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Problem with back burn  RSS feed

 
Posts: 11
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Hi guys,

I am also having a problem with back burn. I'm running an 8" system with a cast core and heat riser built off specs from this you tube.


I have about 33 feet of horizontal pipe with 3 90s in the system. with a 90 at the end then 4 ft of vertical pipe going out side from there.

Cob is mainly dry although the bench isn't done. As this is my first one I wanted to work out any kinks before closing it in with the bench.

I think the problem I have is that my pipe is going out on the windward side of the house. The system seems to run fine with no wind. If I get the slightest amount of wind coming by it will smoke back and blow the fire out. I have to exit on the windward side due to the fact, that's the side where the living area is.

My building is 30x40 cordwood masonry with a loft and a very open floor plan. The only interior walls are for the 14x6 bathroom/ laundry room. I hoping this system will heat the whole building.

My question is: I have a very tall ceiling, its 25' vaulted. Is that going to be too tall for the system to vent it out the roof? That's a lot of pipe to buy, Will that fix my problem?

Thanks Chris
 
Chris Williams
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Also should the vertical pipe be 8" the same as the system or smaller?

Thanks
Chris.
 
gardener
Posts: 1271
Location: latitude 47 N.W. montana zone 6A
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Chris; Once your pipe goes vertical it can be as tall as needed. Vertical pipe does not count against your 50' limit. You can also install it closer to wood framing than a normal stove because the temps will be 130 or so. As far as cost goes ,stick with 8", because of the low temp after passing thru the mass you can use hvac pipe (foil tape all joints) Hvac comes in 5' lengths at apx . $11 a stick. You can start using this within a few feet of your horizontal transition area. Use hvac everywhere except use black pipe to enter your transition area ,exit your mass and when you go out thru the roof jack. Going thru the roof is the the answer to your blow back problem. As far as heating an open floor plan and cathedral ceilings I suspect that you will be burning longer than if you had a lower roof, but as long as your insulated well should be fine. As far as your cast core... they work great , get hot instantly, however be very careful with adding wood to the feed tube as they are fragile. Keep some fire clay & sand handy to patch any wear spots... keep your feed tube 7.5 "x 7.5" it will want to grow larger if you force wood into it.
 
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You mention having only 4 feet of vertical pipe ? What is happening is the wind is pressurizing the entire side of your house, and the only place it can go is into your chimney . . .

With my rocket stove, I added 6 feet of pipe too clear the closest obstruction within 6 feet . . and put a cap on it . . .

Now whenever the wind blows, it acts like a vaccum, and the stove just rockets ! I actually look forward too a stormy day . .

Add enough pipe too clear the roof-line, and it should work for ya.

Edit : keep the pipe all the same size from one end to the other.
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gardener
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Location: Southern alps, on the French side of the french /italian border 5000ft high Southern alpine climate.
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Chris, you realy have to vent out above the roof peak. If it can help, you can make your own insulated chimney, with rockwool/roxul, and sheet metal helf by wire. If your 33ft of pipe are bare, i bet it condenses in there. So, insulating temporarily the horizontal pipe might even be needed for a moment, till you get the thing to draw properly. Then start your mass slowly, removing the next 5 ft of insulation, only after the mass you already build has completely dried etc.

If you are in a hurry, use a refrigerator fan (you know the fans on industrial cooling units for restaurants or supermarkets) They're usualy IP67. At the end of your chimney. That will get you by untill you have enough dried up mass around the pipe, and enough vertical chimney to get the thing working properly.
 
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Dave Lot wrote:
With my rocket stove, I added 6 feet of pipe too clear the closest obstruction within 6 feet . . and put a cap on it . . .

Now whenever the wind blows, it acts like a vaccum, and the stove just rockets ! I actually look forward too a stormy day . .



Which type of chimney cap did you use? As we also have a problem with blow back and a very gusty location, I tried to research suitable chimney caps last year, and just got confused. For any given model that somebody promoted as really preventing or reducing blowback, I found other people saying that model does nothing or worse than nothing.

Obviously you have to get your chimney above the highest point of the roof, but even with that we're getting blowback so which model of cap might help in a gusty location, at least most of the time?
 
