Tuesday, February 10, 2009

Wood Gasification Boiler

Whew, for over a month, I've been immersed in the "theory and practice of modern hydronic heating." My task was to install our wood gasification boiler, build a heat distribution manifold (the circulatory system of our house), and get our radiant heat floors going. I'm happy to report that I have accomplished my mission - it's a balmy 66 degrees inside the house now!

The heart of our hydronic heating system is a "Tarm Solo Plus 40" wood gasification boiler. This unit is made in Denmark and distributed by BioHeat USA (formerly known as Tarm USA). Unlike the outdoor wood boilers that are gaining popularity (and unfortunately notoriety in some areas) in the United States, this boiler is made to work indoors, and burns with little or no smoke. By cooking the wood and drawing gas off the wood to burn in a separate ceramic chamber (at nearly 2000 degrees!), the boiler is able to extract more than 80% of the BTU's available in the wood. By comparison, most outdoor boilers and fireplace inserts do well to achieve 50% efficiency. Less wood to cut - sounds good to me - where do I sign up? The other benefit of the super high combustion temperatures is that very few unburnt byproducts are released into the air. It's odd to see steam coming out of my chimney, but that's what's going on as I type this!

There simply aren't many tradespeople familiar with radiant heat and wood boilers in eastern Kentucky, so I decided to install the unit myself. It turned out to be a lot more complicated than I first imagined, but fortunately, I found a vast resource of friendly people and information on an internet forum devoted to wood boilers at hearth.com. The Tarm boiler comes with instructions, but every installation is so unique that the instructions are more or less "general guidelines." Furthermore, the instructions already assume that your radiant heat manifolds (or air handlers, or baseboard radiators) are already installed and hooked to an existing fuel oil (or natural gas) boiler. I had to start from scratch.

I began by plumbing the accessories that go directly on the back of the boiler. This is fairly standard, so the Tarm manual was helpful here. In this picture, the four copper lines pointed in the air are (from left to right), the supply line, the emergency return line, the emergency supply line, and the return line. The emergency loop is for dissipating the heat in the boiler if your power should fail (and your circulator pump stops) while a fire is in the boiler. A valve on the emergency loop opens when the power fails, and hot water can thermosiphon through the emergency loop if it is designed properly.

The red item is a Grundfos 60 watt circulator pump, and the brass "T" shaped item with red arrows is what they refer to as a "tempering valve." Its purpose is to temper the cold(er) water returning to the boiler, by mixing in the appropriate amount of hot supply water. This keeps the boiler running at the proper temperature regardless of the heating loads. After I finished plumbing all of this, I pressure tested my threaded fittings and sweat joints and everything looked good to go, so I rolled the unit into its permanent location. (Spatially, it is in the basement, directly under my fireplace but it uses a dedicated flue in the chimney.)

*** warning - skip this paragraph if you're not fascinated by pipe thread trivia ***

I'm leaving out a lot of the _really_ boring details, but one detail that deserves mentioning is the pipe-sealant that comes with this European boiler. Whereas in the U.S., we use tapered threads (aka NPT) that get snug after a certain number of turns, the Europeans use straight threads that never get tight no matter how many times you turn one pipe within a fitting. (please, someone from the EU correct me if I'm wrong!) The systems are otherwise similar in that the threads have the same pitch and diameter on both continents. Fittings and pipes will interchange. In the US, we commonly apply teflon tape and sometimes pipe dope to help seal the tapered threads. And this is where it gets interesting (if it ever does)... The boiler manufacturer sees fit to include a unique pipe sealant better suited to the straight threads on their boiler taps. With their system, you wrap the pipe threads with long dry fibers that smell like wet dogs stewed in sour milk (hemp? horse hair?) , and then apply a generous portion of some squishy substance which is partially composed of "animal lipids," before twisting the pipe into the fitting. It was so delightfully archaic that I used their system on all of the fittings I could until I ran out of the stuff. My wife and I were cracking up at this stuff. (if I could have over-ridden my gag reflex, I might have tricked her into smelling those fibers too). Was it a joke on stupid Americans, or would it work? The joints performed fabulously during the pressure test and subsequent operation - no leaks whatsoever. Now I know what to use for pipe sealant in a post-apocolyptic, non-industrialized society should I ever find myself in such a scenario.
*** end pipe thread trivia warning ***

*** warning, veering off topic, gratuitous Appalachian scenery ***
Midway through my installation, we were hit with an ice storm here in Kentucky (reminiscent of the 2003 ice storm that felled all of the trees that became our house). The miserable weather just provided me with more incentive to get the heat functioning in the house! I took this picture from a second floor window, looking south. Fortunately, very few trees fell in this latest ice storm.

