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Posted By slackercruster,
Oct 26, 2011 at 12:27 AM
Why does the longer the chimney make for a better draft?
Generally speaking... longer chimney = higher up in the atmosphere.
Higher in the atmosphere = greater differential pressure
pressure differential = draft. Therefore greater pressure differential = greater draft.
Of course, there are plenty of other things that can affect draft.... but the ultimate reason for chimney height is to create that column effect which causes negative pressure (draft). There are other reasons too... like getting the exit point of the chimney away from the roof surface (fire risk).
Why does chimney length matter . . . because everything is better when it's bigger . . . V-8s are better than V-6s . . . 3/4 ton trucks are better than 1/2 ton trucks . . . a 800 cc engine in an ATV is better than a 450 cc engine in an ATV . . . a 60cc chainsaw is better than a 30 cc chainsaw, etc.
Nice try! Vent an EPA wood stove into a 10X12" unlined masonry chimney...
Interesting calculators can be found here: http://www.engineeringtoolbox.com/natural-draught-ventilation-d_122.html
As inside temp rises (fire started in stove), density drops quite quickly. Temperature differential appears to play a much bigger part in draft than height does (at least when looking at the height range that a residential chimney has) . When starting a COLD stove, the taller chimney will have a much bigger pressure differential and will draw better than the shorter one.
That makes sense. It seems the short chimneys are really only a giant PITA when starting the fire, so it lines up with your findings.
"The opening in the chimney through which smoke passes is called the flue. To provide adequate room for smoke passage and draft development, flues must be carefully sized in relation to stove capacity and chimney height. In general, flue size should be 25 percent larger than the size of the stove pipe, which connects the stove to the chimney. This means a stove with a 6-inch diameter pipe would require at least an 8-inch flue; an 8-inch stove pipe requires a 10-inch flue, etc." from: http://extension.missouri.edu/p/G1732
Strange I thought it has to have the same size!
Those rules only apply to brick and mortar etc, steel liners and Class A steel chimney you want the same size as what's on the appliance. Or at least that's what I've heard.
My stove has a 6" opening, to which is fastened 6" single-wall stove pipe, going up to a 6" i.d. double-wall stainless chimney, per manufacturer's recommendation. I thought that the "flue" was the part that connected the stove to the chimney, but apparently "stove pipe" connects the stove to the chimney, and the "flue" is the piping inside of the outer stainless chimney. Interesting...
That's correct. Smoke travels up the chimney in a circular fashion so a portion of the 8x8 flue is wasted. And the tile is thick, so the actual interior cross-section is smaller. An 8x8" OD tile will be about 6.5"x 6.5" ID. For a metal chimney the pipe size should match the stove's flue size in general. Though sometimes if the flue is extra tall it helps to drop down a size as a way to control strong draft.
The woodheat website is a treasure trove for good information on chimneys:
Interesting, so a chimney that is 2 times as tall will have roughly twice the pressure difference in draft.
The temperature effect is more complicated but basically the bigger the temp difference, the more the draft.
Sorry I just couldn't resist this one. So a bigger stove is better than a small one? Kinda contrary to the whole EPA stove concept isn't it? :cheese: