Hunting Trailer Mini Stove

[bholler]

You also have to keep in mind what UL testing is for. It's for a worst case scenario where some unknowing homeowner installs the stove with absolute minimum heat shield up against a cedar shake wall with fur balls and old newspaper accumulating behind it.

My installation is unique and I know that my heat shield will far outperform a chunk of tin somebody finds at the scrapyard (I've seen installs like that). This is 1/16" thick polished aluminum spaced exactly 1" out with non combustible minimum contact spacers.

UL gives a heavy margin of safety for the minimum install. This stove and trailer will never be sold or used by anyone other than me.

They test for worst case scenario because it happens. We have all lit the stove then got distracted forgetting to shut it back at one point or another. The thickness of the sheet metal doesn't matter much. The fact that it is polished probably helps a little but not much.

I am just trying to give you advice to keep you safe based upon years of seeing things done wrong and what can happen in that event

 

[Blitzkrieg]

The air gap around the insulated chimney will definitely be done dont worry. And I'm not taking anything near any limits. It's just at some point these safety margins are getting ridiculous...

Someone says 117 over ambient, so I decide 100, then someone says no more than 170 and I say ok, then someone somewhere thinks they maybe heard it should be 80... where does it stop?

For what it's worth the stove is running right now at a flue temp of 400F and the insulated chimney is 150F at its hottest point. But again like I said I will be spacing it off any combustibles first thing. Especially since the overfire tests are likely to raise that reading.

 

[Blitzkrieg]

Bholler, I can guarantee you that the polish on the aluminum makes a huge difference. During my first tests with the stove in my garage I had that same aluminum behind the stove and a sheet of 18 gauge steel (satin finish) beside it. The steel was quite warm to the touch but the aluminum is always stone cold.

You're probably right about the thickness not having anything to do with heat transfer but it decreases the chances of it getting deformed and reducing the air gap behind it.

 

[Blitzkrieg]

So back to my original question, does anyone think that lining the 4" flue with a 3" double wall SS would allow me to run the stove cooler while maintaining high flue temps and avoiding smoke?

I tried removing the grate and all it did was slow down the ramp up time. From what I've learned about this stove it likes to be ramped up to a 350F flue temp before sealing the door and then I let the flue rise to 400F before closing the intakes down to almost closed. Then it rolls away happily with no smoke and slowly decreasing flue temp as the wood is consumed.

 

[bholler]

So back to my original question, does anyone think that lining the 4" flue with a 3" double wall SS would allow me to run the stove cooler while maintaining high flue temps and avoiding smoke?

I tried removing the grate and all it did was slow down the ramp up time. From what I've learned about this stove it likes to be ramped up to a 350F flue temp before sealing the door and then I let the flue rise to 400F before closing the intakes down to almost closed. Then it rolls away happily with no smoke and slowly decreasing flue temp as the wood is consumed.

We didn't design the stove. So honestly we have no clue what the vent diameter should be. Going smaller could cool it down by reducing the volume or the corresponding increase in velocity could make it run hotter. There is no way for us to know.

When I said 80 I meant 80 degree rise over ambient temp. I think that is the ul limit. But I didn't look it up begreen could be right at 90. And those temp rises are in a worst case scenario over fire not normal operation.

The temp of the heat sheild doesn't matter. The temp of the wall behind it is what matters. And yes it can be warmer than the heat sheild. The difference in temp you felt probably had allot more to do with the thermal properties of the 2 metals than the finish.

 

[Blitzkrieg]

Alright then, 90 over ambient, not exceeding 170 it is. And yes as an absolute maximum with an overfire. As it stands right now with the stove top at 400F the temp of the wall behind the side shield (I have access to measure the backside) is sitting at 71 with ambient at 85. That's an interior wall panel so it's not cooling from the outside air.

 

[Blitzkrieg]

It appears that the draft pressure created by the stack effect is solely related to temperature differential and stack height. Obviously a larger flue diameter will flow more as there is more area for that pressure to act upon. What I need (or what I assume I need) is a stronger draft pressure at lower stove temps. In theory reducing the flue diameter should create this effect. The same amount of fuel consumed will heat the flue hotter because of reduced surface area, creating a higher pressure at reduced flow. The only question remaining is if that reduced flow (both exhaust and intake) will provide adequate air for clean combustion and enough pull to keep the smoke out of the room.

