Wood Boiler Efficiency

[tom in maine]

I am wondering if anyone out there has actually done any testing of the thermal performance of gasifier wood boilers.
This would require weighing the wood and its moisture content and then seeing how that related to what a storage tank
actually received as heat. (with no heat being removed for a testing period)

A lot of manufacturers talk about 90% efficiency, but that is what they say, not what field experience shows.

We have done this at the University of Maine and were quite pleased to hit 70%. I wonder if this was primitive compared to today's boilers
or is everyone being extremely optomistic?

Tom in Maine--where rain is the new sun!

 

[stee6043]

I think Nofossil has some pretty interesting data on this topic. From what I've read "achieving" and "sustaining" 90%+ efficiency are two very different things. The output of our boiler is more curve shaped than linear. Efficiency depends on so many things including fuel type, water content, draft conditions, water temp of the boiler, temp of the refractories, etc and so on. Not to mention the amount of tuning you can do to vary performance....

 

[Nofossil]

There have been very methodical independent tests done in Europe - weighing the wood, measuring the moisture content, measuring water flow rates and temperature rise. Quite expensive and time consuming, but the end result is 90 - 91% efficiency.

This is under unrealistic conditions, however. If I remember right, it's 35gpm at 130 degrees, with a fairly low temperature rise. Not typical residential operating conditions.

I take a snapshot of all system temperatures every 30 seconds, and I've done my own tests which have their own flaws. I weigh the wood and measure the moisture content, but I can't measure flow through the boiler. Instead I measure the heat delivered into my various heat loads. Each measurement is flawed in it's own way:

1) I assume that the manufacturer's specs for my baseboards are correct, and derate them based on actual water temp. I ignore heat that gets into the living space by means other than the baseboards (loss from plumbing, for instance), even though that's actual heat that the boiler is producing.

2) I assume that my hot tub is actually 550 gallons, and that it's thermally pretty well mixed by the little filter circ that runs all the time.

3) I assume that my DHW tank temp is well represented by a single temperature measurement taken at the midpoint.

4) I assume that the three sensors in my storage tank allow me to calculate the average temperature.

5) I ignore the heat that goes into heating the boiler itself - essentially, I assume it is the same temperature at the end of the run as it was at the beginning. Since I do short burns, this is probably a large error source on a percentage basis.

Based on all of that, I calculate the heat delivered into the various heat loads and calculate a system efficiency. I usually get numbers between 55% and 70%.

 

[tom in maine]

Hi Nofossil,
This is helpful. I know when Dick Hill (the Godfather of wood gasifiers) hears about 90% efficiency, the first words out of his mouth is "I don't believe it".

Thanks,
Tom

 

[SolarAndWood]

Based on all of that, I calculate the heat delivered into the various heat loads and calculate a system efficiency. I usually get numbers between 55% and 70%.

Is that your overall system efficiency or what happens between the wood being put in the firebox and temperature difference between the inlet and outlet of the boiler?

 

[Nofossil]

Based on all of that, I calculate the heat delivered into the various heat loads and calculate a system efficiency. I usually get numbers between 55% and 70%.

Is that your overall system efficiency or what happens between the wood being put in the firebox and temperature difference between the inlet and outlet of the boiler?

Three kinds of efficiency:

1) Combustion efficiency - how well the boiler converts the chemical energy into heat energy. For a properly configured gasifier, this is around 99%. The loss at this stage is any unburned hydrocarbons in the flue gas.

2) Unit efficiency - How much of the chemical energy in the wood is delivered as hot water at the boiler outlet. This is combustion efficiency minus heat lost out the sides of the boiler and up the flue. With normal wood as fuel, this can't be higher than about 94% unless you have a condensing boiler, because there's heat energy necessary to vaporize the water in the wood and that's lost up the flue. The very best gasifiers achieve very close to 94% during the peak burn. The EKO 80 was tested at 90 to 91%, for instance.

3) System efficiency - how much of the chemical energy in the wood is delivered into the desired heating loads. This is average (not peak) unit efficiency minus any other losses to the environment (buried lines in a system with the boiler in an outbuilding, for instance).

What I measure is actually an understatement of system efficiency, since there's lots of heat that gets from the boiler into the house without being measured.

I suspect that during the peak burn, I'm getting a combustion efficiency near 90%. Based on historical oil consumption, I calculate a system efficiency of 80% - 85%, and I expect that I get better than that during peak burn.

