Valuable lessons, learned the hard way

I read with interest the reactions to outdoor wood furnaces. Let me give my background so you know where I am coming from. I was an outdoor wood stove manufacturer and went out of business and have no intention of going back into business. This gives me two unique qualifications to comment on outdoor wood furnaces. I have a great deal of experience in the design, installation and operation of outdoor wood furnaces and I have no vested interest in any design or manufacturer.

We went out of business about four years ago out of a combination of too much money spent in design, some bad business decisions and some unforeseen circumstances. We would have run into warranty problems also if we had stayed in business a couple of years longer. My weakest area of design was in the metallurgy and our mild steel boilers in two of our designs were prone to corrosion after about five or six years.

We have recently purchased a home and I will be installing an outdoor wood furnace (which I will build) and have been researching the current market. I must say I am appalled at the lack of progress in outdoor wood furnace design. Almost all of the furnaces on the market four years ago were very inefficient (in spite of their claims) and very smoky. Most of the manufacturers have not improved their design at all!!!

One of the largest obstacles to an efficient design (70% or higher efficiency is possible) is the amount of money needed to develop the design and the cost of producing such a furnace. Most consumers can not justify 8 or 10 thousand dollars for a furnace. Our last production model was a wood/oil combination which was very low in particulate emissions (smoke) and very efficient (72% on wood and 81% on oil). This is not a trumped up marketing ploy, (remember I am no longer in the business) it was a scientifically verifiable number. It did have some other problems which I dealt with in the next generation design that never got into production and which I will finally get to build for my own home. I am saying this to say that it is possible for manufacturers to build an efficient, reliable, easily operated stove. However, be prepared to pay for it.

As for the current situation and what to do about buying an outdoor furnace.... I still believe it is the most comfortable, healthy, easiest and practical way to use wood heat. If you are in a rural location and the smoke won't bother you or your neighbours by all means buy an outdoor furnace. Shop carefully. Some manufacturers (one of the largest) lied about their true efficiency ratings. Some had bad design. Some are not reliable (unfortunately some of our stoves fell into that category).

A simple firebox and baffle design (which most are) does not reburn any significant amount of flue gasses and cannot be efficient. Stainless is the only way to go for the firebox and tank. The installation is critical, with special attention needed in insulating and waterproofing the underground lines. Pump location is also critical in proper operation of the furnace. Insist on talking to several owners who have had the model you are interested in for at least two years.

Terry

October, 2002

One of our visitors sent us this report.

Our neighbor installed an *******, a 650,000 btu furnace on his 1.9 acres.  The furnace is four feet from our lot line, 200 feet from our bedrooms.  We are sick of the STENCH!

He burns 3 ft diameter, 4 feet long, unseasoned wood.  He even burns pressure treated (the green ones) utility poles.  Not sure where he is getting the poles from.  We have pictures of his burning piles.

Our situation is very similar to Nancy from NH article you have posted. Our terrain is the same.

Our neighbor burns 365 days a year, heats a 2500 sq ft shed, 1500 sq ft indoor pool and a 3000 sq ft home.

Townships are not prepared to deal with these monster units.  Our neighbor hired a lawyer to argue with the town's building inspector, who tried to shut him down many times.

This past December, we went to the Circuit Court to seek an injunction to stop burning and a temporary injunction has been issued.  It's just terrible that we had to go through this expensive process.

Attached is a picture out our bedroom window of what we wake up to on quite a few mornings.

Thanks for your site!
Nancy

Bob installed his outdoor boiler in late 2000. This is his update.

Firebrick floor is placed. Note the creosote-coated steel walls.

This is Bob's second report on his experiences with his outdoor boiler. You'll find his first report here.

I reported my experiences with my outdoor wood boiler to the woodheat.org site in late 2000.  The webmaster said, "We hope he'll send us an update as he learns more." So here it is.

I managed to get through the Winter of 2000-2001 with my wood boiler, but was not at all happy with the smoke and the 22% net delivered efficiency. During the Fall of 2001, I decided to void my warranty in the interest of science and made three basic modifications to the unit.  First was to line the combustion chamber with a refractory material.  I decided on "regular" firebrick for the bottom, back and sides, and alumina fiberboards for the top baffles.  I also made a secondary combustion chamber by adding a false roof of the same alumina fiberboard panels, supported by the firebrick sides. I did not add a means for providing air directly to the secondary chamber, figuring I would just increase the draft to give excess air in the primary chamber, which would then carry through to the secondary chamber to oxidize the smoke and other unburned gases. Finally, I added a chimney top draft inducer with a variable speed motor control, so that I could adjust the chimney draft anywhere from .02 inch to .15 inch W.C.

