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redbourn

making a vent hood for small gas kiln

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It's a 28" wide olympic updraft, so round.  I'll vent through a metal roof.  I read somewhere that I need 10" double-walled pipe for the chimney.  I assume I can find this through an HVAC place or a woodstove place.  I'd like to make a 40" hood for the chimney to fit into--that's what Olympic recommends.  I don't think it matters what shape it is as long as it is wider than the kiln.  I need to meet code requirements.

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Is this for home use or a commercial space? For a commercial space you'll need to have a licensed HVAC person do the work. Ultimately it comes down to what the code says. Does it say how much the hood has to overhang the kiln? Does it say anything about how much pitch (angle) the sides of the hood have to have? The code should also specify if the duct needs to be double or triple wall. Sometimes code just says "to manufacturer's specs", but then the manufacturer says "refer to local building codes"......

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15 hours ago, redbourn said:

It's a 28" wide olympic updraft, so round.  I'll vent through a metal roof.  I read somewhere that I need 10" double-walled pipe for the chimney.  I assume I can find this through an HVAC place or a woodstove place.  I'd like to make a 40" hood for the chimney to fit into--that's what Olympic recommends.  I don't think it matters what shape it is as long as it is wider than the kiln.  I need to meet code requirements.

This is a natural draft hood and I would suggest you use tables available for such. Selkirk provides tables for all their ‘B’ vent which is a pipe within a pipe. The size of the pipe will be based on the final elevation of the pipe above the hood (feet) and the total BTU of the kiln. The table is a great start and using all pre made Bvent products improves your clearance requirements. You will also need to provide combustion air in a specific way meaning air from outside that will serve the appliance and the dilution air that will travel up the hood. There are tables for this as well and they are based on the BTU load

Since these are all manufactured products, there are easy ready made penetration products available  for almost every roof.

Properly designed natural draft hoods will operate in the 400-500 degree range so no  exotic duct is needed.

see picture below of an 800,000 btu hood, it uses a 12” BVent taken during testing and a sample design table. The column you would use is labeled “Nat”

The biggest mistakes I see is oversized pipe. Bigger is not necessarily better. Proper draft is everything and make up air,  (combustion air) is very important and required by code as well.

As for the hood: symmetry, center draw and sufficient appliance overlap as well as mounting it high enough to accommodate reduction firing and as the manufacture suggests.

I am going to add because I have seen this often from HVAC companies, this is a natural draft hood for a gas appliance (propane included) type B vent is the appropriate product. I have talked to folks wanting to make this out of stainless, stove pipe, you name it. This tells me they have no idea of how natural draft hoods work. This thing will operate in the 400- 500 degree range even though your kiln will be 2000 plus degrees. As long as this is natural draft, it is made to stay cool with the dilution air. Directly connecting something to your kiln would be an entirely different story. And  usually not as effective as a natural draft hood.

even if you do not diy this, I suggest reading through the Bvent material and combustion air material you can readily find on the internet before subcontracting this out. I have seen plenty of crazy designs that cost enormous amounts of money and were flat out wrong. Knowledge is a good thing. Your desired use is single appliance, natural draft, ........ not multiple, not force draft, not induced draft, not pellet stove.

 

A3F8F6D2-E089-48D9-A0BD-899774C0BB6A.jpeg

 

 

 

5055740F-9086-4056-A6E0-C9BA315D73F2.jpeg

Edited by Bill Kielb

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I’m not necessarily recommending this as a solution but you are asking if anyone has done this and if it worked.  I made a hood system for cheep for my 16 cu’ Alpine updraft on a outside kiln setup with a metal roof and mostly metal framing. For the hood I used a galvanized metal wash tub and just your standard sheet metal single wall ducting. I lined everything with fiber and just used 4 fairly lightweight chains to suspend the assembly from my structure over the kiln.  Although it could be a tiny bit bigger It’s worked perfect for the last 11 years this is not something I would consider venting inside through a structure or inside where you need to vent all the gasses. The same fiber lined pipe is on the home made down draft kiln  except it’s direct connect vent pipe with no hood and both have home made sheet metal fiber lined vent caps.

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16 hours ago, Bill Kielb said:

This is a natural draft hood and I would suggest you use tables available for such. Selkirk provides tables for all their ‘B’ vent which is a pipe within a pipe. The size of the pipe will be based on the final elevation of the pipe above the hood (feet) and the total BTU of the kiln. The table is a great start and using all pre made Bvent products improves your clearance requirements. You will also need to provide combustion air in a specific way meaning air from outside that will serve the appliance and the dilution air that will travel up the hood. There are tables for this as well and they are based on the BTU load

Since these are all manufactured products, there are easy ready made penetration products available  for almost every roof.