However, my measured system efficiency will always be lower. For instance, there's very little useful heat delivered during the first half hour, because the boiler has to get up to temperature. The boiler leaks heat into the house for hours after it's done, and I don't measure that or give credit for it in my system efficiency calculations.

 

[jebatty]

Take a look at the data tables in this link. Notes at the bottom of some of the data tables show the pounds of wood burned.

https://www.hearth.com/talk/threads/36503/

 

[Rory]

The important thing to me, no matter what the numbers are, is that several folks have reported cutting their wood use in half after switching from OWBs, and no smoke and very little particulate pollution comes out of the chimney once it is burning full bore.

 

[boilermanjr]

Efficiency (U.S.) over 90% for Biomass equipment is not possible without condensing the exhaust. A lot of confusion has been created by the fact that European boilers are tested using the low heating value for wood fuel, where here in the U.S., we use the high heating value for wood fuel in our tests. In layman's terms, this means that the Europeans account for the water in the fuel, assume a certain amount of heat is used to turn that water to steam, and increase their efficiency ratings accordingly. In the U.S., efficiency testing generally is done with wood of a certain moisture content range and no allowance in the efficiency numbers is made for heat used to turn water to steam. It is actually possible with condensing European oil and gas boilers to have efficiencies ratings over 100%.

We have an explanation of the differences in the two efficiencies on our blog at bioheatusa.com. There is also a chart that shows U.S. efficiency data as interpolated from actual European test data. The two can be compared side by side.

ASTM has a test method for outdoor hydronic heaters that has been used on several products. This test verifies boiler output, efficiency, and emissions quite scientifically, but not realistically. It is notable that the ASTM method is very different from the method used in Europe, EN303-5.

 

[Piker]

Input/output efficiencies of greater than 90% are definitely reserved for condensing boilers. A good gasifier (wink wink) will see combustion efficiency of 93%, and thermal efficiencies just below 90% for a no BS input/output in the 80 to 84% range under normal conditions with fair wood. You can easily skew those numbers to look a little better by adjusting test parameters such as moisture content, etc.

cheers

 

[Nofossil]

I'm very interested in understanding the European tests. I'm looking at an EN303-5 test report, and it shows a value of 15.65 MJ/kg for birch with 15% water content. In the calculations, it determines that the energy available from burning 19.5kg/hr is 85.77kw/hr. That almost matches - if you crank the numbers backwards, it works out to 15.83 MJ/kg, or 6820 BTU/lb.

My understanding is that dry wood has a fuel value of about 8600 btu/lb. I think that 6820 is pretty close to the 'low' value.

Based on calculating that there was 6820 BTU/lb of available chemical energy in birch at 15% mc, they arrived at a unit efficiency of 91% - the boiler was burning 42.9 lbs/hr of birch and producing 269,880 BTU/hr.

To get this efficiency, they ran 34gpm of 130 degree water through the boiler, with an outlet temp of 145 degrees. Not realistic home operating conditions, but it seems like a valid test of unit efficiency.

 

[heaterman]

I am wondering if anyone out there has actually done any testing of the thermal performance of gasifier wood boilers.
This would require weighing the wood and its moisture content and then seeing how that related to what a storage tank
actually received as heat. (with no heat being removed for a testing period)

A lot of manufacturers talk about 90% efficiency, but that is what they say, not what field experience shows.

We have done this at the University of Maine and were quite pleased to hit 70%. I wonder if this was primitive compared to today's boilers
or is everyone being extremely optomistic?

Tom in Maine--where rain is the new sun!

I'll believe 90% when I see a condensate drain piped out of one. In the real world with variable wood quality and moisture a good one will do an honest 75% consistently. If you have one with a variable speed draft fan, carefully designed negative pressure firebox, and lambda control you would likely see 80-85% with good regularity.

 

[Piker]

I am wondering if anyone out there has actually done any testing of the thermal performance of gasifier wood boilers.
This would require weighing the wood and its moisture content and then seeing how that related to what a storage tank
actually received as heat. (with no heat being removed for a testing period)

A lot of manufacturers talk about 90% efficiency, but that is what they say, not what field experience shows.

We have done this at the University of Maine and were quite pleased to hit 70%. I wonder if this was primitive compared to today's boilers
or is everyone being extremely optomistic?

Tom in Maine--where rain is the new sun!