Chris Williams
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Thanks for the info guys. I was able to add pipe out through the roof this weekend. I now have a total of 25ft of vertical pipe. That gives it about 3 ft above the highest point. I Also found a witch's hat type cap at a second hand store that seems to aid in suction. We had a windy day this weekend and the stove really roars when the wind is blowing.

This seems to have created a small issue. Now it seems things might be moving to fast through the system even on non windy days. Before my barrel would get so hot I couldn't get my hand within about 10 inches of the top and about 3 to 5 inches to the sides. Now I can touch the top of the barrel fora second before I have to move my hand, even after its been running for hours. However the fist section of pipe after the manifold seems to be the hottest part of the system. I can barely touch that sucker. Yes I know I should get one of those stick on thermometers.

Could this be Due to the suction added by the flue moving the air through the system to fast?

My barrel is about 3- 3.5 inches above the top of the heat riser, with about 3 inches clearance around the barrel between the heat riser.

Once again thanks for the help. I'm building this with out ever seeing one in person, only through the book and you tube. I did purchase the videos through the last kickstarter but they must still be in the works.

Chris
 
gardener
Posts: 2581
Location: Upstate NY, zone 5
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It does sound like you are now getting too much draft and excess air in your system. Have you tried a brick or two partially covering the top of the feed tube? You can adjust these to allow just as much or little air as you want, with the side benefit of speeding up the air velocity entering the feed tube.
 
Chris Williams
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Yes Glenn, I had to use a brick when it was vented through the wall, to get it to draft well. I haven't used the brick since we moved the vent to the ceiling because it draws so well now. If I restrict the feed tube opening will I get a hotter burn that will heat my barrel for a more radiant heat?
 
Chris Williams
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RMH before venting through roof.
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RMH before venting through ceiling
 
Satamax Antone
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Location: Southern alps, on the French side of the french /italian border 5000ft high Southern alpine climate.
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Chris, do you cook or boil water on the barrel? If not, it doesn't matter much. Things are moving faster, and may be the bench gets a bit more heat that way. If you need more fast heat, two solutions. Add a barrel on top of the existing one, or may be at the end of the bench, as a bell.

Realy what you need to know, is the end temperature. Anything less than 60C° is risky. But if you have let say 120C° at the end elbow going to the chimney. You could put an end bell there, you could even cob it.

You have an equivalent of 48ft of horizontal pipe. Prety much on the verge. May be you could add something for testing purposes. A bell, as i said. At your first cleanout, you could also do a dead end bell. Burned barrels are very good for testing bells. 1.86m² for each one of thoses The first thing to get, a thermometer, to monitor your end temperature.
 
Glenn Herbert
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Location: Upstate NY, zone 5
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Yes, if you have a lot more air going through the system than you need for full combustion, the extra will just dilute the heat and cool down the system. The brick will restrict the amount of air that gets in, and make your burn hotter.

I suspect that when you had poor draft, the brick's effect was mainly to speed up the entering air and "blow on the fire".
 
Dave Lot
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Rebecca Norman wrote:

Which type of chimney cap did you use? As we also have a problem with blow back and a very gusty location, I tried to research suitable chimney caps last year, and just got confused. For any given model that somebody promoted as really preventing or reducing blowback, I found other people saying that model does nothing or worse than nothing.



I am not sure what type of cap it is, but I picked it up at my local home hardware store in their wood stove section . . it does not cost much, so if it don't work for ya, your not out many $$ . . .

Let me know how it works for ya . . .
Can't really see it very good in the pic, - it has a rounded top, with 3 flat steel "legs" attaching it too the round pipe attachment ring . . nothing too fancy . .

P.S. - so if an airplane wing provides lift because it is rounded on the top, does this thing suck the air out of the stove because it's rounded on top ? hmmmmm......

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Dave Lot
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Since the chimney was interfering with the output of the solar panels, it had too be shortened.

Good news is, here is a better picture of the chimney cap.

I will let you know how the stove runs with the shorter pipe. .

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Chris Williams
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Been super busy lately. Hunting season hit and I was spending all my time getting meat.

I wanted to post an update. The heater is running great now and heats our house fairly well considering the size. Thanks for all the help..

Chris
 
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we had some problem at first with back "burning".  added about 10 feet of flue on the roof.  never a problem since.  works perfectly.
 
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