out, drop the water temperature to 140, then weigh and burn an amount of wood (for the optimizer 250 I found that 20 lbs. was a good amount) and see what the increase in temperature is. Do this with the heat load off of the stove but leave the circulator on to make sure the water is being evenly mixed. I have found that around 20 lbs. of wood would raise the 240 gallons of water approximately 40 degrees, from 140 to 180.
Use softwood for this experiment so the wood burns up fairly quickly. Don't worry about the last bit of small hot coals as they don't hold much BTU compared to the rest of the wood. You can also use this type of test to see what the true BTU output of your stove is by timing how quickly it burns the wood.
So here are my calculations:
BTU's to heat the water: 240 gallons X 8.35 lbs. X 40 degrees = 80160 BTU.
BTU's to heat the metal and masonry in the stove: 1500 lbs. X .12 BTU per pound (specific heat of steel) X 40 degrees = 7200 BTU.
Total BTU's required: 80160 + 7200 = 87360.
20 lbs. of wood has 6191 BTU per pound at 20% moisture content, so it has 6191 BTU X 20 lbs. = 123820 raw BTU.
So the efficiency calculation using 20 lbs. is 87360/123820 = .7055
So the efficiency is around 70%.
You could run this experiment multiple times to get a pretty accurate rating for your stove. Overall I think I comfortably get around 70% honest efficiency average from the Optimizer 250. It may be as high as 75% under some conditions.
If you don't have a large water capacity stove, you could run the same experiment drawing off a set amount of heat if you measure the flow and delta T of your load.
Off topic, but for the REAL nerds in the group, using molecular weights I have calculated that burning 1 lb. of wood (at 0% moisture) will create 0.6622 lbs. of H2O and 1.70 lbs. of CO2. (Obviously using 1.3622 lbs. of Oxygen from the atmosphere.)
That's a lot of water generated for potential condensation corrosion!
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Also, when it is tuned for efficient operation I have found the true output of the stove to be 125,000 BTU absolute maximum. It is officially rated for 250,000 BTU but I would suspect that rating to be wood consumption when running wide open, not actual BTU's into the water.
Still enough BTU's for my purpose, but I'm guessing all manufacturers are guilty of this practice.
FYI, I'm not bashing P&M in any of my posts. I think they are one of the best out there and would absolutely recommend them over others.
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I think to be fair you have to somehow factor in idle times. I think that is what kills the efficiency on these boilers.
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I don't think the efficiency of my style of gasification stove is really affected by idle times. Very minimal fuel is consumed between cycles as long as everything is tight and there are no air leaks. With the large refractory brick, startup between cycles is quick.
Long idle times does make it more messy inside the stove with condensation and creosote buildup though, but this can be minimized with a proper 12 hour wood load instead of stuffing it full regardless of the heat load for the day.
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Hi Jreimer,
I like your approach to determining efficiency. I am curious as to the equation you used in determining the O2 and CO2 outputs via molecular weights. Also, with respect to the amount of water formed as a result of combustion, I am curious as to the mass of water that's simply run through the stove with the forced draft fan (assuming the 250 has one) relative to water vapor produced via combustion.
Cando
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efficiency? Whats that, with the ole home built tank in a tank, throw in a wheelbarrow full at 5 pm and go back out at 10 am and take a guess on what you will need for the rest of the day.
I worried about efficiency when I was paying for every btu I burnt. Now, thanks to some sea container that was likely bringing in some fake garments from some sweat shop from some Asian country that landed somewhere in the good ole USof A, that little emerals ash borer has killed off about my whole bush (except for what the dutch elm disease gets) and has supplied me with so many trees to cut....WHO CARES about efficiency......not ME!
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I like your approach to determining efficiency. I am curious as to the equation you used in determining the O2 and CO2 outputs via molecular weights. Also, with respect to the amount of water formed as a result of combustion, I am curious as to the mass of water that's simply run through the stove with the forced draft fan (assuming the 250 has one) relative to water vapor produced via combustion.
I've got the calculations back at the office so I'll get those to you next week.
With the cold, dry air up here in winter, I don't think the mass of water brought in with the incoming air would even register in the equation. Also, the incoming air moisture is already in gas form, so there is no phase change to suck additional energy, unlike the evaporating water in the wood. However, I'm thinking that bringing the incoming air (and moisture) up to the exhaust gas temperature is where a good portion of the 30% efficiency loss would be. If only there was a good way to bring the incoming air up to 250 degrees without cooling the exhaust temp too low... Oh well, it's not illegal to dream... yet.