I'll believe 90% when I see a condensate drain piped out of one. In the real world with variable wood quality and moisture a good one will do an honest 75% consistently. If you have one with a variable speed draft fan, carefully designed negative pressure firebox, and lambda control you would likely see 80-85% with good regularity.

There is definitely a difference between real world numbers and lab test numbers... Not to say that lab test numbers are not achievable in the field, but I doubt that every homeowner uses 4x4 white oak blocks at exactly 23% moisture content with a perfectly clean heat exchanger and burn chambers for every burn.

cheers

cheers

 

[Gooserider]

What I'm wondering is why we aren't seeing any manufacturers taking that logical next step and making a condensing wood boiler... Seems to me like it would be possible without all that much difficulty - take the standard current gasser design, and add a downward loop of stainless pipe running through a water jacket that the heating system return water is flowing through, then vent back up through an insulated stainless stack, maybe even direct vent?
(think something like a stainless sink trap)

Should keep the temps in the main part of the boiler about where they are now, but allow the smoke to cool when it enters the stainless trap part of the exchanger, and pre-heat the boiler return water. Put a condensate drain at the bottom of the exchanger. Any water that condenses in the chimney would run back down into the exchanger trap (not the boiler) and go out the drain the same way - have the drain either dump into the sanitary sewer (if that's permitted, I'm not sure if it would be or not) or one of those AC condensate pumps....

Wouldn't work with a traditional boiler since they aren't clean enough, but I keep getting told that a gasser runs nearly as clean as a dino-boiler, so I can't see why it wouldn't work...

Gooserider

 

[heaterman]

What I'm wondering is why we aren't seeing any manufacturers taking that logical next step and making a condensing wood boiler... Seems to me like it would be possible without all that much difficulty - take the standard current gasser design, and add a downward loop of stainless pipe running through a water jacket that the heating system return water is flowing through, then vent back up through an insulated stainless stack, maybe even direct vent?
(think something like a stainless sink trap)

Should keep the temps in the main part of the boiler about where they are now, but allow the smoke to cool when it enters the stainless trap part of the exchanger, and pre-heat the boiler return water. Put a condensate drain at the bottom of the exchanger. Any water that condenses in the chimney would run back down into the exchanger trap (not the boiler) and go out the drain the same way - have the drain either dump into the sanitary sewer (if that's permitted, I'm not sure if it would be or not) or one of those AC condensate pumps....

Wouldn't work with a traditional boiler since they aren't clean enough, but I keep getting told that a gasser runs nearly as clean as a dino-boiler, so I can't see why it wouldn't work...

Gooserider

maybe its not cost efficient,eh
did hear from prof this morn that jetstream/hill configuration will burn @ 30% moisture content

Part of it is engineering, part is what can be gained and part is just plain physics.
The engineering part involves a fair amount of extra expense and doubtlessly more maintenance. We're talking pieces that have to be added that would be made out of very high grade stainless steel to withstand the stress and acidity. Think $$.$$ ......and for what gain? Maybe an actual 3-6% which would take a very long time to pay back the additional expense.
That brings us up to the what can be gained, which when you do the math with a fuel as inexpensive as cordwood, equates to very little in terms of $$ spent on fuel. If wood goes to
$400 per cord then Yah.....maybe. The last part is the physics of condensation. To collapse the vapor back into liquid the flue gas has to be brought below its condensation temperature. This would mean flue gas temps in the range of probably 225* or less (I'm just guessing on that temp). The issue presented is that it's difficult to get the flue gas down to that range when dealing with a system water temp of 180* or more. Using a gas or oil fired boiler as an example, condensation pretty much quits when boiler water temps exceed 140* or so. That pretty much eliminates about 90% of the heating systems out there as they demand higher water temps than that. If your boiler was set up to condense and was connected to a radiant floor system you could effectively drop the water temp to force condensing to take place. At higher temps there would be little condensing even if the boielr was set up for it.

 

[Rob From Wisconsin]

I find this thread very interesting.
In our area boilers have become rather popular (almost a "fad").
I see many on my long trip to work every morn.
Lots went "idle" shortly into winter (no smoke & wood pile gone).
I don't think a lot of them realize just how much wood it takes to "feed" them.

At our local County Fair, numerous displays were present for outdoor boilers,
and the sales people were very quick to throw-out very high efficiency numbers,
and low wood consumption.

