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Post by dyanna63 on Oct 14, 2013 16:11:05 GMT -8
I am wondering if anybody has played around with the optimal length of the exhaust for a 6 inch system. Assuming the stove is built properly to all the correct dimensions, what would be the optimal length of the exhaust including horizontal and vertical pipe? Also, what is the lowest dept. that should be achieved at the end of the exhaust to reach maximum efficienty?
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Post by birchcreek7 on Oct 15, 2013 20:08:22 GMT -8
forty feet with 6" fifty feet with 8" elbows count for 5 ft. I heard this at a workshop with Ernie and Erica. I just built my stove with 8" flu and 25' of flu. because of space confines. It rockets like crazy and the vertical flu going through the roof is 140 to 160 degrees. Better than the 300 or so from our old wood stove but I am wondering if there is any thing that I can do to harvest more heat from my horizontal runs.
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Post by dyanna63 on Oct 16, 2013 22:11:02 GMT -8
I just did a test tonight,, I wanted to see the temps inside the pipe so I drilled a bunch of holes and used a digital meat thermoter. Not exactly scientific but what the heck.. Anyway my finding were this: I tested after a 1/2 hour burn, a 2 hour burn and a 3 hour burn. I tested at 3 feet from the manifold, 11 feet, 17, 26, 33 and 38 feet. These are the results in order: Starting cold at 3 feet from manifold it was 73F. after 1/2 hour burn 185F, 2 hour burn 253 and 3 hour burn 262F. At 11 ft. the numbers were 139-167-177F. At 17 feet 115-136-143F. at 26 feet 98-107-109F. at 33 feet 88-95-95F. at 38 feet 82-88-88F. The manifold clean out at 2 hours was 277F and at 3 hours was 313. Keep in mind it took a lot of fire to get my mass heated up so the first couple hours were slowly heating the mass. Also notice that the last sections, at 33 feet and 38 feet there was no increase in heat But these sections are not cobbed yet. So my thoughts are that at my 26 foot point I am gaining no more heat storage so that is a good place to go vertical.
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Post by dyanna63 on Oct 16, 2013 22:22:35 GMT -8
birchcree7 You may have answered my big question, I think that I have too much horizontal and potentially too much mass to heat up and that is slowing down my draw. My thoughts are that if I shorten my horizontal then the mass I have should heat up quicker because the fire will burn hotter and faster therefore burning cleaner?? Trying to answer all these questions in my head. As it is right now my exhaust is way too cool. I think it is slowing down everything.
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Post by matthewwalker on Oct 17, 2013 8:20:52 GMT -8
Dyanna, I'm not sure how much of that flue is cobbed, but there is a large performance difference between bare flue and cobbed in my experience. The cob flue will initially put a large load on the system as it drags heat from the exhaust, but it will start to warm up and your end temps will slowly climb over time. Bare flue will just continually radiate the heat, so in my opinion you can go a little longer if the system is cobbed. It just might be a little finicky starting from a dead cold mass, but for me that only happens once a year.
If your bench is still wet, performance will improve as it dries.
Birch, one way to keep more heat in the house is to add a small bell at the end of the system. It can be a metal radiator or a masonry bell. This will not only keep some of the heat inside that is currently going out the flue, but will slow down the loss of heat up the chimney when the stove is not burning.
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Post by birchcreek7 on Oct 18, 2013 13:10:30 GMT -8
Mathew, After doing a bit of research on this forum and the interwebs at large I am not sure how the bell would incorporate itself into our system. Could you give a sketch or picture of a bell at the end of the system. the radiator is intriguing as well, still not sure about the interfaces there. One thing I neglected to say and you did answer it a bit in your reply to dyana and that is our stove is only about 50% covered in cob so I am figuring that the fully encased system will draw more heat out of the horizontal flu as well. I must say your youtube channel gave me a lot more helpful ideas and advice than a certain unnamed 8 hour DVD set I recently slogged through, oh and good luck and congrats at the woodstove decathlon!
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Post by matthewwalker on Oct 18, 2013 17:00:17 GMT -8
Thanks so much man, I can't tell you how much that means to me. I could go on, but I'm going to show some restraint here....ugh, that's really hard to do! Lol. Okay, on to the bell...
As for the bell at the end of a system, well, it can be as simple as an empty barrel with the flue entering low in the barrel, and the exhaust exiting slightly lower. The same configuration could be made out of bricks for a bell with some mass. There is a limit to how much heat you can pull out of the system and still save a bit for the stack to help things along, so it might take some experimenting, but I think it's a good way to harvest the most heat from a system. I'll see if I can get around to making a little sketch, but it's a dead simple thing once you get your head around it.
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Post by matthewwalker on Oct 19, 2013 10:18:21 GMT -8
Hey Birch, here's a quick sketch of what I had in mind.
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Post by birchcreek7 on Oct 20, 2013 19:29:07 GMT -8
Thanks a bunch Matthew, I've got pretty limited real estate for something like this, it would definitely cut down on bench space. I will move forward tomorrow getting all the thermal mass in place and see how that affects overall performance and if we're still losing a lot of heat I will consider my options. I was just reading another thread where you and others were talking about bells and you mentioned that you had built a number of metal barrel bells and at some point you wanted to build a brick bell, if I do add one to my system it will be brick or cob, I will let you know. Thanks again
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