Q&A About Outdoor Boilers
A satisfied ex-outdoor boiler owner
About eight months ago I contacted you about wood gasifiction boilers. I had bought an outdoor furnace the previous year and was very dissatisfied by it's performance. I had after one season decided to find a better way or install propane in my house. I searched long and hard and found only three options: Tarm, Blue Flame and AHS. After seeing both the Tarm and AHS (Alternate Heating Systems), I chose the AHS since the Blue Flame was not available in this country, and the Tarm needed the wood to be cut pretty short.
As you said they were not gasifiction boilers but downdraft boilers. I now am pleased with heating with wood again. It is amazing after having an outdoor boiler how much wood wafted away as smoke. I compared 7 day usage between last year and this year during cold snaps both averaging 25°F during the day and 9°F at night. I used one full cord of white oak for the outdoor furnace and 1/3 cord for the new unit and smoke has never been a problem compared to the smokescreen I used to have.
I just want to thank you for your quick response to my questions on this subject and am grateful help such as you exist.
Common sense versus technical knowledge in boiler design
I haven't read everything on your site about this topic so may have missed this suggestion: what would be the expected smoke problem from an outdoor boiler if it was used to heat water in a large storage tank, and the stored heat then extracted over several days while the boiler was off and cold? This would allow the boiler to be run hot and (relatively) clean, reducing if not eliminating the smoke problem.
I have considered building one of these myself for a Panabode house I'm putting on a 5-acre property I'm moving to in the next few months; the shop is under construction right now. I build pottery kilns and assay furnaces for a living and therefore am competent working with firebrick making chambers that stay at over 2000F for months at a time for many years. Destruction of these occurs from corrosion of the 70% alumina firebrick, not from heat. So would I be able to build a firebrick combustion chamber with the exhaust gasses passing through a conventional many-tubes-in-a-tank heat exchanger and achieve a clean-burning system?
I've learned over the years of burning with wood that efficiency is simpler to achieve than it would appear at first glance: buy top quality, keep it smaller and burn it hotter, rather than bigger and cooler. It took me three stoves to find that out, finally falling in love with a Petit Godin, which is what will go into the panabode. I bought mine (1979) just before the requirement for CSA testing so it is not labeled, but I have subsequently found that the model I have (3720) was tested and certified and I have obtained the CSA file for it, so am hoping that the inspector sees the world the way I do! We'll see shortly.
This looks like an excellent website, especially to someone who can grow enough wood on his 5 acres to keep himself warm for the foreseeable future, gas prices notwithstanding. It will be fun watching the popularity increase again like it did in the mid-70's, and listen to all the old ideas being re-discovered. I grew up on a farm where wood was the only heat we had, and have always wondered what all the fuss was about! There's a lot more common sense to it than rocket science.
I agree with much of what you say, but I'm not so sure about your take on common sense and wood burning. I've seen too many fire hazards caused by the application of 'common sense' by the homeowner during the installation of a wood stove and/or chimney. And I suspect that the stove manufacturers who collectively spent many millions of dollars back in the late 1980s developing combustion technologies to meet the EPA emissions limits would not agree that low emission, high efficiency wood burning technology development is just common sense. Over the years I've taken the view that common sense is highly overrated in the field of wood burning.
However, I do agree wholeheartedly with your assessment of outdoor boiler technology. Yes, burning at high rate and storing the heat is one way to reduce emissions. The big mistake made by many outdoor boiler manufacturers is to surround the primary combustion chamber with a heat exchanger in the form of a water jacket. I think it is amateur 'common sense' to focus on heat transfer as the key to efficiency, when experienced combustion engineers know that good quality combustion must come first and is the big challenge, and after that heat transfer is easy.
What I read in your message does not sound like common sense as much as a depth of technical knowledge about combustion and heat transfer. And I think you are right.
In answer to your question about boiler design, yes an insulated primary combustion chamber and multi-tube heat exchanger is the right general design. But there is a whole lot more to it than that which is why we don't recommend that anyone build their own systems.
I've got a broken outdoor boiler
I found your article very interesting, as well as the feedback from others. I recently purchased a house in a rural setting which has an outdoor boiler system installed, with a backup forced air electric furnace. The unit was purchased from *******, who no longer seem to be in business. Just before we purchased the property (last Dec), the owner indicated the boiler developed a leak in the firebox, so he shutdown and drained the system. I ended up using the electric furnace last winter. We have a wood stove in the unfinished basement and an old Empire wood stove in the kitchen. Neither seemed very effective as a primary heat source. My plan was to see if the leak in the boiler could be repaired, or if not, the cost to replace the unit. Can you provide any reputable contacts for these options. Based on your article, I'm wondering if I might be better to replace my furnace with a wood burning/electric combo furnace. Any comments on how efficient these units are? Any help would be appreciated.