*** end warning, back on topic ***

With the boiler plumbed, I focused my attention on the heat distribution manifolds. Some day, I'll make a proper diagram of the whole system and post it here (my wife insists I make one some time before I die), but for now, I'll just point out some of the highlights that might be interesting to someone designing their own hydronic heating system.

First, the architecture is "primary-secondary." In hydronic-speak, this means there is a single primary loop that circulates the heat in the system, with several secondary loops tying in at various points along the way in order to extract heat for various functions in the house. With ths architecture, the pressure and flow in each of the secondary loops is isolated from the primary loop (and therefore from each other). One of my secondary loops is the radiant heat loop. Another of my secondary loops is my indirect domestic hot water heater. I may add other secondary loops in the future (e.g. a huge hot water tank to store heat, a solar hot water collector, a clothes dryer that uses hot water for heat, etc.) . It's very easy to add on to this architecture and that's one of the reasons I chose it. The more widely used alternative is to have one giant supply manifold and one giant return manifold, but that architecture has drawacks that I won't go into now.

Another important aspect of this design (and virtually all hydronic radiant heat designs) is how to obtain the proper temperature for the radiant heat zones. Specifically, the boiler heats water to 180 degrees whereas water somewhere between 110 and 150 degrees is appropriate for radiant heat floors. There are several sophisticated ways to achieve the temperature drop, but I chose the rather simple method of a three way thermostatic mixing valve. In this picture, 180 degree water from the primary loop enters the top port of the mixing valve, cooler water returning from the floors enters the left port of the mixing valve, and 130 degree water magically exits the bottom port of the mixing valve. The valve is entirely mechanical (no electrical components), and presumably fairly reliable. This one is made to be taken apart and cleaned, should that ever be necessary. Incidentally, the copper T in the upperleftmost of the same photograph is where the rest of the return water from the floors is injected back into the primary loop.

At least one more aspect of the system deserves mentioning... Because the entire heating system is using off-grid solar/battery power, I was very critical of the power consumption of the circulators (pumps), zone valves, and the boiler fan. My concern was heightened by the fact that the heating system is most in demand when my solar resources are at a triple minimum... (1) during the winter... (2) during cloudy days, and (3) at night. The boiler requires only a small circulator because it does nothing but circulate water through the relatively short primary loop. I chose a Grundfos 15-58, which consumes only 60 watts. It has a low speed setting that I might get by with that uses only 48 watts. The fan on the Tarm boiler only uses about 50 watts. These are manageable, but not insignificant numbers (50 watts + 60 watts). With these components pretty much "fixed," I focussed my attention (i.e. budget!) on the large circulator that pushes water through almost one mile of 1/2" pex pipe in the floors of the house. For this task, I found a German (they are more energy conscious than us Americans) made WILO Stratos pump that is super efficient. In fact, it has a microprocessor that will vary the pump speed based on how many zones are calling for heat. (The technology to control the pump speed is called ECM for "electronically commutated motor.") The motor draws between 9 and 130 watts, and in my application, I think it will usually be drawing about 80 watts based on my measurements. A conventional circulator pump for this application would probably draw at least 150 watts constantly.

Well - that's it for now. I could go on about how to check for leaks, sweat joints, choose a pex manifold, store heat, bleed lines, etc., but most of that stuff is already on the internet. I've included just enough detail here to bore most folks to tears, while not enough detail to reproduce my results and avoid my mistakes. If you have questions about this system, or questions about a system you are designing or modifying, I am glad to offer what little knowledge I have picked up while building this system. I'm still tinkering and will post updates later on. For now, the house is warm and I can work in my T-shirt! On to other unfinished, inside projects...


Anonymous AJG said...

Amazing Thomas - both the views from the windows and your installation of the boiler. I'm impressed by both! : )

February 11, 2009 at 9:36 AM  
Anonymous Jim K in PA said...

Good job on the Tarm, Tom. I wish I had gone with the Wilo pump for my house secondary loop. I am using a Grundfos 26-99 and I need to vary it based on Delta T. Is the vaiable Wilo you have Delta T or Delta P based? I am going to add a Tekmar 157 to the house pump and it will do what I want, but at additional cost.

So have you started gathering some propane tanks for your storage? There was a guy on eBay selling 1000 gallon "blems" for cheap recently. I think he was in TX though.

How is your wood supply? You want nice, dry wood for best performance, as I am sure you know already.