So, when I look at "efficiencies", I like to look at the whole package.
- Loss when the boiler is at Idle (smoking).
- Heat loss to the outside world at the boiler
- Transfer loss from the Boiler to your house

From what I know, that puts most outdoor boilers down near 50%.
And it makes sense to me, because most of my Boiler neighbors need 10+ chords of wood to survive a winter,
which is 2 to 3 times more than most wood stoves (this doesn't take into account electricity to make everything happen).

Rob

 

[Nofossil]

So, when I look at "efficiencies", I like to look at the whole package.
- Loss when the boiler is at Idle (smoking).
- Heat loss to the outside world at the boiler
- Transfer loss from the Boiler to your house

From what I know, that puts most outdoor boilers down near 50%.
And it makes sense to me, because most of my Boiler neighbors need 10+ chords of wood to survive a winter,
which is 2 to 3 times more than most wood stoves (this doesn't take into account electricity to make everything happen).

Rob

Someone (Michigan, if I remember right) did system efficiency tests for OWBs. I think they ranged from under 20% to around 35%. System efficiency is a tough number, but it's the right one to measure. I suspect that the best gasifiers give a system efficiency around 70% to 80%.

My brother had a conventional indoor wood boiler which he ran for years, tweaking it to get the best possible efficiency. He kept a journal the whole time, weighing his wood every day. I have no doubt that it was WAY better than the average boiler. It never idled. Almost no smoke at all, and no creosote.

When it finally died, he built a downdraft gasifier modeled loosely after the EKO.

His wood consumption per degree day dropped by 40%.

 

[heaterman]

So, when I look at "efficiencies", I like to look at the whole package.
- Loss when the boiler is at Idle (smoking).
- Heat loss to the outside world at the boiler
- Transfer loss from the Boiler to your house

From what I know, that puts most outdoor boilers down near 50%.
And it makes sense to me, because most of my Boiler neighbors need 10+ chords of wood to survive a winter,
which is 2 to 3 times more than most wood stoves (this doesn't take into account electricity to make everything happen).

Rob

Someone (Michigan, if I remember right) did system efficiency tests for OWBs. I think they ranged from under 20% to around 35%. System efficiency is a tough number, but it's the right one to measure. I suspect that the best gasifiers give a system efficiency around 70% to 80%.

My brother had a conventional indoor wood boiler which he ran for years, tweaking it to get the best possible efficiency. He kept a journal the whole time, weighing his wood every day. I have no doubt that it was WAY better than the average boiler. It never idled. Almost no smoke at all, and no creosote.

When it finally died, he built a downdraft gasifier modeled loosely after the EKO.
His wood consumption per degree day dropped by 40%.

The test you are referring to was conducted at a lab in Wisconsin. There were 7 or 8 OWB's tested along with some other types and several government entities were involved along with the Michigan EPA air quality division. Kiln dried oak, square cut to 4"x 4" was used as fuel, not really field conditions eh? The results from the tests on the OWB's showed efficiency ranging from 28 to 41% under those test parameters. One can only guess what they are like during actual use. Probably 10-15% less than that. Particulate emissions were monitored closely as this was mainly an air quality thing and the amount of particulate and unburned fuel exiting the OWB's was shocking according to my contact who has access to the test data.

 

[Nofossil]

The test you are referring to was conducted at a lab in Wisconsin. There were 7 or 8 OWB's tested along with some other types and several government entities were involved along with the Michigan EPA air quality division. Kiln dried oak, square cut to 4"x 4" was used as fuel, not really field conditions eh? The results from the tests on the OWB's showed efficiency ranging from 28 to 41% under those test parameters. One can only guess what they are like during actual use. Probably 10-15% less than that. Particulate emissions were monitored closely as this was mainly an air quality thing and the amount of particulate and unburned fuel exiting the OWB's was shocking according to my contact who has access to the test data.

I'm not sure that's the same test, but it tracks pretty well. The one you're talking about measures unit efficiency, so it doesn't take into account losses to the outside under normal operating conditions (really cold, buried pipes to the house, etc.)

On an apples-to-apples basis, the test you mention is pretty equivalent to the EKO unit test that I referenced earlier - it came out at 90% unit efficiency.

There's another OWB test somewhere that looked at emissions and efficiency as a function of idling percentage. It wasn't as methodical, but no surprises in the results. If I remember right, a unit with a peak output that's four times the average load (typical for OWB installations) idles around 75% of the time and delivers a system efficiency under 20%.

