Rick has done some deep thinking about his boiler. 

He lives in Eastern Ontario, Canada.

Those pictures in your article on outdoor wood furnaces are realistic. That is just like my outdoor wood furnace which is now into its fourth heating season.

It works as described. It regularly produces enormous quantities of smoke especially in fall and spring. When used in summer, it is terrible. Our nearest neighbour is nearly 500 metres away, and the next is nearly 1km away. This is obviously a rural setting. If we were in a populated area, or near a road, there is no doubt there would be many complaints and it would be totally irresponsible to use it in the summer. Running one of these in a city or town in summer would be, at best, extremely inconsiderate.

Overall, almost all the claims made by the manufacturer were absurdly exaggerated. Long periods between burns, such as 72-96 hours? Ridiculous. As to that, in mid-winter at -30degC, we load it 3 times a day and it burns cleanly with little or no visible smoke. The claims for 96 hour burn times are absurd. This is might be true if you were not pulling any heat out of the unit, and it will indeed go 72-96 hours in July without reloading, no problem, if filled with dry seasoned oak, but this is rather pointless. The joke is that I have seen these units sold at Farm shows and it is always the same line of misrepresentation: the dealers have one set one up and are bragging about how they have only put in one armful of wood since setup three days ago because the unit is just so efficient... Since they haven’t heated anything except the furnace itself, the statement about an armload of wood is completely meaningless, even if it were true.

Efficiency? Not likely. There are numerous manufacturers of these outdoor furnaces, but I have seen no evidence that any of them has done any serious scientific engineering design of their units. There is no evidence any of them have any concept of the physics of convection, heating or cooling, nor of fluid dynamics. These are basically all just metal boxes containing a fire surrounded by a water jacket. However, as long as the source of heat energy is available and not costly or is free, then efficiency is a moot point.

Burning Green wood? At this it excels. In fall and spring we burn garbage wood such as Manitoba Maple pulled from off fence lines and elsewhere on our property (all green, all unseasoned). Why not? This junk wood heats our water and when the weather is mild (anything above zero) the furnace does a fine job heating the house--there just are not enough BTUs in low quality wood to use it when it gets really cold as it goes through a load too quickly. I appreciate that some people will think it is BAD to burn green wood (bad for the environment, that is). Well, what is the alternative? Here it is: build multiple brush piles out of all your junk wood, wait for an inch or two of snow in the fall, and watch all those BTUs go up to the sky. That is just reality. So, it is very fine to talk about grams per hour from a furnace or some nice air-tight wood stove with a catalytic converter in it, but how many grams per hour of carbon are there coming out of a brush pile fire? Probably thousands or tens of thousands or more.

You mention setting fire to your fields with it or setting fire to your house.  This is not surprising.  The manufacturer of our unit claimed no problems at all with sparks, but we needed to add our own spark arrester because so many sparks came out. Without a spark arrester the unit shoots lots of sparks unless the wood is high quality and very dry. However, with the spark arrester in place we can burn any day we like, anything we like. We do not need to worry about tending the furnace or starting brush fires.

Are there Problems? Yes.

Problem 1. These furnaces require work. Muscle power. The manufacturers ignore this detail in their exaggerated claims. You need to get out there and put the wood into it. If going from any form of automated heat to an outdoor wood furnace: THINK VERY CAREFULLY FIRST.... Wood heating can lose its romance quickly when it is a day-in-day-out operation for 6 months. The outdoor wood furnace will be one of your major winter hobbies. If going from an indoor wood-furnace to an outdoor wood-furnace (as I did) then the benefits are huge. But, lets be realistic: not very many people heat their houses with indoor wood furnaces. I would think less than 1 in a thousand even in rural areas.

Problem 2. You need a BIG wood shed. Otherwise, you are going to be burning green, or wet, or snow-covered wood. Wet and/or green wood will burn, but you will waste a lot of wood getting that wet wood up to ignition temperature. And it is very hard to get a wet-wood fire going. Most people do not have the space for the required woodshed nor space to store all the wood they will need to burn for a winter. Let alone space to store wood for the next two or three winters. Very few people have any idea how much wood it takes to heat a house for a winter. It takes a lot.

Problem 3. A lot of wood is needed. This is because these outdoor wood furnaces are very inefficient. There is no way the efficiency of an airtight is matched even remotely. Ours produces huge amounts of heat, but we go though at least 7 bush cords (i.e., 7 piles of 8 ft logs where each pile is 4ft high and 4ft wide) of dry hard oak per year. The cost for us for to buy this much hard oak (white or red oak) or hard maple (sugar maple) and have it delivered (if we buy it as logs) is $600.

Problem 4. The units are extremely expensive. Small ones are $6K. Big ones can be $30K or more. Approximately two-three times the cost of a top-quality high-efficiency propane/oil boiler to heat the same sized space. The extra money could buy a lot of propane/oil. The economics need to be considered carefully.

