Gas kiln flue gas temperatures and draft

[hitchmss]

So as many of you know I am in the process of building my new studio (which is coming along; photos and detailed post to come at the end of it). I am going to move the 65 cu ft downdraft LPG car kiln from the current studio to the new one and I'm wanting to change the way the chimney exits the building. 

 The current chimney has a custom built roof jack which is a series of metal baffles, air space, IFB's, and kaolwool which protects the trusses it's installed in between and the roof sheathing/shingles. It does reduce the chimney from a roughly 12" square (inner dimension) chimney to an 8" round flue pipe, but we've never had any issues with reduced draft on this approx 18' tall chimney. 

Now, the new studio with have a metal roof, and short of making a new roof jack, which will closely follow the metal panel ridge profiles, and relying on caulk and other flashing methods to maintain watertightness, I am not coming up with a readily available, and cost effective way of make the transition through the roof. Because of this I am wanting to run my chimney through the side wall, by using an "offset" all fuel style chimney pipe kit. My plan is to carry the IFB chimney up to about 7-8' (2-3' short of the bottom of the trusses), then use a chimney pipe cap, and two elbows to shoot a 10-12" round chimney pipe through the wall, out past the soffit/gutter, and straight up about another 4-6' above the gutter, topped with a rain cap. 

Now I have two concerns which I'd like your advice on; chiefly, what kind of temps have you measured on your chimney flue gasses at different heights above the damper? Duravent who makes the chimney pipe, make a number of different models, some of which will handle continous temps of 1700*F, and peaks of 2,000* for 30 minutes. This assuredly would handle my temps, but I also don't want to spend more than I have to if this is overkill.

I8I know I can see the inside of my chimney from my damper opening, and towards the end of my firing I definitely have red-orange heat below, and a inch or so above the damper, but not sure how hot it is from there up? I could take an IR thermometer or thermocouple up to my chimney top and measure there, but still won't know how hot it would be 4-6-8-10' above my damper. I could drill some holes in my chimney and test with a thermocouple but don't want to make holes if I don't have to. Does anyone know what kind of temps I should be expecting on a cone 10-12 Firing lets say 4' and higher above my damper?( Damper is 2' above floor of kiln.....middle of total chamber height of 48"). 

Second, this new chimney pipe will not reduce from 144 square inches (roughly)  to 50 square inches as the current one does, instead it will go from 144 to 80-100 square inches, and the chimney will be about 6' taller than the current one. My inclination is to say that I will have adequate if not excessive draw/draft (which can be controlled with damper) with this new chimney as compared to the current, but ill also have (2) 45* elbows, and about 6-8' of 45* angled straight pipe compared to an all straight chimney, which may impact more than I predict. Should I be concerned about my natural draft /Venturi style big Bertha burners not drawing well with this new chimney setup? 

 Thanks in advance folks!

  Sam

[Bill Kielb]

A lot of design to be sure and your seat of the pants operating experience is likely as accurate as the set of approximations that I would come up with. An accurate one line drawing of what you have and any temperatures that are known would be helpful. I am not an offset guy and prefer a roof tall cone approach  made for the corrugated. I do like terminating in factory fittings complete with spark arrestor etc.. and allows the opportunity for triple wall prefabricated design if necessary and  reliability and asssembly ease.

again likely not helpful but my best suggestion is a one line of what you have and as much temp data as possible to develop what we believe to be the draft and energy whereby you could begin to calculate your next design leaving some room for the inevitable adjustability.

for stuff I design I always go back to see if my assumptions were true and to reflect how things perform in the real world just to help me along with my next designs. Your current knowledge locked in your noggin about what really is going on and maybe your present ability to measure  one more firings would be super helpful. Then just synthesizing a design from tables knowing your Burner output using a realistic one line drawing is a great start in my view.

the picture below is an example of how I look at a completed design to see how my design velocities and temperature assumptions ended up. Not sure if you have this kind of info available or can measure it  for your existing configuration.

with a simple drawing  and real quantities as well as measurements I think  you could get some helpful opinions here. Else it is hard for me to comment except to say I am not a fan of offsets in stacks and prefer designed penetrations for their longevity to water penetration.

 

[Mark C.]

I have seen many a stack penetrate metal roofs-they all have the same issue

I would like to know how high up the roof is from the damper in your building? since you are adapting to standard wood stove pipe the flashings are pretty tight.

In terms of the metal roof they make a metal V dam that goes in above the penetration so water-snow melt-rain is mostly not at hole and then the standard dura vent flashing with heat tape can work as long and this is my main point its pretty far away from the damper.Check out all the penetrations pieces for your brand of metal roof-if its corrugated you are out of luck but if its standing seam or flat then up then flat ups are in luck  as they have good water proof fittings.

I have some issues with the two 45s-as they will slow some draft in stack unless they are very close to one another-no long sraights between them.

How far away from the damper will the 1st 45 be? 6 feet or 8 feet?

as to duravent get the best double wall stainless you can get unless you want to replace every few years.No such thing in this aplication as overkill only undersell .

