Outdoor Boiler User Report

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.  We hope he'll send us an update as he learns more. 

Bob did send an update 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.

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