Newbie wondering about system length.

[Fire_n_Stories]

Hello all,
I am a newbie woodburner, as a result of having just purchased a 430sq ft cabin at about 8,800 ft in Colorado. I was given a Trailblazer with a 6in flue, and am struggling to put together what I read was the minimum system length of 15 ft. I am guessing that this minimum is related to the hot air having enough time to cool before exiting the chimney top, but am not sure. My cabin has 7ft ceilings at the highest, and about 5.5ft at the slope drop against the wall.

I just purchased all the parts for the system, including a 4ft chimney, but that puts the length at only about 11ft long, from the top of the stove, and maybe 13ft from the floor.. Buying a chimney that was longer seemed sorta ridiculous, but maybe I was just being nutty about it? Any advice or explanation of why this length is so important/necessary would be much appreciated.

 

[Niko]

Its all about draft for the stove. From what i have learned on the forum myself. Not sure but i think higher elevation works against you so a taller chimney would be needed. if the instructions are confusing i would suggets checking out you tube videos of installs. If its not your cup of tea then get a buddy to help or a pro to do it.

 

[Fire_n_Stories]

Its all about draft for the stove. From what i have learned on the forum myself. Not sure but i think higher elevation works against you so a taller chimney would be needed. if the instructions are confusing i would suggets checking out you tube videos of installs. If its not your cup of tea then get a buddy to help or a pro to do it.

Actually, I think I remember reading that. So do you think that I need to go longer than 15? I do want to be safe...it's a high fire area, and the last thing I want is to cause a disaster.

And my buddy is putting it together for me. I think I'm pretty square on construction, we just couldn't figure out why the 15' requirement. But it's all coming back to me now.

Thanks!

 

[JA600L]

It is mostly draft related. You also don't want to smoke out your cabin everytime you open the stove door. The fire may be sluggish and hard to get it cruising. Not many people complain about having too much draft. 15ft is already at the minimum so you might as well at least get it to that point.

 

[Fastdonzi]

I don't think the point of the required flue is to give the gasses time to cool, it is so the heat going up the flue has time to keep pulling from the stove. I'm no Thermo-Dynamicist, But (in my mind) is seems like the pipe should be getting smaller the higher it goes, increasing the velocity ...it may have been tried before, If I had height limits I would try it... ( Don't try it on my accord ) maybe others can help

 

[JA600L]

I don't think the point of the required flue is to give the gasses time to cool, it is so the heat going up the flue has time to keep pulling from the stove. I'm no Thermo-Dynamicist, But (in my mind) is seems like the pipe should be getting smaller the higher it goes, increasing the velocity ...it may have been tried before, If I had height limits I would try it... ( Don't try it on my accord ) maybe others can help

It would increase the pressure of the stack and reduce the flow. Neither one would help. Otherwise, a Diesel truck would be putting smaller pipes on rather then bigger to increase performance.

 

[Fastdonzi]

But a diesel is pushing air through the pipe and restriction is bad , it may be the opposite of what I was thinking, maybe a diverging pipe is better. my thought is there is probably some way to get the most bang from a short stack...and me being me i'd try them.. I cant leave anything alone

 

[JA600L]

But a diesel is pushing air through the pipe and restriction is bad , it may be the opposite of what I was thinking, maybe a diverging pipe is better. my thought is there is probably some way to get the most bang from a short stack...and me being me i'd try them.. I cant leave anything alone

Anytime you add an orifice pressure increases and flow drops. Creosote buildup is a perfect example of why that would never work.


Your best bet is a well insulated chimney . Keeping the chimney warm causes the natural stack effect increasing draft.

 

[Fastdonzi]

I'm with you on that.. with my old stove a little creosote build up at the very top of the pipe Greatly affected draft.. After a fresh chimney cleaning when I open the doors it sounded like a rocket motor. With the build up- up top not so much.

 

[Isaiah53]

It would increase the pressure of the stack and reduce the flow. Neither one would help. Otherwise, a Diesel truck would be putting smaller pipes on rather then bigger to increase performance.