February 11, 2009 at 11:54 AM  
Blogger oldmilwaukee said...

AJG, heat in the house means we'll be making pizza again soon... let us know what toppings you all like.

Jim, from the factory, the WILO will let you pick "constant (delta) P", "proportional (delta) P", or "constant RPM". It also has a night set-back function that I don't understand yet. You can also slave two pumps together.

According to the manual, there's something called an IR module (never seen one for sale in the US - not sure if IR has anything to do with Infrared) that you can plug into the pump's control board and it will let you do delta T or control the pump with a 0-10 V signal. I would like to have a tiny Wilo pump with delta T to do the temperature mixing function (replace the thermostatic mixing valve). Their smallest pump is just delta P though.

I saw those blems on Ebay. If he had been closer, I probably would have bought one.

I did not realize how much of a difference dry vs. green wood makes in my boiler at first, but now I see it is huge. With slightly wet wood (well seasoned, but it was under the snow), the unit struggles to get up to temperature. With dry wood, it is amazing. First thing this spring, I'll be splitting next season's firewood. Fortunately, this year I have enough timber cut-offs from the house frame. Been seasoned for 5 years - they burn awesome. Most are sitting outside uncovered though. I have a couple pallets of them sitting in the basement drying out right now.

February 11, 2009 at 7:54 PM  
Anonymous Jim K in PA said...

Tom - thanks for the info on the Wilo. I think I am going to use a delta P based VS pump for my house zones, so I will look into the Wilos.

I have been burning wood that is almost TOO dry. I want to get a moisture meter to find out what the content is, but it will probably be all gone before I do. I had a problem with the ultra-dry wood burning too fast, causing the GARN to "puff", or essentially be gasping for more air. This is with a 3/4hp Dayton jet pump fan motor pulling in hundreds of CFM!!

I am also burning several cords of scrap wood left from the farmhouse renovation and new addition construction. I love being able to heat the house with what would have otherwise been landfill or burn barrel fodder. I mix it in with the cord wood.

February 12, 2009 at 1:35 PM  
Blogger Kineret said...

Hi, I'm a devoted reader from Israel. Love your blog (and of course your project). Thanks for the inspiration!

Those "long dry fibers that smell like wet dogs stewed in sour milk" are fibers from a plant called flax (actually linen is produced using the same fibers, as well as linseed oil which is a great natural wood protector and sealant.


Here in Israel (and probably in Europe as well), these fibers are widely used for all metal plumbing threaded fittings. They do smell bad, but work great due to their expansion properties over time in humid or damp conditions (just like wood) - thereby tightening the seal.

February 14, 2009 at 2:40 AM  
Blogger oldmilwaukee said...


Thanks for the low-down on those stinky fibers! They just seem so weird - everyone in the US seems to use "teflon tape." After reading the article you linked, I wonder if cotton displaced flax in North America to the degree that we "lost the art" of sealing threads with flax? And we get linseed oil from the same plant?! Sounds like I need to sow some of that in my fields. :)

February 14, 2009 at 6:26 PM  
Anonymous ScottM said...

The amount of work and planning you've done is simply astounding! I've long thought about building my own tiberframe house - after building a TF pergola i'm now planning a TF shed before looking into designing a house. Thanks very much for providing me with the inspiration to transform my dream into reality!

March 6, 2009 at 12:40 PM  
Blogger surya said...

I have just purchased a wood gasification boiler that I will put in a shed about 150 feet from the house - piping the heat to a 100,000 BTU copper tube heat exchanger in a 900 gallon thermal storage tank. I have an oil furnace currently with about 6 zones to the baseboard in my old farm house in Maine. I am wondering about how to design the system. I have heard that it is better to hook the twp systems up so the wood boiler heat does not run through the oil boiler. This is much more work and more complicated and wonder if there really is much heat loss in going through the existing boiler.

May 29, 2009 at 4:50 PM  
Blogger Mano said...


You have got a pleasant blog with educational posts. Cheers for being so informative regarding steam boilers.

October 13, 2010 at 2:26 AM  
Anonymous Mike from Wood Stove Wizard said...

Great post - my wife and I are considering one of these setups if/when we get our new home. The place is currently an empty shell so we can do what we like.

Have you installed a thermal store of some sort since you posted this? That seems to be the main attraction - one hot fire heats the thermal store up for the day.

November 29, 2010 at 6:12 PM  
Blogger Unknown said...

Great post. Awesome work. My wife has been begging me to put in a new Wood Boiler. Thanks for the helpful blog post!

May 10, 2012 at 10:53 AM  

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