Chimneys...

[new smoke]

A question about chimneys came up while talking with my welding guy who is working on our build.

We are using that spreadsheet to figure sizes on things.

When figuring chimney length, can we count the length of chimney in the cooking chamber, or do we need to count just the portion outside the chamber. Want to get the proper draft.

Thanks.

[aczeller]

i would just use the outside. the chimney draft is induced more by the temperature differential, rather than just the temperature. there are other factors to consider for the draft as well, but i would just count the outside.

Later,
Andy

[new smoke]

That is what my dad and the welding guy were talking about. Shortly after I posted this AM, I came across this from hearth.com.

"Chimney Height- The taller the better ! A taller chimney will have more draft than a shorter chimney (all other things being equal). Its relatively easy to decrease the draft in a chimney which is too strong, but improving the draft on a short chimney can be more difficult.

Chimney Diameter- Chimney diameter also has an effect on the draft of your chimney. In this case, however, having a chimney that is too wide (as compared to the outlet on the stove) can be a problem since it allows the smoke to cool excessively, thereby slowing it down. If possible, the chimney diameter should be similar to the diameter of the flue outlet on the stove. In no case should it be more than 3 or 4 times the cross-sectional area of the stove outlet. If your chimney is oversized, you may consider relining it with a smaller diameter flue.

Chimney Location—The early settlers in this country knew some things we have forgot. They located the chimney inside the structure, where it would stay warm and draft better. This indoor location also meant that the warm chimney added some heat to the home. Take a hint from our forefathers…locate your chimney inside if possible!"

I talked with the welding guy earlier today. With a 6" diameter chimney, we need approx. 61" length. We are going to put 30-36" inside the chamber and the remainder outside. Then we are going to hinge the outside portion, so I can fold it down for traveling.

Seemed like a good solution, we'll see how she breathes when we're done. With the hinge, we can always add more length if needed.

[aczeller]

Originally Posted by new smoke

That is what my dad and the welding guy were talking about. Shortly after I posted this AM, I came across this from hearth.com.

"Chimney Height- The taller the better ! A taller chimney will have more draft than a shorter chimney (all other things being equal).

that part is false. for what you are talking about, yes... the taller the better. however, on buildings, there is a maximum height on chimneys and flues... as the gasses rise, they will cool off, becoming heavier, putting more back pressure on the hotter gasses below. this is the reason that wood-fired stoves in homes will smoke when they first start up. after the chimney warms up and the draft starts functioning properly, the smokes clears and everything is dandy.

like i said, for your application, you shouldn't have any issues... BUT, when you are talking about some major distances (over 10' or so), you can potentially run into some issues.

I think your 30 or so inches (external) should be more than enough... that should get it above face level for windy-day applications, and you should have a fine draft after it all gets warmed up and running.

Later,
Andy

[new smoke]

Originally Posted by aczeller

that should get it above face level for windy-day applications, and you should have a fine draft after it all gets warmed up and running.

Later,
Andy

I shouldn't have to worry about face height, but a great point.

The top of the cooking chamber is about 7'. I'm only about 6'4", so another good reason for the hinge. Without it, I'd be about 10' going down the road.