Part of the problem is that many dealers encourage the 'bigger is better' idea, so the boiler ends up much larger than necessary. An OWB running flat out with dry wood is a much different animal, and the best ones probably do reach unit efficiencies of around 40%. I still haven't seen real independent data on the newer models that claim to be gasifiers, though I talked to someone near here who had one and had gone through 20 full cords by some time in February. The smell outdoors and the creosote streaks leaking down the outside of the flue joints provided a clue that for that one at least, secondary combustion was not entirely effective.

 

[Gooserider]

What I'm wondering is why we aren't seeing any manufacturers taking that logical next step and making a condensing wood boiler... Seems to me like it would be possible without all that much difficulty - take the standard current gasser design, and add a downward loop of stainless pipe running through a water jacket that the heating system return water is flowing through, then vent back up through an insulated stainless stack, maybe even direct vent?
(think something like a stainless sink trap)

Should keep the temps in the main part of the boiler about where they are now, but allow the smoke to cool when it enters the stainless trap part of the exchanger, and pre-heat the boiler return water. Put a condensate drain at the bottom of the exchanger. Any water that condenses in the chimney would run back down into the exchanger trap (not the boiler) and go out the drain the same way - have the drain either dump into the sanitary sewer (if that's permitted, I'm not sure if it would be or not) or one of those AC condensate pumps....

Wouldn't work with a traditional boiler since they aren't clean enough, but I keep getting told that a gasser runs nearly as clean as a dino-boiler, so I can't see why it wouldn't work...

Gooserider

maybe its not cost efficient,eh
did hear from prof this morn that jetstream/hill configuration will burn @ 30% moisture content

Part of it is engineering, part is what can be gained and part is just plain physics.
The engineering part involves a fair amount of extra expense and doubtlessly more maintenance. We're talking pieces that have to be added that would be made out of very high grade stainless steel to withstand the stress and acidity. Think $$.$$ ......and for what gain? Maybe an actual 3-6% which would take a very long time to pay back the additional expense.
That brings us up to the what can be gained, which when you do the math with a fuel as inexpensive as cordwood, equates to very little in terms of $$ spent on fuel. If wood goes to
$400 per cord then Yah.....maybe. The last part is the physics of condensation. To collapse the vapor back into liquid the flue gas has to be brought below its condensation temperature. This would mean flue gas temps in the range of probably 225* or less (I'm just guessing on that temp). The issue presented is that it's difficult to get the flue gas down to that range when dealing with a system water temp of 180* or more. Using a gas or oil fired boiler as an example, condensation pretty much quits when boiler water temps exceed 140* or so. That pretty much eliminates about 90% of the heating systems out there as they demand higher water temps than that. If your boiler was set up to condense and was connected to a radiant floor system you could effectively drop the water temp to force condensing to take place. At higher temps there would be little condensing even if the boielr was set up for it.

Makes sense on the economics side of things, but not sure about the condensation side. What you say about dropping the temperature makes sense, along with the problem of high water temps reducing the condensation, except that if that is the case, then why are people saying such great things about the mod-con gas and oil boilers - what are they doing that allows them to condense at high temps that the wood boilers can't do?

Gooserider

 

[heaterman]

What I'm wondering is why we aren't seeing any manufacturers taking that logical next step and making a condensing wood boiler... Seems to me like it would be possible without all that much difficulty - take the standard current gasser design, and add a downward loop of stainless pipe running through a water jacket that the heating system return water is flowing through, then vent back up through an insulated stainless stack, maybe even direct vent?
(think something like a stainless sink trap)

Should keep the temps in the main part of the boiler about where they are now, but allow the smoke to cool when it enters the stainless trap part of the exchanger, and pre-heat the boiler return water. Put a condensate drain at the bottom of the exchanger. Any water that condenses in the chimney would run back down into the exchanger trap (not the boiler) and go out the drain the same way - have the drain either dump into the sanitary sewer (if that's permitted, I'm not sure if it would be or not) or one of those AC condensate pumps....

Wouldn't work with a traditional boiler since they aren't clean enough, but I keep getting told that a gasser runs nearly as clean as a dino-boiler, so I can't see why it wouldn't work...