My stack about 5 feet about the damper cracked standard wood fire house chiminey bricks from to much heat-its 12x12 at that point.I ran standar high fire bricks up to the 7 foot level then switched to cheaper house chiminey bricks then at 10 feet went to 3/16 x12 inch diameter  round stainless steel  from a pulp mill salvage yard. I hafe two metal roofs diverenting rain off the stack and trimed with a standard aluminum bonnet style rain guard. It still lets some water in but heck the kiln is outside,.That rood is corrugated steel as well-very hard to stop water.(you can fabricate a box to divert water wich I did on my other updraft thru same roof and its dry.

You will be operating at temps above wood stove venting for sure.It may work as long as you get it up and away as far as youi can from damper-how far above are you thinking before the transition???

go with brick even IFB or hard brick as far up as you can before this transition to wood stove pipe(duravent)

what type of metal roof (profile) will you be using?

I see the benifit of a side vent but need more details-

I once capped my venturi kiln stack about 20 feet-the cap was stainless and about 1.5 feet above the top and it slowed down the draft to a crawl-it was open on all sides as well just the top had a cover. Took it off after 1 fire.Draft can be affected more than youi think with a sideways diversion

 I think it will work as long as the 45s are close together. My salt kiln has a bend in the stainless 10 dia inch stack at top but its less than 45 with no issues

[Bill Kielb]

A couple fairly reliable simple terminations I have used  below. Especially with a tall cone for structure, although the picture penetrate in the middle of a  channel, I would prefer splitting if possible to trap as little standing water as practical.

 

 

[Mark C.]

Bill these look like pretty new penetrations .Have any failed and leaked yet and how many years have they been in service?

Also your kilns have hood and these vent the hoods which run a  whole lot cooler than a straight natural draft chimney . I have a vent hood and that penetration is about 1/2 as cool over my straight chiminey stack penetration 

[Bill Kielb]

 

@Mark C.

No doubt, part of the design is the dilution air and scrubbing velocity in the picture of the hood. This is not intended to imply your design is as such, just that I return to my designs to see where my assumptions were correct.

As to the terminations I have used these over the years in several high temperature applications, remember we have million plus BTU boilers in the Midwest as well as incinerators etc...

my point for you was a one line drawing gets you into the proper table for your design and then you can pick sizing and terminations by what is possible. For whatever reason kiln designers tend toward bulky full firebrick designs which usually end up difficult at the point of penetration. Engineered designs generally perform better longer.

your knowledge of how this thing actually runs with it’s existing plumbing is super key as well, meaning temperatures and total Btu as well as observerd or inferred velocities and draft.

its almost certain that you will need to pick double or triple wall for your final design and your materials in all likelihood will be high temperature. Minimizing these by design is usually the issue. A one line drawing with all measurements would go a long way as well as temperature measurements worst case as well as in the low temperature range to ensure positive ventillation early on. Without that info, it’s probably going to be difficult for folks to comment in a meaningful way.

the termination in the picture in my case has been dependable for years. Prior to the ready made stuff, these were fabricated with EPDM but when I use it I usually prefer to waterproof around a tall cone which provides airspace through the penetrated area and supporting structure to the flue pipe. The cone extends through the roof, the pipe extends through the cone, the flashing waterproofs the tall cone and has a storm collar just above where the flue pipe exits the cone very standard dependable maintainable detail.

not sure that answers your question but if I was planning my first step would be a one line drawing with measurements (distance) fittings, and min and max  BTU  to start looking at table solutions, then use my current operating knowledge to confirm the table assumptions are correct.

example tall cone below

 

[neilestrick]

I'm out of town until this weekend, but when I get back I'll check and see if I can find an old flue temperature chart I used to have from my Alpine kiln days.

[Bill Kielb]

I am not sure  about Alpines design tables but Selkirk has a pretty standard design guide. I believe your configuration is direct vent which I have excerpted the relevant pages. Their ultra temp seems interesting. If you have a firing left in the existing kiln a draft guage and a few temperatures would tell us a bunch. Might even give you an opportunity for a super smooth barometric design  that maintains a constant draft throughout the firing. How cool is that?

link to Selkirk pdf : http://www.selkirkcorp.com/-/media/selkirk/reference-documents/common/file/resources/chimney/sure-temp/chimney--venting-sizing-handbook.pdf

 

 

 

 

[hitchmss]

On 3/26/2019 at 2:25 AM, Mark C. said:

I would like to know how high up the roof is from the damper in your building?

Top of arch is approx 5'; damper is located about middle of the chamber, or about 2.5' from the floor. Bottom of my truss is at 10', however, the roof line will be about another 6-7' from the bottom of the truss. I say "about" because while I know where the kiln is going to be sitting, and how much pitch there is in my roof (4/12), without some graph paper and some time, Im spitballing. Also, Im going to try and dodge the roof purlins with my chimeny; until the kiln is in place, not sure how far off my exterior wall the chimney stack will be, which can dictate the height to roof line.

The bottom cord of my truss is lower to accommodate a 10' finished ceiling height, but my eave height is 13'....hence why there is another 6-7' above bottom cord before the roof metal. Best estimate there is 13-15' of chimney ABOVE the damper, before the roof.