Problem 5. The units are not durable. They corrode, even though anticorrosion additives are put into the water. The manufacturer of our furnace went bankrupt within one year of our purchase (so much for the 10-year warranty) when someone went after them for a faulty unit. We learned this when a faulty weld started to leak. We had to fix it ourselves, at our own cost. To do so I ripped all the casing off the front, pulled all the insulation off, drained the unit, and brought in our mobile welder at $50 per hour plus mileage etc. And at that point the real guts of the beast was sitting there before us, not just the pretty furnace, covered in decorator colour aluminum siding, that was delivered to the yard. No, in truth it is just a very poorly welded box, slapped together, apparently in a hurry. We changed the insulation before the siding went back on as the manufacturer was so sloppy in surrounding the boiler that you could see parts of it. That was just ridiculous. We had to do the same thing about 18 months later. It has sprung yes another leak, so I need to do it again. Absurd.

Problem 6. The units require daily monitoring. Also regular adjustment, regular adding of water, and that gets to be a pain. It takes very little time, but it needs to be done. No one who has one of these can just shove wood into it and forget it. For this reason alone, an outdoor wood-furnace is not for everyone. If you do not enjoy or are not good at playing with machines, then I would suggest not getting one. If you let the water level go down, and you burn out the fire box (I known one person who did this) then there goes your $6-8K investment. Scrap. Compared to the ease of the standard propane or natural gas boiler (at a third of the initial price) with the slide of a finger to do everything, the outdoor wood furnace is often one big headache (especially when it is minus 30 C outside and you would rather just stay inside and be with your wife/girlfriend but unless you get outside and feed the furnace the fire will go out, the water temperature will start to drop and the house will start to get cold).

Problem 7. Electricity is needed. The outdoor wood furnaces require electrical power to operate: power for the fan (if it has a fan), power to open and close the air vent to get it burning, power to run the circulating pump, etc. If you have a power outage, you have no heat. You probably want a secondary source of heat (i.e., indoor wood stove, or propane heater, etc.).

Problem 8. There is a risk of freeze damage and pipe bursts. If there is an extended power outage, the unit itself can probably be kept warm. Perhaps by wedging sticks into the dampers to permit airflow into the firebox, and thus a low level of combustion, during power outages if it is necessary (we did this during the Jan 1998 Ice Storm, but we were worried about the underground lines freezing). Even if the underground lines are below frostline (most installation guides appear to recommend only going down 1-2ft, well above the frost line in Ontario), the underground lines MUST come above frost-line to connect to the furnace. At that point, if there is no electricity to operate the circulating pump, the lines can freeze.

Problem 9. You are a slave. Would you like to go on vacation someday in winter? Then you need an alternate way to heat the house and to heat the outdoor wood furnace. Forget about getting someone else to baby-sit the outdoor wood furnace for you while you are on vacation. No one in their right mind would do this. I would not, and I have had one for four years. They are just too unpredictable, and who would want the responsibility of keeping someone else's house from freezing after the outdoor furnace has gone out from wet-green wood too long on the smolder? If the furnace goes out in the winter, and it is not heated in some way, it will freeze unless you drain it. Draining the outdoor furnace in winter is something you probably do NOT want to do. So, you need to have things set up so that provide heat to it from the house, and this is most commonly by reverse heating from your hot-water tank. This can be a major downside.

Problem 10. Smoke. The low/no smoke claims of some manufacturers are Ridiculous. There is lots of smoke unless in mid-winter when burning good dry hard maple or hard oak with frequent cycling. The units without fans smoke the worst. The talk about natural draft is hogwash as there is no stack. There is just no draft to speak of. The fan is what makes things burn well, and the fan is what allows you to burn whatever you want. The smoke usually is not a problem for us (personally) as the wind is very predominately away from our house.

Problem 11. They smoke far too much in the summer to use. It would not be much trouble to feed the machine every 2-3 days in summer just to heat hot water. However, when the only heat be drawn out of the unit is to heat a domestic hot water tank, the units sits and smolders most of the time. When the unit starts to burn after smoldering for many hours the volume of smoke is just unbelievable for about 30 minutes or more (depending on what is being burned). If we have clothes out on the line when that happens, or the windows of our house are open and the wind is toward the house, we or our clothes get fumigated! So, after that happened a few times we decided not to use it outside of heating season. It is just too much hassle and makes too much smoke. Our unit goes out in June and we fire it up again in September. The exception is burning garden weeds without having to bother tending the fire. We save oak and seasoned hardwood for winter.

Problem 12. These units need to be near the house. Claims in product brochures and manufacturer's websites that these units can be 500ft from the house are totally unbelievable. Virtually every unit I have seen is within 50ft of the house. Heat is lost underground unless the pipes are extremely well insulated. It is hard to see much heat energy being left after a run of 500ft.