Gooserider

maybe its not cost efficient,eh
did hear from prof this morn that jetstream/hill configuration will burn @ 30% moisture content

Part of it is engineering, part is what can be gained and part is just plain physics.
The engineering part involves a fair amount of extra expense and doubtlessly more maintenance. We're talking pieces that have to be added that would be made out of very high grade stainless steel to withstand the stress and acidity. Think $$.$$ ......and for what gain? Maybe an actual 3-6% which would take a very long time to pay back the additional expense.
That brings us up to the what can be gained, which when you do the math with a fuel as inexpensive as cordwood, equates to very little in terms of $$ spent on fuel. If wood goes to
$400 per cord then Yah.....maybe. The last part is the physics of condensation. To collapse the vapor back into liquid the flue gas has to be brought below its condensation temperature. This would mean flue gas temps in the range of probably 225* or less (I'm just guessing on that temp). The issue presented is that it's difficult to get the flue gas down to that range when dealing with a system water temp of 180* or more. Using a gas or oil fired boiler as an example, condensation pretty much quits when boiler water temps exceed 140* or so. That pretty much eliminates about 90% of the heating systems out there as they demand higher water temps than that. If your boiler was set up to condense and was connected to a radiant floor system you could effectively drop the water temp to force condensing to take place. At higher temps there would be little condensing even if the boielr was set up for it.

Makes sense on the economics side of things, but not sure about the condensation side. What you say about dropping the temperature makes sense, along with the problem of high water temps reducing the condensation, except that if that is the case, then why are people saying such great things about the mod-con gas and oil boilers - what are they doing that allows them to condense at high temps that the wood boilers can't do?

Gooserider

Simple answer............they don't condense at those temps. The advantage comes from using one with outdoor reset of the water temps in the shoulder seasons allowing the boiler to operate at much lower temps., 150* or less especially on a radiant floor design. How often do you really need that 180* water temp? Pretty much at design conditions only.

 

[DaveBP]

why are people saying such great things about the mod-con gas and oil boilers - what are they doing that allows them to condense at high temps that the wood boilers can’t do?

My understanding of gas mod/cons is that the exhaust gas is exposed to a second heat exchanger surrounded by the cooler RETURN water after passing through the SUPPLY water that all the standard boilers use. But with a typical baseboard system the return temps may not be low enough to get efficient condensation. Radiant systems (and snow melt for sure) would get better use of all the extra cost of a condensing boiler, I think.

 

[brad068]

Guys, You could make a gasifier go into condensing but you better make the condensing coil/exchange readily accessible for cleaning. Gassers do burn clean WHEN they are up to operating temp but during startup/warm up the creosote and unburned particulates do exit the boiler which, unless you could bypass the condenser until operating temp is reached I can see the condenser getting very dirty. And it might clean off during operation but I don't think so

I think thats why wood fired appliances want an insulated chimney. Not only is it to protect the structure but it is also to keep the exhaust hot and not solidify/condense in the pipe and keep a draft going. Thats my take on it.

 

[tom in maine]

I suspect part of the reason we have not seen condensing wood boilers yet are several fold:

1. The exhaust from wood boilers is composed of a lot more complex emissions than oil or gas. As such, it is extremely aggressive to many
metals.

2. Condensing boilers do not last as long even with those "simpler" fuels. And they are more complex.

3. Wood fuels are already cheap b comparison to fossil fuels, so the motivation is not as great.

4. Stop and think about the wood boiler "industry". We have a lot of big metal boxes that have no secondary burn
features. OWB have been a great example of this. They are inane, primitive and even the gasifiers are still subject to very large
heat loss by being outdoors, but they are still be built and sold aggressively.

All that being said, I am sure we will see condensing wood boilers. Just be prepared to pay a lot for them!
And let's appreciate the fact that we can get our fuel from the backyard or back 40.

 

[Piker]

All that being said, I am sure we will see condensing wood boilers. Just be prepared to pay a lot for them!
And let's appreciate the fact that we can get our fuel from the backyard or back 40.

It always boils down to the dollar. The market determines the point of diminishing returns. If we could gain an additional 15% input/output efficiency by condensing a wood boiler, the average homeowner using 5 full cord of wood with their downdraft gasser would save around $110 per year in wood (at $150/cord), but spend probably several thousand dollars more for a unit that would likely have higher maintenance costs and/or a shorter overall life.

There's no doubt in my mind that we could develop the technology needed for a condensing wood boiler... but at this point, such a unit would not be marketable because the price could not be justified for such a complex unit... not yet anyways. Just my $.02

All THAT being said... i think it would be a blast developing one.

cheers