More bricks line the sides and rear and the top baffle was added. The baffles are made from alumina fiberboard sheets.The improvement was amazing.  On the first trial from a cold start, I had a net delivered efficiency of 42%, this even with a lot of heat taken up by the firebrick mass.  As in previous tests, I let the system cool down by operating the house circulators until the water jacket temperature was back down to 120 degrees F.   Since the firebrick was now warm, the second trial was even better!  I got at least 48% efficiency.  I say "at least" 48%, because at that point, the water temperature had reached the set point on the aquastat, shutting down the draft inducer and closing the intake damper.

Here is the false ceiling installed. The exhaust leaves the firebox at the back of the false ceiling, flows forward and does a 180° turn around the leading edge of the top baffle and goes to the back of the boiler and out to the chimney. Most of the heat transfer happens in this last run to the exit.Another improvement was the delightful lack of smoke!  To be sure, while starting up, the boiler did smoke, as does any wood fire, but once all the wood had "caught", it was possible to adjust the draft so that there was no visible smoke.   I found that it was best to start with the draft at maximum until the chimney stopped smoking.  This would usually take 20 minutes from a cold start, 5 to 10 minutes after adding wood to a hot bed of coals.  After that, the best results were obtained by backing off the draft to the point where I just started to get smoke and turn it up a bit.  The best efficiency was obtained by adjusting the draft as the fire progressed, turning it down to minimum when the wood was near the ember phase of the burn. If I didn't feel like going out every half hour and fiddling with the draft, the best compromise draft setting seemed to be .05" W.C. once the fire was going.

The front baffle encloses the secondary combustion chamber where the flue gases do the 180° turn.I did have some problems with the high heat cracking the original calcium silicate panels, on the third trial, the false firebox ceiling and baffles collapsed into the firebox. I had to wait half a day for everything inside to cool down enough to safely handle. I rebuilt the inside using alumina fiberboard panels and they have stood up for three seasons now.

These modifications reduced the amount of the wood that can be handled in one load to about one half of original, but because of the improved efficiency, the boiler will still go for about 6 hours before reloading as before.  On above freezing days, I have to give it 3/4 loads, else it will cycle off and I'll lose all the efficiency in a smoldering fire. I also found out that very small loads (less than half) don't work well, as the temperature never gets hot enough to light off the smoke and gases. One way to avoid this problem would be to add a heat storage means, such as insulated water tanks in the basement.  That way the unit could even run in the spring and fall at maximum efficiency and little smoke.

Bob in Pennsylvania

Footnote:  Method of measurement of net delivered heat efficiency and btu/hr.

The system is known to contain 150 gallons of water.  Water weighs 8 pounds per gallon, for a water weight of 1200 pounds.  The BTU is defined as the amount of heat required to raise one pound of water 1 degree F.  The btu/hr rate was determined by timing how long it took to raise the water temperature by one degree, under the test conditions stated below. The boiler comes equipped with a digital thermometer with a stated accuracy of 3%

The refractory bricks and panels run so hot the carbon burns off.The net delivered efficiency was determined by loading the boiler with a known weight of wood, typically 40 pounds. The wood for each trial was red oak, cut from the same tree, seasoned and stored in the same manner.  Before each trial, ashes and coals from previous fires were removed from the boiler.  The heat from the system was removed by allowing the house circulators to operate until the boiler temperature was down to 120 degrees F.  At this time, the house circulators were turned off and the valves to the house heating system closed.  The power to the oil burner was also switched off.  The circulator for the wood burner was left running, so as to prevent stratification of the water inside the wood boiler. 

Heat in the firebox was so much more intense that the draft damper solenoid on the outside of the door failed. Ceramic fiber board shields were added to the door to protect it. This method also inserted the transmission loss through the piping system into the measurements, since the water circulated from the wood boiler, to the house through the inactive oil boiler and back to the wood boiler. There may have been some extra losses through the inactive oil boiler, but these are believed to be negligible, as the boiler was well insulated and the measured stack temperature of the oil boiler while inactive was the same as the ambient temperature.  The wood was then ignited and allowed to burn until the boiler temperature no longer showed an increase.  At that time the temperature was recorded.   The embers and unburned wood, if any were weighed.  The heat output was determined as detailed in the previous paragraph.  The input was determined by multiplying the weight of the wood consumed by 6000 (an average value stated for seasoned red oak).  Efficiency was determined by dividing the BTU output by this input value.

No smoke while in operation.Bob, Pennsylvania, USA

Bob installed his outdoor boiler in late 2000

He has used fairly simple, but reasonable methods to estimate its efficiency.  This is a literate and technically competent report.  See Bob's update in sent in November 2001.

I placed one of these outdoor wood burning boilers in service two weeks ago, and here is my experience to date.

The good news:

The transmission loss from the boiler to the house is minimal.  If the boiler is operating at 180 degrees F, the temperature delivered to the house system is 177 degrees, for a transmission efficiency of 98 percent.  This is with the water lines in an uncovered trench.  By the time that the 2" thick high density foam insulation arrived, the back fill was frozen solid.  The trench and lines are covered with a tarp, and the only time that the snow melts over the trench is when the air temp is above freezing and the snow is starting to melt in places anyway.