I was just hoping that there was some golden rule that everyone here knew about flue diameter vs stove operating temps. I guess you just normally go with the manufacturers flue collar size.

I think I'll test out my theory this weekend.

 

[bholler]

It appears that the draft pressure created by the stack effect is solely related to temperature differential and stack height. Obviously a larger flue diameter will flow more as there is more area for that pressure to act upon. What I need (or what I assume I need) is a stronger draft pressure at lower stove temps. In theory reducing the flue diameter should create this effect. The same amount of fuel consumed will heat the flue hotter because of reduced surface area, creating a higher pressure at reduced flow. The only question remaining is if that reduced flow (both exhaust and intake) will provide adequate air for clean combustion and enough pull to keep the smoke out of the room.

I was just hoping that there was some golden rule that everyone here knew about flue diameter vs stove operating temps. I guess you just normally go with the manufacturers flue collar size.

I think I'll test out my theory this weekend.

Your theory about reducing the flue size could be absolutely correct. Or the reduced volume could make it impossible to run the stove efficiently. We just don't know. There is a reason teams of engineers are used to design stoves using complicated computer modeling. There are tons of variables that go into it.

 

[Blitzkrieg]

Well I for one am willing to put the time required in to find out. The only other real option I had to get a wood burner in this trailer was to buy a cubic mini and all their accessories for a safe (according to them) install. This would have cost well over $1000 and even then the system would be unlisted with no true guarantee of safety. I'll let you guys know how it works out.

 

[electrathon]

My suggestion to you would have been before you started was to reverse engineer off of a mini stove. Use what they are doing and go from that starting point. If you are looking to make a better wheel it is best to look at other wheels as a start point.

 

[Blitzkrieg]

Yeah I would have loved to have access to one for reverse engineering before hand but I dont know anyone who has one. Also my goal was never really to make a better stove than the cubic mini as it is a fantastic product from what people say about it.

My design was limited by certain dimensions beyond my control such as fire bricks, affordable flue pipe, the max width of material for my hydraulic press brake.

I have got the materials to test out the 3" flue theory so I'm gonna give it a shot this weekend.

 

[begreen]

Bholler, I can guarantee you that the polish on the aluminum makes a huge difference. During my first tests with the stove in my garage I had that same aluminum behind the stove and a sheet of 18 gauge steel (satin finish) beside it. The steel was quite warm to the touch but the aluminum is always stone cold.

You're probably right about the thickness not having anything to do with heat transfer but it decreases the chances of it getting deformed and reducing the air gap behind it.

Right now you are flying blind. The short chimney may not be drafting strong enough to pull a decent flow of air through the secondaries. Figure out how to put a window on the stove so that you can observe secondary burning, even if it's just a porthole.

PS: There are a few good threads in the Classics forum about converting old stoves to secondary burners. They have been commented on by some resident experts. I'd recommend visiting them for tips and lessons learned.

 

[Blitzkrieg]

A window is definitely on my list of upgrades. The ceramic glass is just so expensive I wanted to make sure that my door wasn't going to warp or something before I cut the hole for it.

 

[begreen]

Doesn't have to be a large window. The Elm stove used a pyrex pie plate.

 

[Blitzkrieg]

When I put a window in it will be ceramic glass, tempered glass can be a dangerous thing on a stove that isn't specifically designed for it. I wouldn't want the window to shatter into a million pieces leaving a gaping hole in front of a raging fire.

 

[Blitzkrieg]

I tried out the 3" flue yesterday and it was hit and miss. The stove did appear to be more controllable, I could set the air intakes at a range of settings instead of simply wide open until good flue temps then down to minimum. The ambient temperature also climbed much slower in the trailer and the exterior of the 4" outer flue and 6" insulated chimney were far cooler as expected.

The down side is that when starting the fire smoke seeped out of the secondary air intake (minimally when closed but still noticable). After the fire got going the secondary intake didn't smoke anymore but with the door wide open the occasional tuft of smoke would enter the room.

I believe that the increased draft pressure due to higher flue temps is what made the stove more controllable and the decreased flow is what caused the smoke issues. So my next step will be to go back to the 4" flue and add my last 2' section of pipe on top of the chimney to try and get the best of both worlds (more draft with the same flow). The uninsulated 4" section above the chimney will surely lose some heat, dropping average flue temps but hopefully a 33% increase in overall length will overcome that.

