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Old 11-12-2017, 09:32 AM   #1
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Woodstove Chimney - Out Window

(apologies if this is a repost, my first post seems to be invalid now?)

Hey Folks,

Hoping for a little advice on improving my wood-stove efficiency.

Long story short I was running a little 14" woodstove, vented out the window with 4" stovepipe. There is a 2' section coming out of the stove, 90degree elbow, 1' section going out the window, 90degree outside with 2 2' sections going straight up above the bus roof. This worked quite well, however the major drawback is that I could get a max of about 2.5 hours burn time before I'd have to reload it, or risk having to relight it. Also it would loose it's heat pretty quickly after going out.

I was given a better/larger woodstove which I have moved in and hooked up. It holds heat a lot longer with the firebricks, and I can increase the burn time by a couple hours and it holds coals much longer. However after a couple of -15(c) nights, I see there are some problems... I can't get it burning hot enough. It stalls out at the back where unburnt material piles up. I have to constantly move stuff from the back to the front. I also get a lot of blowblack and now I seem to be getting a lot of creosote build up, it's dripping off the rain cap. I have some very nicely cured Ironwood which I can't seem to get going well at all now. Seems like I need to increase the air draw.

The new stove has taller legs and uses 6" stovepipe so now I only have a 1' section coming out of the top which connects to the 90degree, then connects to a 6" to 4" reducer, which then connects to the existing 4" pipe going out the window, and the existing oustide setup of a 90degree and 4' stack. Sorry I don't have a photo of this.

The big difference from the old setup being, 1' out of the top of the stove instead of 2'. And then using a reducer to go from 6" down the 4".

Seems like there are a couple things happening which have slowed down the air flow. Hoping to make a quick change so I can still use it for winter, but the cold weather had already arrived. I'm thinking if I remove the reducer and repipe the whole thing with 6", that might help? I'm still also concerned that my problem could be going down to 1' before I make that first 90 degree turn?

I'm too scared to cut a hole in the roof, although I realize a straight vertical stack would be my ideal.

(old stove)




(new stove)
- taller stove = reduced height before elbow, 1' instead of 2'
- 6" exhaust reduced to 4" after the elbow instead of 4" all the way.

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Old 11-12-2017, 09:51 AM   #2
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The two 90 degree bends are hurting you. If you can take the pipe straight up, and clear the roof by a couple of feet it will provide a better draft that you can control with a damper.

The woodstove we have in the house burns extremely cleanly, and for a long time on a full load. But it has a straight pipe of about 15 foot.
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Old 11-12-2017, 10:07 AM   #3
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This question always comes up around this time of year. You're not the first to have a short, elbowed chimney that doesn't perform well. There's really only one solution for such a short chimney: straighten it out.
I'd also upgrade to a 6" chimney all the way up for that stove. 6" is a common and affordable size for chimneys.

Grab yourself a flexible, high-heat flashing piece and cut a roof hole. You'll be glad you did.

I used one of these:
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Old 11-12-2017, 10:16 AM   #4
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Here's the manual for the Waterford 104 Mk II wood stove: http://woodburningdocuments.com/wp-c...ove-Manual.pdf

It appears to have a 5" exhaust, but when purchased new they supply a 5" to 6" adapter. I suppose either size would be appropriate, but 6" chimney pipe will likely be easier to find.
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Old 11-12-2017, 10:18 AM   #5
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I definitely see the advantage of going to 6" pipe just for the simple reason of being able to buy replacement pieces everywhere. The 4" pipes and elbows are very tough to find. Although I saw in an old thread here someone was saying they had best results going down to a smaller 5" pipe. But every install is different...

Cutting a hole in my roof is scary, but seems like what most end up doing for best results. I guess I'd really have to commit to my stove location. Not even sure where to begin, but I suspect I can find a post somewhere here about cutting the hole. I think technically it's two holes, I believe there is a roof cavity?

I'm tempted to just switch back to the small stove for this winter and get used to short naps...
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Old 11-12-2017, 10:21 AM   #6
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Quote:
Originally Posted by jazty View Post
Here's the manual for the Waterford 104 Mk II wood stove: http://woodburningdocuments.com/wp-c...ove-Manual.pdf

It appears to have a 5" exhaust, but when purchased new they supply a 5" to 6" adapter. I suppose either size would be appropriate, but 6" chimney pipe will likely be easier to find.
Thanks, for some reason I've been unable to find it.

