Attached is a moderately pathetic drawing of the potential design of the ductwork in the mass of my heater that as yet is unstarted. Each long section represents a 5 ft section of duct. I am wrestling with a few issues in my head (As I always do) about what I've read and how I'm interpreting it. In other words, I've read on here that an 8" rocket can push gasses through about 50 foot of horizontal pipe. What I need to do is go through a mass, then come back up by the barrel, and then vertical for around 15 feet out the roof. It is my understanding that the 15 vertical feet do not count towards this 50 ft number. Is this correct? On my drawing on the left, it doesnt have as much mass or ducting so it will let gas leave hotter then the one on the right, but would it have a better draft? On the drawing on the right, does the elbow at the end turning upward also count towards the 50 foot? In other words, would 50ish foot of pipe PLUS the chimney be too much, or just right?
I just want to make sure that I am not misunderstanding the concept and mess up. Thanks for your answers and lack of laughter.
Location: Canton, NC
posted 5 years ago
Just to be clear, when I say chimney, it is not a standard brick and mortar, it will be a vertical section of round ducting just like the mass ducting. I'm just making a verbal distinction between the horizontal and vertical sections.
Matt Sorrels : Looks O.K., I do not count the last 90º turn from horizontal to vertical -which should be a 'T' with a clean out ! The sketch on the left a simple "U''
I counted as 30 feet of horizontal and the one on the right i am call ing a Double ''L'' is 50 feet.
On the double L where you make your 180º turn a short length of pipe evens short as 8'' will make that flow a little easier, consider too that there is a good
location for a clean out! That short length of pipe will make adding cob, and urbanite and cob, easier and the more dense rock and stone you put in the Thermal
Mass the Less Cob you will have to make.
When you bring your pipe back nearby the barrel it will pick up additional heat and help provide a steady draft !
You are saying that from near the floor of your living space to the top of your final vertical chimney will be 15', as long as it Tops the peak of the structures
roof by 4' - 5' so you should be good !
For the good of the craft ! Big Al
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For an 8-inch system size, start with your 50-ft, subtract the length of piping, as you have. But then each elbow subtracts 5-feet from that original 50; that's minus 10-feet for a 180-degree turn (two 90-degree elbows).
Your vertical rise not only does not count against you (sorry for all the negatives! heheh) some folks calculate it adds, but I don't recall about that really. Try finding some of Ernie Wisner's posts, where he talks about this. He rattles these rules of thumb right off the top of his head, he's done it so often.
But really, I think you are good with either layout.
With regard to the chimney, the point to be taken (other than fire safety, and construction details in making the through-ways) is to get the proper height. Best is a minimum of 2-feet higher than your highest roof ridge; reasonable, and for most areas I've seen, also by code, is to find the highest roof line, or other substantial object, within a 10-foot radius of your chimney, and make the chimney height a minimum of 2-feet taller; in other words, the top opening of the chimney ought to be at least two feet taller than anything within 10 feet of it.
And put a rain cap on too. You can get directional one's for like $50, or you can make a low pressure one out of three T's, forming an H-like shape. All of this assumes you do not have something unusual, or a giant tree blocking all the wind there, or something strange going on in the vicinity of your proposed chimney. If you have any doubts, pay a chimney sweep or other fireplace/chimney expert to look at your site.
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