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Telescoping stovepipe as adjustable bypass in split barrel bench

 
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When I got my first RMH assembled enough to do a test fire I immediately encountered the reality of operating a low temp exhaust system in a marginal climate.  If your exhaust temps are around 150 F then your draft is not that strong.  If you live in a climate like mine, where winter temps can swing from the 20's to the 50's and include rain and wind, then a RMH can be pretty finicky and hard to start on mornings when temps are warmer and/ or the wind is blowing.  You can deal with this by priming the flu, but a bypass seems to be a more sure bet.  I realized this a bit late into the game, and after considering various options decided to try making my exhaust flu adjustable where it exits the split barrel bench bell.  Lift the pipe to the highest position as a bypass, so the hot gasses bypass the bench entirely, and then lower it once draft is primed.  I was wary of the idea of trying to slide a pipe up and down through cob - I suspect it would be challenging to maintain a good seal over time, and the pipe is going to expand and contract at a different rate than the cob.  So I used a telescoping stove pipe.

I cut the outer sleeve as shown below and bolted it to the barrel, the inner sleeve slides up and down through it.  It's functional, but still needs some tinkering to make it easier to use.  I'm able to raise the flu for starting, and then lower it once the RMH is drafting well. (I only need the bypass when the RMH is cold or when weather is windy and/or warmish).  One challenge I have is that the exhaust stove pipe is not straight, it jogs a bit through two 45 degree elbows between the bench and the ceiling, so it's hard to keep the upper part of the stove pipe stationary while I slide the adjustable sleeve up and down.  I fabricated some brackets to try to hold the pipe firm, but there are several angles and they did not come out quite right, I need to fabricate something adjustable so that I can dial in just the right amount of support and tension on the flu, not too much, not too little.  I'll be fiddling with this some more in the weeks to come, for now it works fine.

Not all telescoping stove pipe is created equal.  Duravent makes 2 models of telescoping stove pipe under the brand Durablack.  I found the one offered by Northern Tool & Equipment to be the best: https://www.northerntool.com/shop/tools/product_200447737_200447737?cm_mmc=Google-pla&utm_source=Google_PLA&utm_medium=Heaters%20%26%20Stoves%20%2B%20Fireplaces%20%3E%20Heater%20%26%20Stove%20%2B%20Fireplace%20Accessories&utm_campaign=DuraVent&utm_content=710700&gclid=Cj0KCQjw28T8BRDbARIsAEOMBcz_hwIUmxLmf_e99w6rTFc_scGV44nvhjH2TrkfBjA7v_X31yMRbPIaAuI7EALw_wcB&gclsrc=aw.ds  

There is another made by Duravent under the Durablack label that is much lower quality, the sleeve does not fit as tightly and I would bet it's subject to leaking.  I'm including labels of the two in the photo below.  

Would love to hear more about what other folks are doing for a bypass.  I'm surprised I don't see more posts and discussion on bypass ideas and designs on this forum, is this because most RMH users are in cold climates that don't require them?  I'm considering another post on RMHs in marginal climates...
IMG_4357.JPG
This shows the assembled bypass. The smaller duct on the lower right is a cleanout.
This shows the assembled bypass. The smaller duct on the lower right is a cleanout.
IMG_4349.JPG
I cut the outer sleeve to contour the barrel.
I cut the outer sleeve to contour the barrel.
IMG_4353.JPG
then flattened the tabs and attached them w/ sheet metal screws.
then flattened the tabs and attached them w/ sheet metal screws.
IMG_4363.JPG
Inlet from riser on right, exhaust duct lowered in operating position on left. Raising the exhaust duct to the top of the barrel bypasses the barrel.
Inlet from riser on right, exhaust duct lowered in operating position on left. Raising the exhaust duct to the top of the barrel bypasses the barrel.
IMG_4410.JPG
I covered the attachment and flue w/ ceramic fiber blanket for expansion. Coated the blanket w/ clay slip to minimize airborne fiberglass.
I covered the attachment and flue w/ ceramic fiber blanket for expansion. Coated the blanket w/ clay slip to minimize airborne fiberglass.
Bypass-telescoping-duct.jpg
I recommend the 6TBK-TL, the lower label, which is higher quality: NOT the 6DBK-48
I recommend the 6TBK-TL, the lower label, which is higher quality: NOT the 6DBK-48
 
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Location: Westbridge, BC, Canada
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Hi Mark,   In your 4th photo you say "exhaust duct lowered in operating position on left."  Looking at the photo, the pipe extends all the way to the floor leaving no room for the cooled gasses to escape into the vertical telescoping pipe - BTW which on permies is often referred to as a plunger tube. I'm assuming you forgot to raise the pipe up off the floor for the photo?

I was going to do this type of bypass as well for my build but found that an uninsulated pipe just allowed too much heat to escape out the chimney until I insulated it. Of course once insulated though, I lost the function of my bypass. Instead, now I just leave the pipe about 2 or so inches off the floor and use the cleanout closeby to act as a priming port if needed.  
 
Mark Dumont
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Hey Gerry:

Hard to tell in the photo, but the pipe is about 2 inches from the ground.  I took this photo during construction, before the bypass was operable, so I was just guessing at the placement.  Since using it I;ve kept it between 5" and 2" from the floor, when not in the full bypass mode (all the way raised).  I find once the flu has heated up it's difficult, if not impossible, to adjust the flue due to expansion, so I do my adjustments fairly soon after the stove is primed and leave it.

Clarification question for you -

I was going to do this type of bypass as well for my build but found that an uninsulated pipe just allowed too much heat to escape out the chimney until I insulated it. Of course once insulated though, I lost the function of my bypass. Instead, now I just leave the pipe about 2 or so inches off the floor and use the cleanout closeby to act as a priming port if needed.  



I would assume the reverse - that an uninsulated pipe would disperse more heat to the room and less up the chimney, unless you are referring to surrounding the pipe in mass?  

Good to know that you leave it 2" from the floor.  How often do you need to prime it?  Is priming simply a matter of burning a little bit of newspaper in the flu, or is it more elaborate?  Does the priming create much ash over the long term?  Where are you located?  I'm just curious about the varying needs for bypass and priming by climate.  

 
Gerry Parent
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Location: Westbridge, BC, Canada
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When I re-look at that photo again, I can see that its an optical illusion making it appear the pipe is on the floor. Whoops!

I can see your confusion about my description. When I said that my vertical pipe was insulated, I just meant the part that is within the bell, not the whole pipe outside of it. When I left it uninsulated, it was acting very similar to how placing the vertical exhaust pipe close to the barrel assists in heating up the pipe to help increase draft (which I didn't want).

I only use the priming port function when the mass is stone cold or during warm conditions like you mentioned. From an old metal pickle jar lid and a coat hanger to act as a handle, I made myself a little burning tray to reach under the exhaust opening through the cleanout port. This way my hand/arm doesn't get dirty from having to reach in there and also a way to remove the ash that paper/cardboard/birch bark etc. tends to make lots of for this purpose.

I am in southern BC Canada in zone 4b.  Generally, anything above 40F outside could mean I need to use the priming port which is generally only during our shoulder seasons.
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