Yeah it does have that adapter on the top of the stove. Dang, so I guess technically I'm going from 5", to 6", then back down to 4". I suppose that's probably not great...

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Old 11-12-2017, 10:31 AM   #7
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Quote:
Originally Posted by CyberHippie View Post
Cutting a hole in my roof is scary, but seems like what most end up doing for best results. I guess I'd really have to commit to my stove location. Not even sure where to begin, but I suspect I can find a post somewhere here about cutting the hole. I think technically it's two holes, I believe there is a roof cavity?
I say just go for it! It takes no time at all and there's so little you can screw up.

As you mentioned, there are two layers of sheet steel you'll need to go through with a cavity between. I figured out where the chimney would go through from the inside, marked the spot with a permanent marker then drilled through the whole roof. This way you can easily see the center from top and bottom. I then drew a rough circle about 1/2" bigger than the chimney and grabbed the angle grinder. The hole on the outside doesn't have to be pretty. It'll be covered by flashing. You'll probably want to take your time with the interior hole.

After setting the chimney up I used a handful of little L-brackets to screw the chimney to the roof. They are, of course, all set at a 90 degree angle so they need to be slightly bent in or out to properly fit the shape of the roof.

The flashing was sized big enough to fit over the whole mess. Follow the installation instructions and you'll be ready to go.

Quote:
Originally Posted by CyberHippie View Post
I'm tempted to just switch back to the small stove for this winter and get used to short naps...
I'm tellin' ya, you'll be so glad once it's installed right! 2.5 hours at a time is a tough way to sleep..
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Old 11-12-2017, 10:58 AM   #8
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Awesome tips, thanks! I worry most about screwing up a non-leaking roof. But I guess that's what flashing is for... For the support screws you put into the roof, did you put any type of sealant on the screw?
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Old 11-12-2017, 11:07 AM   #9
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Steps for cutting that hole nicely:

You need a fairly big, low RPM drill like a Milwaukee hole hog. You need an appropriately sized hole saw with piloted arbor, maybe an extra 1/4" drill bit to make a long pilot, a 1/4" drill long enough to go through both layers of roof, cutting oil, possibly a set of tin snips and a plum Bob.

First get your bus nice and level.

Use the plum Bob to mark the center of your chimney hole on the interior of the roof. Make sure the chimney isn't going through a roof rib or anything else important. You should be able to angle it a bit if needed.

Drill a 1/4" hole as vertical as possible through that center point and go through both layers of roof.

If the roof is curved where you are cutting, a long pilot bit in your arbor will help keep you centered.

Use oil and light pressure when drilling your roof hole. If the saw teeth grab too much the torque will try to rip the drill out of your hands. This isn't going to be fast or easy.

If the curve of the roof won't let the hole saw cut all the way through, bend out the piece blocking it and trim it away with the tin snips, then continue.

If drilling from inside is too difficult, you could try from the top after you've drilled your pilot hole but inside might be safer (no risk of being thrown off the bus).

Cheap arbors and hole saws are never a bargain when cutting metal. Get a good beefy arbor that won't strip out or round over and get a bi-metal hole saw. Don't attempt this without at least a heavy duty 1/2" chuck low RPM drill. A Milwaukee Magnum Hole Shooter is probably too small. If you don't own one, borrow or rent.
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Old 11-12-2017, 11:08 AM   #10
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Quote:
Originally Posted by CyberHippie View Post
Awesome tips, thanks! I worry most about screwing up a non-leaking roof. But I guess that's what flashing is for... For the support screws you put into the roof, did you put any type of sealant on the screw?
No need, since all of those screws will be under the flashing.
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Old 11-12-2017, 11:13 AM   #11
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You need an appropriately sized hole saw with piloted arbor,
Yeah, this is how I intended on doing the hole until I looked at the price for a big hole saw. $70+ dollars.. For potentially one use... Maybe renting is better?

Another option is a jigsaw with sheet metal blades. I've had good luck with those. A piece of clothe under the base plate keeps the metal from getting scuffed.
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Old 11-12-2017, 11:34 AM   #12
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I know school buses are not exactly air tight, but is it possible that you are not getting enough make up air ?, you could try to fire it with one of the windows open to see if this is th problem
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Old 11-12-2017, 12:03 PM   #13
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You could try a used hole saw from eBay. I bought a bunch of hole saws at a garage sale and it had a few big ones. I used one of them for a job last year and managed about 3 1/2 cuts in steel before it dulled to the point of futility. It was pretty far gone before I started but I got the whole lot for $20.

