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[CRJR]

I'm pondering on building a gasifyer and was curious is it possible to pull to much heat from the exhaust gas? Would it cause condensation? I would like to hear some thoughts on peoples experiences as far as can you have to much cfm in fire box on non-gasser or a gasser? i dont think my  home built traditional owb is getting enough air to burn as clean as it should. I feed the air from under the fire through the grates.  It seems to burn cleaner when it has not much wood and has a few good air holes blown through the coals. but when its cold and i put a good load in the fire box for the day with a full bed of coals the smoke coming out the stack is dirty (dark or black) not nice and white or almost none any like when it has good  holes through the coal bed.  I am think that i need more cfm to penetrate the coal bed better

[Scott7m]

When stack temps drop below 300, condensation can occur and will depending on various environmental factors.  I've checked mine numerous times, when things are nice and clean it's 325-340, higher when it's dirty.  Be leery of anyone telling you that you can safely run under 300 degree exhaust, that's a condensing furnace and as of now there is no such thing in regards to wood boilers. 

I deal with this stuff daily and can only say that companies with hundreds of thousands to spend in development of a gasser rarely get a design that's user friendly, some wont hardly work at all, the task of doing it as a Lone Ranger would be monumental at best.

[jreimer]

Scott,

I finally drilled a hole and stuck a candy thermometer in the exhaust of my optimizer 250.  I'm not seeing temps above 280-300 under full gasification.  Typically they are between 260-280.  Where would the condensation occur?  In the chimney?  I have an insulated 12 foot Selkirk chimney attached to the stove.

[martyinmi]

James,
Your temperatures are fine. I've experimented with mine and I typically won't get any at all until temperatures get down in the 230* area while burning seasoned Ash @ 18 - 20% MC.
Condensation formation in this area is affected by a few things, and probably the most important would be the dew point, which can be controlled for the most part by the moisture of your wood.
In other words, if you burn wood that has a MC of 30% or higher, you could feasibly get condensation even with EGT's as high as 280 - 300 degrees.
If you don't make it a habit of burning wood that has a MC of more than about 22 - 23%, you'll likely never have a condensation issue in that area.

[Scott7m]

The manufacturers have to build some wiggle room into these units..  You have to allow for different altitudes, different moisture contents, different atmospheric conditions etc..   

Now if your someone who is absolutely meticulous with your wood quality and such you can go under 300 safely, but when you go much further your def reaching the point where bad things can occur, it will condensate in the tubes, or in the chimney and rain back in.   Manufacturers tho have to allow for a whole slew of potential differences to make a forgiving stove.

12' of stack is a lot for sure

[slimjim]

Yes you can bring down exhaust temps to far and create condensation. if you want to bring the temps down to around 300 degrees you would have to be burning very clean first or you will have one nasty mess inside the heat exchanger. most of the conventional stoves on the market today run an exhaust temp of 700-1200 degrees for that same reason. if you have a conventional boiler and want more heat from the exhaust then maybe build a coil into the stack and run your return  water from your home to the coil and then return to your boiler, I got a 10 degree water temp rise just from the heat going up the stack. Use caution though as it makes a great place for creosote to collect and chimney fires to happen. just curious, were you trying to design your own unit(research and development) or just did not want to spend the money for a factory built unit. Suggestion would be ,look at the different models out there, pick their brains, compare notes, and then decide if it is worthwhile building your own or buying one already built, tested and warrantied. A great boiler to look at is the one that we sell but I don't think it would be right to advertise the name here, however if you are interested in learning more about it you may certainly E mail me
Richard