homemade secondary air injection questions

[akwood]

Building a new wood stove, dimensions as follows :

54" tall; 30" in diameter; hot water coil takes up top 24"; bottom 8" taken up by shaker grates,
which leaves me with a firebox approx. 30" wide and 32" tall

Would like to use fan-induced draft but not sure, may be normally aspired -- would like to add secondary air for combustion,
was thinking of using stainless barbecue grill flame manifolds for secondary air injection manifolds

1) When running secondary air for combustion, what percentage of air do you think goes secondary versus primary ?
2) Do you start out with all primary and switch some over to secondary at a certain time in the burn ? (would need to know to fab air induction valving)
3) Don't have glass door so do I need to add a peephole to see if secondary combustion is working or not ?
4) Should my secondary injection manifolds be above or below a baffle ? (which I don't have now but could install if need be)

Any and all comments or suggestions would be greatly appreciated. Thanks for help, trying to get this project buttoned up and burning.

 

[Corey]

Well, beyond the fact that this seems like a death trap...be ABSOLUTELY sure you use plenty of pressure and temperature relief valves (I probably wouldn't rely on just one) for the water coil in the top of this thing...

1) When running secondary air for combustion, what percentage of air do you think goes secondary versus primary ?

It's going to depend on how wide the primary air is open, basically how hard you are running the stove, and on the design of the stove itself. If you are building from scratch, it would probably be easiest to make both primary and secondary air fully adjustable. That way you're in control. Secondary air should never be completely closed, you will probably find some optimum level then 'set it and forget it'

2) Do you start out with all primary and switch some over to secondary at a certain time in the burn ? (would need to know to fab air induction valving)

I think most manufactured stoves, they have done the R&D;so that the secondary air is metered to optimum levels simply by how many and what size holes are drilled in the secondary pipes. It is non adjustable. To start up (or reload), you'd have primary air wide open, then close it down after the wood chars

3) Don’t have glass door so do I need to add a peephole to see if secondary combustion is working or not ?

Probably so. If you open the door to look, the firebox will change how it was burning because the airflow will be different through the open door.

4) Should my secondary injection manifolds be above or below a baffle ? (which I don’t have now but could install if need be)

Most all modern designs I have seen have the injection below a baffle - and in a fully firebrick lined (insulated) firebox (except for the front door obviously). The idea is to retain the heat from the fire in the firebox then inject extra fresh air in the top and burn up the smoke. This superheated air then spills out over the top of the baffle and heats the stove. If you plan to use forced draft, you could make quite a long path for the flue gasses to take and extract maximum heat. If you're relying on natural draft, you have to balance heat extraction with convection...you need the flue gas to be hot enough to create a good draft.

Additionally, you could use the BBQ grills as flame manifolds - they may or may not have the optimum number and size of holes and they will need to be replaced periodically as they live a pretty rough life with the extremes of temperature at the top of the fire. Simple stainless tubing might be a better option, cheaper and easier to replace.

 

[Chettt]

Can you go take a look at a "store bought" stove in a neighbor's house to see how everything works? Are you connecting the hot water coils to your house system? The type of questions you are asking makes it sound like you haven't much experience with a wood stove and that you are trying to build one that is extra complicated. I hope someone here can give you all the information needed for this.

 

[begreen]

Moved to boiler room for better advice on draft induced burning and working with hot water. By all means pay attention to safety. Steam under pressure is an extraordinary, powerful force.

 

[akwood]

thanks for the comments. will absolutely play it safe. will have t+p valves and also will be an open system. if I don't like the way it works out it will become a large waterless woodstove. have plenty experience with woodstoves, just not with cats or secondary air. seems to me not being able to see whats going on in stove could be one of my main problems. thanks