If you can find a used hole saw with teeth that still look sharp and most of it's paint still on the blade, it should be good enough and can be sold later.

Worst case, over a few winter's that $70 will start to look pretty cheap.
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Old 11-12-2017, 12:43 PM   #14
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Thanks for the Tips Drop Out and Jatzy much appreciated

Quote:
Originally Posted by Kubla View Post
I know school buses are not exactly air tight, but is it possible that you are not getting enough make up air ?, you could try to fire it with one of the windows open to see if this is th problem
Yeah when the windows are open it does work a little better. It was working better when I was testing it late summer, but all the windows were open. I was hoping it would draw better once everything was shut down, but I guess that's not how it works. I think it would work better even if I had a little fan in front of the door.
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Old 11-12-2017, 01:02 PM   #15
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Quote:
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Thanks for the Tips Drop Out and Jatzy much appreciated



Yeah when the windows are open it does work a little better. It was working better when I was testing it late summer, but all the windows were open. I was hoping it would draw better once everything was shut down, but I guess that's not how it works. I think it would work better even if I had a little fan in front of the door.
Fit a vent in the sidewall, behind the stove.
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Old 11-18-2017, 01:34 PM   #16
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hi

we have a wood stove through our window and it works. We do have to clean the elbow and pipe a few times every winter. What we found out was when we used pipe that was not double wall outside it got stopped up alot. We have two sections of double wall and a t and wall bracket mount we used. It has been working just fine now but still need to clean a few times each winter. We bought the whole six inch setup from Menard's .com in stainless for under 3 hundred dollars. Hope you get it worked out.
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Old 11-18-2017, 01:51 PM   #17
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Interesting. I have a feeling that my outside pipes are a bit of a mess, as I haven't cleaned them at all in a couple years, which is probably why black stuff is oozing down from the rain cap. I was thinking maybe it could be stopped up, or even have a nest in there.

Before I give in and just go straight through the roof I'm going to try and rejigger a few things. Switching all my pipes to 6", and swapping my two 90degree elbows to 45degree elbows. I do have a section of double walled that I could potentially use. Maybe that makes sense to use outside so it doesn't cool as fast. The main problem I was thinking is that those pipes are much heavier and will require more bracing. Which I guess probably means putting screws into the roof... But then I also have to buy fewer pipes since I already have at least 6' of unused double wall, plus a nice rain cap.
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Old 11-18-2017, 01:59 PM   #18
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If one is going to use a holesaw make sure you use rigid cutting oil (or any oil} when cutting or they will become useless after even one cut. if you aren't used to working with a 1/2 inch drill and holesaw practice on something to get the feel for it before attempting your project. they grab and will snap your wrist quite easily.
No way was I putting a hole in my roof for the chimney pipe or using a window so I ran mine out the left rear where the upper flashing light once was.
Creosote is hard to avoidat ythe best of times so I just run my stove mostly wide open. Even so I'm getting drips of creeossote outside on the left rear window mostly. I think I'll just paint the back of the bus a dark brown to cover.

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Old 11-18-2017, 02:27 PM   #19
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Quote:
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Switching all my pipes to 6", and swapping my two 90degree elbows to 45degree elbows.
That's a good compromise. The 6" pipe and looser bends should help a lot.

I have a wood stove that uses 2x 60° elbows so that the chimney could be routed through an already existing hole in a mobile home roof. It draws pretty good/ok. I have a friend in a near identical venting situation with a similar sized wood stove, but with 2x 90° elbows instead and his vents terribly. He gets a blast of smoke indoors every single time he opens the stove door.

Quote:
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if you aren't used to working with a 1/2 inch drill and holesaw practice on something to get the feel for it before attempting your project. they grab and will snap your wrist quite easily.
I typically enable the drill slip clutch when cutting with hole saws. Especially at the beginning and when nearing the end. Once the hole is started they don't typically grab much until you pierce through the other side.
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Old 11-18-2017, 04:45 PM   #20
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"I typically enable the drill slip clutch when cutting with hole saws. Especially at the beginning and when nearing the end. Once the hole is started they don't typically grab much until you pierce through the other side."

Very wise jazty, thanks. Guess you have never been bitten by a holesaw. Takes me back to being a green apprentice electrician cutting 4" holes with a holesaw. There is a knack. Easy but firm pressure on the drill at all times and knowing when to let go of the trigger should the holesaw grab. Not for amateurs.

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