Properly designed natural draft hoods will operate in the 400-500 degree range so no  exotic duct is needed.

see picture below of an 800,000 btu hood, it uses a 12” BVent taken during testing and a sample design table. The column you would use is labeled “Nat”

The biggest mistakes I see is oversized pipe. Bigger is not necessarily better. Proper draft is everything and make up air,  (combustion air) is very important and required by code as well.

As for the hood: symmetry, center draw and sufficient appliance overlap as well as mounting it high enough to accommodate reduction firing and as the manufacture suggests.

I am going to add because I have seen this often from HVAC companies, this is a natural draft hood for a gas appliance (propane included) type B vent is the appropriate product. I have talked to folks wanting to make this out of stainless, stove pipe, you name it. This tells me they have no idea of how natural draft hoods work. This thing will operate in the 400- 500 degree range even though your kiln will be 2000 plus degrees. As long as this is natural draft, it is made to stay cool with the dilution air. Directly connecting something to your kiln would be an entirely different story. And  usually not as effective as a natural draft hood.

even if you do not diy this, I suggest reading through the Bvent material and combustion air material you can readily find on the internet before subcontracting this out. I have seen plenty of crazy designs that cost enormous amounts of money and were flat out wrong. Knowledge is a good thing. Your desired use is single appliance, natural draft, ........ not multiple, not force draft, not induced draft, not pellet stove.

 

A3F8F6D2-E089-48D9-A0BD-899774C0BB6A.jpeg

 

 

 

5055740F-9086-4056-A6E0-C9BA315D73F2.jpeg

Bill, If your set-up used a 12" pipe and had 800,000 BTU/hr input, wouldn't the chimney have to be hundreds of feet high, according to the chart?  Or am I mis-reading it?  My kiln is 280,000 BTU/hr.

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The answer is no, your table does not show 12” the table above stops at 10”.  A 12" duct at 8' feet was fine. See below:

Let me know your likely vertical height  and I will work up a quick approach for you. It will not be your design, but will illustrate how one might approach this. For your design you will need to do your own or at least you will be informed a bit more if you get someone to do it for you. Keep in mind, when we do these design and installations we fully permit them and they are inspected prior to use and confirmed in operation after with the infrared just to ensure our assumptions in design were correct.

The 12" table is shown below, notice the big jump in capacity. This stuff is not intuitive for most so using the tables are advised.

The proper size duct will draft correctly, the aluminum inner duct will heat quickly creating an updraft and the velocity up the stack will strip moisture from the inner duct making for a long life less corrosive operating environment.

BTW the tables were developed in the 50's and have been used successfully for 60 years. Some questions for you:

  • Estimated vertical Height starting at the top takeoff of your hood including a description of where on the roof?
  • Will it run vertically or will there be any horizontal offsets? 
  • 280,000 BTU/h is maximum?
  • Where are you located or what is your local altitude? For me: Chicago < 800 ft. above sea level
  • Profile of roof?
  • Will the flue be at least 10' away from ANY intake duct, etc....
  • Combustion air will likely require two openings 70 Square inches each free area (1/4000) which translates to a 10" Simple single wall duct if piped through the roof with one high vent of the same size to prevent stack effect. Do you have a location where this can be done at least 10' from ANY discharge?

 

 

 

241448963_2019-08-30(7)_LI.jpg.aee44351d58efd4d7c27016a0a720c94.jpg

Edited by Bill Kielb

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Bill, you are so generous with your help!  If you do this for a living I will happily pay you for a design.  What I misread in the charts is that the numbers are per thousands of BTU's.  A big mistake!  So for 28000 BTU's (yes this is max) I would need a  7 or 8 inch pipe, 6 feet in height.  I'm attaching 2 photos of my set up so you can see the roof shape.  From floor to top of kiln is 44".  From top of kiln to ceiling where it sits now is approximately 75".  I've placed the kiln 3 feet from the inner wall but it could be moved a bit so the dimension from top of kiln to ceiling could be taller.  I would probably hang the hood from the ceiling high enough that it will work when the kiln top is open, another 30" or so.  So from floor to bottom of hood would be 74".  The hood and chimney will run straight up through the metal roof with no bends.  I live in Huntersville, NC which is 813ft above sea level.2121746324_kilnview1.jpg.913e6e990347cf9429c44751633bb28c.jpg

kiln view #2.jpg

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45 minutes ago, redbourn said:

Bill, you are so generous with your help!  If you do this for a living I will happily pay you for a design.  What I misread in the charts is that the numbers are per thousands of BTU's.  A big mistake!  So for 28000 BTU's (yes this is max) I would need a  7 or 8 inch pipe, 6 feet in height. 

Good Picture, The kiln will be outdoors I see so no combustion air needed. Give me a bit and I will walk through this and post the steps that we would take as an example for all. Not necessarily a roadmap but the concepts and steps that one might consider when designing this. And yes, I have seen far too many of these done extremely poorly or using crazy expensive materials for no reason so its time to show some of the steps necessary for natural vent hood appliance venting. 

