Heard one that made me laugh.............

[Piker]

The heatloss from a DD sitting outside under a roof can be staggering too!

Indeed!

cheers

 

[brad068]

Heaterman,

Is the DD in an indoor environment? The air going into a downdraft gasser really needs to be tempered before it goes into the unit. If the secondary air is 0* it can cool the refractory down significantly to the point where gasification efficiency is effected dramatically. For instance, a DD in a three sided shed is a no-no for sure. It's best to enclose the unit inside an insulated room where the heat loss from the boiler itself can temper the air a bit before it enters the combustion path. The difference in fuel usage between a unit inside and one setting outside under a roof can be staggering.

cheers

Piker,

I have often though about this with my garn style unit. Garns pull combustion air from directly out side a most ingenious design and when temps are cold out I kinda though that the cold air was slowing combustion too but only for a little while. I was going to build an exchanger to capture exhaust heat but any colder I get my exhaust and I will be in condensing range. Running @ 240*F right now and that is the temp gauge right at the back of the boiler. The combustion air does get somewhat preheated as the intake passes through the water volume and around the air collar another ingenious design.

But my point is garns move alot of air. And moving all that air leads to great combustion with wood from 15-40% MC. Garns, in my opinion are not as finikey, shall we say as DDs but they do consume alot more electricity that DDs, but thats a price I'm willing to pay for the ease of operation and simplistic design.( simple refractory and electrics)

 

[deerefanatic]

That's true.... Garn has definitely won the "simple" award..... And if you've got simple AND efficient, then cheap is out of the equation! (Though our DD's can hardly be called "cheap")

 

[Piker]

Heaterman,

Is the DD in an indoor environment? The air going into a downdraft gasser really needs to be tempered before it goes into the unit. If the secondary air is 0* it can cool the refractory down significantly to the point where gasification efficiency is effected dramatically. For instance, a DD in a three sided shed is a no-no for sure. It's best to enclose the unit inside an insulated room where the heat loss from the boiler itself can temper the air a bit before it enters the combustion path. The difference in fuel usage between a unit inside and one setting outside under a roof can be staggering.

cheers

Piker,

I have often though about this with my garn style unit. Garns pull combustion air from directly out side a most ingenious design and when temps are cold out I kinda though that the cold air was slowing combustion too but only for a little while. I was going to build an exchanger to capture exhaust heat but any colder I get my exhaust and I will be in condensing range. Running @ 240*F right now and that is the temp gauge right at the back of the boiler. The combustion air does get somewhat preheated as the intake passes through the water volume and around the air collar another ingenious design.

But my point is garns move alot of air. And moving all that air leads to great combustion with wood from 15-40% MC. Garns, in my opinion are not as finikey, shall we say as DDs but they do consume alot more electricity that DDs, but thats a price I'm willing to pay for the ease of operation and simplistic design.( simple refractory and electrics)

I think cold combustion air is a factor in any wood boiler... in order to burn the woodgas efficiently, it has to be raised to some pretty high temps... the higher the temps, the easier it will burn regardless of whether it's a DD, and OWB, or a garn type unit. On an OWB, I don't know that it makes as noticeable of a difference, since the efficiency is already extremely poor, but on a DD it makes a huge difference compared to how the unit operates with tempered air. If you insulate the shed or room that your DD is in, you will more than make up for the heat lost from the boiler itself by increasing the combustion efficiency with tempered air inside the room.

I have often thought of using a double wall type chimney in outdoor installations where you can draw combustion air into the shed through the space between the walls of the chimney pipe. This would utilise a great deal of the heat that gets wasted up the stack, but would also probably condense quite a bit, and produce a poor draft situation... If you could capture that condensate before it runs into the boiler you could at least protect the boiler, but the stack itself would be subject to additional acids.

cheers

 

[heaterman]

Heaterman,

Is the DD in an indoor environment? The air going into a downdraft gasser really needs to be tempered before it goes into the unit. If the secondary air is 0* it can cool the refractory down significantly to the point where gasification efficiency is effected dramatically. For instance, a DD in a three sided shed is a no-no for sure. It's best to enclose the unit inside an insulated room where the heat loss from the boiler itself can temper the air a bit before it enters the combustion path. The difference in fuel usage between a unit inside and one setting outside under a roof can be staggering.

cheers

Piker,

I have often though about this with my garn style unit. Garns pull combustion air from directly out side a most ingenious design and when temps are cold out I kinda though that the cold air was slowing combustion too but only for a little while. I was going to build an exchanger to capture exhaust heat but any colder I get my exhaust and I will be in condensing range. Running @ 240*F right now and that is the temp gauge right at the back of the boiler. The combustion air does get somewhat preheated as the intake passes through the water volume and around the air collar another ingenious design.

But my point is garns move alot of air. And moving all that air leads to great combustion with wood from 15-40% MC. Garns, in my opinion are not as finikey, shall we say as DDs but they do consume alot more electricity that DDs, but thats a price I'm willing to pay for the ease of operation and simplistic design.( simple refractory and electrics)

I think cold combustion air is a factor in any wood boiler... in order to burn the woodgas efficiently, it has to be raised to some pretty high temps... the higher the temps, the easier it will burn regardless of whether it's a DD, and OWB, or a garn type unit. On an OWB, I don't know that it makes as noticeable of a difference, since the efficiency is already extremely poor, but on a DD it makes a huge difference compared to how the unit operates with tempered air. If you insulate the shed or room that your DD is in, you will more than make up for the heat lost from the boiler itself by increasing the combustion efficiency with tempered air inside the room.

I have often thought of using a double wall type chimney in outdoor installations where you can draw combustion air into the shed through the space between the walls of the chimney pipe. This would utilise a great deal of the heat that gets wasted up the stack, but would also probably condense quite a bit, and produce a poor draft situation... If you could capture that condensate before it runs into the boiler you could at least protect the boiler, but the stack itself would be subject to additional acids.

cheers

RE: cold combustion air. I've been doing some cruising on the www looking for articles that deal with that topic because I have a hunch that the temperature of the combustion air can indeed have an effect on the burn process. How much is my question. Realistically a difference of say 70* wouldn't have much influence on the burn but who knows. All I have found is some article dealing with its (cold air) effect in huge gas and oil fired boilers. Huge as in multiple millions of BTU's. Those article deal with it basically on how cold combustion air affects NOX production as the articles are dealing with power generating boilers. I'll keep on looking

Piker: What you are talking about is a concentric style vent where the flue gas exits through a pipe which is within a second pipe through which combustion air is drawn. Very common on some models of Viessmann boilers and it works well to not only help[ insulate the flue but also to preheat (somewhat) the combustion air. Just about every mobile home type F/A furnace made has this type of inlet/outlet flue system. It would take some doing to adapt a typical DDG style boiler to this system but given the willpower it could probably be field engineered. The issue would be how to deal with the required barometric draft control.

Garny: The Garn design does preheat the combustion air because the inlet air passes all the way through the tank on its way to the air collar. Once there it soaks up a bunch of heat through direct contact with the backside of the firebox. Someday I'm going to stick a thermocouple in the collar and find out just how much it gets elevated temp wise. Does your unit incorporate this part of the design?

 

[brad068]

Heater, my unit does incorporate this. During testing of my unit in /around the air collar I did see air temps in the collar up to 350*f especially when it starts puffing and the heat is brought backwards. Remember the ol' turkey thermometers at wal-mart!? I also checked with a infared gun checking the outer metal surface of the air collar.

FYI, I started my unit 11-1-08 and stopped 5-24-09. Heating @4000sf and two ten plates. All in all @ 16fc and alot of that was not 20%mc wood.