The unit is actually capable of heating the whole house during the majority of the day.

The house seems less drafty when heating with the outside boiler, because there is no combustion device operating inside drawing in cold outside air to replace that used in combustion.

The bad news: Where should I begin?

It goes though wood in a hurry, a big hurry!  It went through in a week what I thought would last for three weeks to a month.  It was at that point I decided to make some measurements, and here is what I came up with.  The method of measurements I'll include as a footnote, if anyone finds fault with my method of measurement, please point out my error(s) and suggest a better method.

The net delivered efficiency is horrible, much worse than claimed.  Actually, the manufacturer did not give any percentage figures in their literature, they just compared their unit to others on the market.  If this one is the best, I hate to see the worst!   The best I could get out of it was 22%   This was with only natural draft, I tried a temporary combustion blower, which cut the smoke down a lot, but the efficiency dropped to 19%, because the hotter fire simply went up the stack. I was really hoping for 50% efficiency, my wood supply would last twice as long if that were the case.

The manufacturer also claimed "up to 250,000 btu/hr" for this model.  Well I'd like to know how.   I suspect they reported a peak firing rate when the load was burning the hottest, with very dry red oak.  The best I got was 144,000 btu/hr plus or minus 3 percent burning well-seasoned red oak.  However, this is actually OK for me, since the manual J calculation for my house comes out to 150,000 btu/hr heat loss at 10 degrees F (the temperature used by heating contractors in our area).

I don't try to fill it chock full and let it cycle off, since it would be even more inefficient and smoky to have the fire smolder, this means that I have to reload it frequently, about every five to six hours. I do add a final maximum load just before going to bed.  It does not have enough capacity to make it through the night by itself, at around 4 a.m. the oil burner kicks on to make up for the short fall, but there are still enough live coals at 8 a.m. to start a new fire.

As others have reported on this site, my outdoor boiler smokes a lot. Not nice white smoke either, which would be primarily water vapor, but nasty blue smoke.  Even operating it as I do, with very little "off" time, it still smokes.  That's where all the missing BTUs from my wood are going, up in smoke.  As stated earlier, trying to increase the draft cut down on the smoke, but the stack temperature went way up, and my net efficiency went down.

The manufacturer claims "no wood splitting necessary because of firebox size".  This might work for their largest model, but on mine, any piece larger than six inches in diameter does not completely burn.  Besides, to season well, wood needs to be split anyway.

Bottom line:  Fortunately I have 50 acres of wood lot, which I should be able to harvest in a sustainable manner and I'm out in the country so there's nobody around to be bothered by the smoke, and I wisely situated the machine downwind, so that my family won't be bothered by the smoke.  The state of the art as currently implemented is only suitable for those with access to free wood and the time to cut, haul and split it. As it stands now, I'll only operate the boiler when it is below freezing outside, I'll revert to oil heat during spring and fall, and I'll be experimenting with improving the technology for next winter.

Bob, Pennsylvania, USA 

Footnote:  Method of measurement of net delivered heat efficiency and btu/hr.

The system is known to contain 150 gallons of water.  Water weighs 8 pounds per gallon, for a water weight of 1200 pounds.  The BTU is defined as the amount of heat required to raise one pound of water 1 degree F.  The btu/hr rate was determined by timing how long it took to raise the water temperature by one degree, under the test conditions stated below. The boiler comes equipped with a digital thermometer with a stated accuracy of 3%.

The net delivered efficiency was determined by loading the boiler with a known weight of wood, typically 40 pounds. The wood for each trial was red oak, cut from the same tree, seasoned and stored in the same manner.  Before each trial, ashes and coals from previous fires were removed from the boiler.  The heat from the system was removed by allowing the house circulators to operate until the boiler temperature was down to 120 degrees F.  At this time, the house circulators were turned off and the valves to the house heating system closed.  The power to the oil burner was also switched off.  The circulator for the wood burner was left running, so as to prevent stratification of the water inside the wood boiler.  This method also inserted the transmission loss through the piping system into the measurements, since the water circulated from the wood boiler, to the house through the inactive oil boiler and back to the wood boiler.  There may have been some extra losses through the inactive oil boiler, but these are believed to be negligible, as the boiler was well insulated and the measured stack temperature of the oil boiler while inactive was the same as the ambient temperature.  The wood was then ignited and allowed to burn until the boiler temperature no longer showed an increase.  At that time the temperature was recorded.   The embers and unburned wood, if any were weighed.  The heat output was determined as detailed in the previous paragraph.  The input was determined by multiplying the weight of the wood consumed by 6000 (an average value stated for seasoned red oak).  Efficiency was determined by dividing the BTU output by this input value. Efficiency values for each of two trials under the same conditions were within 1 percent of each other.