NC is beautiful place, I am envious!

Wood above will dry out considerably so a good hood will be an asset. The wood all around is a bit concerning to me, adding a small removable chimney to the kiln might provide better performance and contain wind blown sparks better. Need to ponder this a bit. I will add to this post.

I assume 280,000 BTU (For typo of 28,000 above), and now likely propane I am guessing which is irrelevant but will condense less.

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@redbourn

OK here we go,

If I were to begin contemplating this:

  • 280,000 BTU (from table) gets me 8" diameter Bvent at 6'-8' of rise with 8' being too tall actually so closer to 6'. I would strive to have this hood  as close to the kiln as practical without making it something one could bump their head into.
  • Combustion air is not required as this is an open structure.
  • Intermediate pipe support would be framed with steel stud from wall to header with a top support framed between the roof joists, so two locations of lateral and vertical support under the roofline. All products clamp type - no penetrating screws through the Bvent
  • Standard tall cone, silicone flashing and storm collar would be my penetration detail finished with high temp silicone caulk as needed, the tall cone would be fitted from the bottom so only a hole would be cut in the metal roof and all fasteners would be gasketed installed on top for easy maintenance.
  • The silicon boot waterproofs the tall cone to roof, the storm collar waterproofs the pipe / tall cone interface
  • Rooftop Guy support bracket as needed depending on the area, expected wind and developed system stability.
  • Suspend hood by cable or chain to enhance lateral bracing, not simply straight vertical support chains. This should increase the lateral stability of the whole stack if done at a reasonable angle.
  • A suitable single wall starting collar can be installed into the hood with mechanical fasteners and transition to the Bvent

Clearances to be minimum 1" from combustibles as per code, generally we ensure at least 2" (The tall cone usually provides this automatically at the roofline)

All Bvent to be installed per manufactures requirements

hopefully the pictures below clarify some of the questions

Just a quick crack at this though, your design will likely include things pertinent to your location.

85A09A43-0BC7-409A-81ED-99F89746D776.jpeg.26226f6ff3adff8793d52444f8b276d3.jpeg

 

C8BCA0A2-70B0-49C5-BFCC-E703E3BF2B97.jpeg

5AA7C5FD-DD57-4DEA-865F-7B81C44E03E8.jpeg

2F39C977-A0C4-4AAA-BEFE-A909B0807C67.png

0EE9A281-D85B-4CC3-BB70-95A7CF33F14E.png

C42CD4E9-2D0C-4FC8-A15E-828A5CE0234D.jpeg

ECE70E81-8242-44AE-8522-2002347447AB.jpeg

Edited by Bill Kielb

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11 minutes ago, redbourn said:

Thank you so much, Bill.  This makes sense and looks quite doable.  I'll let you know how we get on!

Just a word of caution / experience, for 8” pipe you likely will end up with a 10” tall cone. Check measurements on these things as often the literature on  the  cones is literal and 8” Bvent (double wall) pipe is roughly 10” outside diameter. Also  suggest check the pitch of your roof and order a matching pitch cone..

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Galvanized sheet metal is common.. you likely can find something pre made. You can always go with something exotic like stainless but I wouldn’t for this use. High heat paint on the outside gets you some protection and a neat look. Drafted right it will max out at about 500 degrees.

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One thing to consider here is that your hood is going to be considerably higher above the kiln than usual because your kiln is a top loader. Most gas kilns are front loaders, and have the hood much closer to the top of the kiln. So by the time the heat from your kiln enters the hood and duct, it will be much cooler than on a kiln where the hood is down close to the kiln.

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2 minutes ago, neilestrick said:

One thing to consider here is that your hood is going to be considerably higher above the kiln than usual because your kiln is a top loader. Most gas kilns are front loaders, and have the hood much closer to the top of the kiln. So by the time the heat from your kiln enters the hood and duct, it will be much cooler than on a kiln where the hood is down close to the kiln.

Something to consider  but in an open area as this not likely an issue. If he reduces in this kiln he needs to have it 18-24” above anyway to prevent flame impingement. The inner duct will still become 400 degrees, it has little mass. The hood overall will generally run cooler than indoors though. I suspect windy days will cause more variability than anything but  natural draft hoods are not designed to mitigate this. If he experiences erratic performance he can add a small say 24” tall removable natural draft chimney to the top of the kiln  leaving room to operate his damper and experiment with that to find what works best for him.

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2 hours ago, redbourn said:

Thank you so much, Bill.  This makes sense and looks quite doable.  I'll let you know how we get on!

A couple you tube clips that may interest you

Storm collars and flexible pipe boots. You will be pipe booting the tall cone.

 

 

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