The right wood heating system depends on the house size and design and your objectives. I don't have enough information to recommend a particular approach.
You should only consider an outdoor boiler that appears on the EPA list. We have no basis for advising people as to the best outdoor boiler to buy, except among the group that is certified.
The failure by leakage of your boiler seems to be fairly widespread so we hear. Fixing it is probably not a good strategy; I think there is a serious design error (beyond the obvious design errors that plague all conventional outdoor boilers).
Oh, No! I just bought an outside wood boiler
Oh, No! I just purchased an outdoor boiler. Now I find your site on the web. I just got this computer and learning how to use it.
Any suggestions? Sell it? I don't feel too good now. Your site is excellent. Wish I had seen it earlier.
I too am sorry you didn't find our site before buying an outdoor boiler. It might not have changed your decision, but at least you would have had more information to work with. We have had a fair amount of traffic on the site and through email on outdoor boilers recently.
You might find Barney's comments useful. He claims that through careful operation, smoke can be minimized, although he admits it still smokes.
Another problem with outdoor boilers
I recently read of a vacation home in the mountains of Central Oregon whose original builders had been persuaded to install an external boiler heating system. They had to replace frozen pipes numerous times, and it seems an obvious point that an outside boiler is not a good choice for a house that is not continuously-occupied, though apparently there were freezing problems even when the system was fired up. Admittedly, some of the freezing problems might have been due to improper installation of pipes, rather than design of the outside boiler.
The second owners of the property suffered through one winter, when in addition to freezing problems, they discovered the boiler had a voracious appetite for fuel, making it very expensive to operate. In the end, they decided to replace the boiler system with a propane furnace, supplemented by a modern indoor wood heating stove. Propane is normally not an economical fuel, but in this instance they found it cost less than the firewood to keep the boiler going, and at considerably less trouble.
This single example can of course not be taken as proof that outside boilers don't work; however, it IS a good illustration of how they are a bad choice in certain situations, and how someone apparently received very bad advice about them in the first place.
Thanks for the note. That is a sad outdoor boiler story. We have heard all those complaints but maybe not in a single example like that. One of woodheat.org's main jobs is to help people to be successful with wood burning, and we have been hearing of problems with outdoor boilers for years, which is why we take a cautious and even critical approach to the technology.
Mind you, for every dozen problems we hear about outdoor boilers, we hear of other people who are satisfied with the performance of theirs. Although the combustion design of conventional outdoor boilers is either crude or non-existent, another serious problem is poor system sizing advice from part-time retailers. A properly-sized system might work reasonably well in the hands of an experienced operator.
A readers analysis of wood boilers
Hi: I am interested in a wood boiler but I am not satisfied with the technology that is available in Canada. Many of the people I talk to who have outdoor boilers all have the same complaints. The complaints being wood consumption, smoke, cracked boilers, leaks, what do you do when you want to go away, what if you want to sell your house?
There are many European manufacturers such as HS Tarm, Mescoli, Perge, Kuenzel etc. that claim to make efficient wood boilers. I am not interested in an outdoor boiler as much as one that I would install in my garage that is attached to my house. My question is are there any manufacturers in Canada that make a high quality unit that makes use of wood gasification technology and are a multi fuel unit? All of the manufacturers recommend that you also install a heat storage system that reduces the amount of time that the boiler is idle. I think that this idea would help to correct some of the smoke problems that many people are experiencing. What do you think?
I personally think that the Europeans are on the right track. What do you think about the technology that they are currently using?
I read an article that talked about a manufacturer in Parrsboro Nova Scotia that makes a unit that uses this technology are they still in business? Have you heard of them?
I thoroughly enjoy your web sight keep up the good work.
You're completely correct in your entire analysis of the current boiler situation. Downdraft boilers like some of those you listed can be efficient and effective. Also, they tend to be available in the right sizes for residential heating.
Outdoor boilers downtown?
I am researching a project. My question is: is it recommended that outdoor boilers be used in urban areas on relatively small tracts of land even if the boilers are installed properly? I'm concerned more with environmental factors, as well as their use in residential areas. Any information about outdoor boiler use would be greatly appreciated. Jeff
I guess it depends on who does the recommending. If you were to ask the bylaw enforcement officers of the many jurisdictions that have banned outdoor boilers because of smoke complaints, the answer would be an emphatic NO! As far as we can tell, correct installation does not resolve the smoke problems because it is the system characteristics of conventional outdoor boilers that causes the smoke, not specifics of the installation. The new breed of EPA compliant outdoor boilers are too new to make an evaluation of how suitable they might be in built up areas.