If I went with the offset chimney pipe design, my first 45* elbow would be 6-8' above the damper. Was planning about a 4-6' straight piece between the two elbows, which would shoot me out past the overhang (1') and gutter (5"), then allow me to continue vertical.

@Bill Kielb I will work on getting some temperatures pulled together from my next few firings with the kiln at its current location. Ill have to dig around and find the old pyrometer, or pick up a new one and/or IR thermometer. Not sure if the IR could read a temp through a 1/2" hole drilled into my chimney or not....

Both your concern and @Mark C. 's concern about the reduced draft in my chimney due to the offsets, and angles, has me concerned as well. Last thing Id want to do is spend $2k on expensive chimney parts to find out that it doesnt work like I planned, and is also very difficult to resell.

So, because of this, you've got me rethinking, and pretty much decided on going back to a straight stack, and exit through the roof, not the siding. Not only will this most readily assimilate my current chimney setup, but it requires less chimney pipe, thus, it is a little cheaper. I figure Ill use my IFB's to continue the chimney all the way up until about 1-2' below the metal roof, and then make my transition to 10-12" double/triple wall insulated or non chimney pipe, which will require only about 8' total of pipe and no elbows.

Ive used the cones to make transitions through roofs for wood stoves in the past, but its always been on a shingled roof with sheathing. The metal roof I have is not corrugated, and not a standing seam in the sense of a raised square seam lap. Metal is a "C-Loc" raised rib panel; ribs are 3/4" high, 9" OC. I think the cone inserted through the roof from underneath will stabilize my chimney pipe, and provide that air movement (capped by a trad storm collar), but I may try a combination of pure silicone caulk, and one of those rubber "boots" like you showed to provide my water tightness. Use the silicone between the flashing of the through the metal cone, and along the seam, and the boot over top.

[Mark C.]

Is this your metal profile ?

https://www.berridge.com/products/standing-seam-systems/cee-lock-panel/

I like the straight thru approach with the IFB as high as possiable

The temps the updraft guys deal with  with hoods is far coolor than any downdraft stack as the hot air is NOT cooled by any air infiltration.I have a hooded upfdraft so I'm speaking from experience as well as a downdraft. Night and day really.

I would go with the  old school cone and a  the newer silicone boot then top it off with traditional storm collar with the high heat putty/tape. If the tape/putty  burns off use high heat RTV silicone caulk

I also would install the metal fee water diversion dam above thge cone to take water to the sides as you will have to cut one of the c-seams as 9 inch flats are not wide enough to get the stove pipe only in the flat.

BY the way 

 (IR could read a temp through a 1/2" hole drilled into my chimney or not....)

My IR lazer gun  puts out a very small spot that  will fit  thru 1/4 hole.I use it on my trailer wheel bearings on boat trailers alot.

[hitchmss]

39 minutes ago, Mark C. said:

Is this your metal profile ?

Nope. https://cmtcomponents.com/metal-panels/

Scroll down on the page, right hand side there are some pdf's which have profiles of the panels.

I like the idea of using the "V" in front of the chimney to divert the water. We dont get HUGE snow loads here in Ohio, but on occasion we do have a couple of feet. Keeping that snow from "ice damming" up due to the heat escaping from the kiln room, around the chimney, and then consequently ripping my chimney off is a slight concern. More probable is that it would melt slowly enough to eventually work its way into any little crack it can find, and then onto my kiln.

Ive seen some people who have some seemingly simple solutions to issues like this; no reason you couldnt leave the space around the brick chimney open, install gutter (under the roof) around the perimeter of the hole, and catch and evacuate all the water that comes in.

I however like the idea of never allowing it a place to get in at all; sometimes the best solutions are ones that use a combination of "overkill" and "barely any effort".

[Mark C.]

I would try to weather tight it 1st choice-before the inside gutter-as thats always an option with a metal saw.

[Bill Kielb]

Just a quick add, if you can measure your ideal draft with a manometer, even home made inclined  one it could provide great insight into your your final size and height necessary.  0-1” spread out to 10”  is likely precise enough. When I taught  I would often encourage students  to measure their systems as often the answers they could get by measuring were more accurate than the assumptions and calculations.

As to waterproofing  the tall cone if installed on the raised seam centerline allows for a simple sheet metal field fabricated upstream saddle bridging the standing seam while providing drainage along each side into the adjacent channel. The penetration is waterproofed with EPDM field fabricated or pre molded. Both are 20 year details if done correctly and require top seam finish with compatible sealant which are urethane based and last longer than silicone generally.

The tall cone overcut will allow at least 2” of airspace around the deck penetration and double or triple wall smoke pipe should limit the temperature at this connection to something very nominal. Waterproofing the tall cone prior to stack insertion is simple with the installation of a cover until the pipe is inserted through the cone later. Prefabricated, - pitched roof - tall cones, are a thing in case you have not used them previously.  Everything can be laser pointer prespot and measured from the ground if you have a good footprint drawing of the existing.

The one miss I often run into is In operation, radiation and convection which could cause condensation and overheating the penetration even though the  outside of the pipe is reasonably cool. Just something to be aware of if your roof deck is 500 degrees from radiation off your kiln.

Just some thoughts.