If the taller flue works out I may make an insulated chimney extension out of the extra 2' flue section and just place it on top when I arrive at my hunting location.

 

[bholler]

When I put a window in it will be ceramic glass, tempered glass can be a dangerous thing on a stove that isn't specifically designed for it. I wouldn't want the window to shatter into a million pieces leaving a gaping hole in front of a raging fire.

Pyrex as suggested is ceramic glass.

 

[Blitzkrieg]

Oh, that's good to know! Thanks for the tip.

 

[Blitzkrieg]

Just did a little research on glass and woah there's a lot to learn... Apparently Pyrex comes in two varieties, older Pyrex (marked PYREX) is borosilicate glass and basically laboratory grade glass but the new stuff (marked pyrex with a blue green tint) is soda lime glass and nowhere near as good with thermal shock. They're not the same as pyroceram or neoceram from what I can tell but I think if I can find an older borosilicate PYREX dish at the thrift shop I should be in business.

 

[begreen]

Just did a little research on glass and woah there's a lot to learn... Apparently Pyrex comes in two varieties, older Pyrex (marked PYREX) is borosilicate glass and basically laboratory grade glass but the new stuff (marked pyrex with a blue green tint) is soda lime glass and nowhere near as good with thermal shock. They're not the same as pyroceram or neoceram from what I can tell but I think if I can find an older borosilicate PYREX dish at the thrift shop I should be in business.

Call a good glass shop and ask if they sell ceramic glass (pyroceram, robax, neoceram) and see what a small piece ( 6"x6") would cost. Maybe they have a scrap end from a sheet that would work if you are flexible in size. It might be simpler to fabricate a proper mount and seal for a square window.

Here are some pictures of Elm stoves that show the pyrex window and details.
(broken link removed to https://benchman.smugmug.com/Business/Elm-Wood-Stoves-and-Slatter/)

 

[Blitzkrieg]

I had always planned on ordering neoceram glass eventually but if I could find a small square borosilicate pyrex dish I think it would look real cute on the stove. Fabrication of an odd shaped retention system isn't a problem for me, I can build just about anything imaginable out of steel.

Going to test out the taller 4" flue later today once I've hung enough drywall to satisfy the wife lol. I'll report back my findings.

 

[Blitzkrieg]

Adding the extra 2 feet the the chimney worked perfectly. Ramp up time from lighting was reduced dramatically and I was able to shut down the air intakes much sooner without smothering the fire.

I installed a probe style flue thermometer (actually a bbq thermometer) and it is way easier to see what is happening with the flue temps now. I can see almost instant reaction on the thermometer when I make changes to the controls. Max flue gas temps reached 700F before I closed the door and it continued to climb so I closed down the air.

Tomorrow I will see if I can light the fire earlier in the day and try and maintain a long low burn for as long as I can. Temperatures of combustible materials are 135F max at the hottest places I can find around the stove, this is while ambient is 95-100F depending where in the trailer I measure.

Thanks for all your help and concern guys!

 

[SpaceBus]

Call a good glass shop and ask if they sell ceramic glass (pyroceram, robax, neoceram) and see what a small piece ( 6"x6") would cost. Maybe they have a scrap end from a sheet that would work if you are flexible in size. It might be simpler to fabricate a proper mount and seal for a square window.

Here are some pictures of Elm stoves that show the pyrex window and details.
(broken link removed to https://benchman.smugmug.com/Business/Elm-Wood-Stoves-and-Slatter/)

Are those still made?

 

[begreen]

Are those still made?

I think Vermont Stoves stopped making them around 1989, but we did have some folks getting them direct after that. wkpoor had one and Bushman bought a new one around 2009? or 2010. Might have been making them one off, but prolly not now. They still do rebuilds and refurbishing last time I checked.

 

[Mech e]

Alright then, 90 over ambient, not exceeding 170 it is. And yes as an absolute maximum with an overfire. As it stands right now with the stove top at 400F the temp of the wall behind the side shield (I have access to measure the backside) is sitting at 71 with ambient at 85. That's an interior wall panel so it's not cooling from the outside air.

If it is an interior wall and ambient is 85, your wall temp must be at least 85 as well to have reached steady state. You need to run the stove at high temp for a much longer period of time to get an accurate measurement of the wall temp.