+1
Draft is what creates the gas flow or velocity, not the other way around. You cannot increase draft with higher velocities. Higher velocity will increase the pressure drop in the chimney and put more back pressure on the firebox (less draft), so you generally want a larger diameter flue - up to a point. If the diameter is too large, the gas cooling effects become greater, and the velocity can even get so low that cold outside air can come down the chimney at the same time hot flue is rising, and really cool down the flue. It is best to stay with the manufacturers recommended diameter and minimum height. The only way to really compensate for a short flue would be to run at higher stack temperatures.

 

[wildlingabovethebridge]

I don't think the point of the required flue is to give the gasses time to cool, it is so the heat going up the flue has time to keep pulling from the stove. I'm no Thermo-Dynamicist, But (in my mind) is seems like the pipe should be getting smaller the higher it goes, increasing the velocity ...it may have been tried before, If I had height limits I would try it... ( Don't try it on my accord ) maybe others can help

You are correct that convergent ducting would increase the velocity of the gas, but in return pressure will build upstream of the duct inlet. These principles are useful in engineering to accomplish certain effects (ie increase thrust in a jet engine, create a venturi in a carburetor) but not to vent your stove. If your idea was true a flue damper would have the opposite effect until fully closed. Props for thinking outside the box though.

In fluid dynamics terms a stove pipe is a motive-flow system, by where an initial energy input ( usually a Δ in pressure and/or temp, like a hot fire in a stove that emits the hot, rising gasses) starts a flow that perpetuates itself by creating it's own suction as long as material and energy inputs and outputs, and the volume of the system remain somewhat homeostatic. In such a system the larger the inertial mass involved the stronger the pull.

IOW, the more material you get moving through a defined volume, the harder it is to stop it. That's why a longer stovepipe provides a stronger draft.

 

[Isaiah53]

This is a technicality, but it has nothing to do with momentum or inertia. Draft is simply the differential pressure that exists between the outside air source and the firebox. It is what drives the air flow into the firebox (pressure difference is what drives fluid flow). The pressure of any fluid (including air) increases as you go deeper due to the weight of the fluid above. That is why submarines are limited to certain depths - the pressure exerted by the water becomes too great for the hull. The relative increase in pressure is simply the density of the fluid times the height of the fluid above it. At the exit of the chimney, the pressure of the outside air and the flue gas are equal to one another. Consider what happens when you move down from that chimney exit reference point, both along the outside air and the chimney path. The pressure of the air side and the flue gas in the chimney will both increase, but because the flue gas is hotter, its density is less and therefore the pressure inside the chimney will become progressively less than the surrounding air the deeper you go down (i.e. chimney height). The difference in pressure at the stove top elevation will be your available draft. Therefore the taller the chimney, the greater the final differential pressure or draft. Larger differences between the air and the flue temperature will also increase the draft. Frictional pressure losses within the chimney reduce the available draft. Bends and other changes in direction increase this frictional loss. Pressure differences between the outside air and the inside of the house (i.e. negative pressure) will also act to reduce the available draft.

 

[JA600L]

We are off topic guys although interesting discussion!

 

[Oldman47]

It really is simple. The hot gasses in your flue are less dense than the cooler air on the outside of the flue. Every foot of hot gasses presents that much more buoyant effect to the flue gas. If you have a long enough and hot enough flue the gasses will flow properly and fresh air will enter the stove to support combustion. At high elevations the cool outside air is less dense than at sea level so it takes a longer flue gas pipe to develop the same amount of buoyant effect. The end result is that a 15 foot chimney requirement at sea level may well translate into 18 or more feet needed at 8800 feet.

 

[Wood Duck]

The longer the flue, the better draft, or pull on the air in the stove, it will have. You need good draft to suck air up the chimney, and as a result pull air into the stove. Poor draft sucks, and makes burning a lot harder. Get a long flue and strong draft. It is generally easier to use the air controls to decrease air flow with a strong draft than it is to find ways to compensate for a poor draft. As Oldman says, go with more than the 15 ft minimum because you're at high elevation.