# Draft flow/top vent calculations...



## indaswamp (Jan 24, 2018)

So, I built a smokehouse in March/April 2017. Here is my build thread for reference pics.:
https://www.smokingmeatforums.com/t...ard-outhouse-smoker-build-pics-addded.261865/

I have been using the heck out of it, but for smoking under 170* when making sausage, I am having to crack the doors (top and bottom) open to regulate the heat. My questions are pertaining on how to solve this issue.

Currently I have a general High pressure propane burner; think it is a 15000 BTU. I am looking at converting to a low pressure regulator and going with this burner from tejas smokers:






https://tejassmokers.com/Cast-Iron-Burners/24

I can pipe the venturi outside the smokehouse to pull in clean air for the flame combustion, and this will also help with draft. 

My question is, I basically used this smokehouse as a guide:
http://lpoli.50webs.com/text_files/build_smokehouse.pdf

But I went 2.5' X 2.5' X 6' instead of 2' x 2' x 6'. I have roughly 36cu.ft. of space. I have (2) 2" holes for top vents, one on either side. I have (2) 2" holes about 6" off the bottom of the smokehouse, one on either side for air intake. As shown in the design plans above.

With those 2" holes, do I have enough draft through the smokehouse? Should I add 2 more 2" top vents? Will adding the more top vents help me with not needing to crack the doors?

I have been trying to smoke sausages when it is cold, below 32*, and that helps a lot being able to control the smokehouse temp., but I am still needing to crack the doors slightly when doing low and slow @ 120~130*.

Any ideas would be helpful. Thanks in advance.

Maybe just going to a low pressure regulator and burner will help fix this issue. Eventually I want to go to a PID controller with an auto valve and pilot. I get nervous about flame outs on windy days and would feel much safer with an auto system with a pilot.


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## indaswamp (Jan 24, 2018)

I am also considering fabricating a 'doughnut' ring that I can use to cover the outer ring of propane jet holes which will effectively half the BTU's the burner is putting out. This way, I can run just the inner ring of jets at a higher output and this would help prevent flame out. Doing some reading on the web, I may need to fabricate an insert for the feed pipe from the venturi to the burner to reduce the dia. of the hole size to ensure good mixing of the gas and air when using the doughnut.


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## daveomak (Jan 25, 2018)

Reduce the BTU output of the burner...  Install a screw in every other one of the burner holes....  self drilling screws would work..   get a low pressure regulator that you can regulate the pressure...  Get a needle valve to finely adjust the gas flow....  Add 2 more 2" outlets at the top of the smokehouse...   Expanding gasses could almost double the needed exhaust holes...  2 for inlets and 4 for exhaust should be close to what you need...
Here is what one member did to solve his high temp problems....


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## indaswamp (Jan 25, 2018)

Thank you Dave for the reply!


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## indaswamp (Jan 29, 2018)

Low pressure system is the way to go....
Picked up a needle valve and the correct orifice today. Hooked everything up and did a test run-no wood. I can turn the heat down to a 1/8" small blue flame and hold the smokehouse @110*. Held it there for about an hour. Then cranked it wide open to see how high I could get it. Currently running @265*, but took about an hour to get there.

I'm using a king kooker WKAF2B 18,000 BTU 11" WC LP burner. Looks like there will be no need for me to block any jets for this burner. I hooked this one up as a test run. I have the Tejas 30,000 BTU burner with external venturi and will install it when I increase the flew size.

Now I know roughly what I need for BTU's. I'm happy.


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## indaswamp (Jan 29, 2018)

@275* on the top, 260* on the bottom now.

This is with no wood and no meat in the smokehouse. If I increase the BTU's by 1/3 to 30,000 I should have enough reserve heat for when filling the smokehouse up with meat. The biggest change though was going to a low pressure 11"WC system. Much finer control over the flame.


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## daveomak (Jan 29, 2018)

indaswamp said:


> Low pressure system is the way to go....
> 
> I'm using a king kooker WKAF2B 18,000 BTU 11" WC LP burner. *Looks like there will be no need for me to block any jets for this burner. *I hooked this one up as a test run. I have the Tejas 30,000 BTU burner with external venturi and will install it when I increase the flew size.
> 
> To keep from having a "flame out"..  You might consider blocking 1/2 the burner jets...  unless you have a flame sensor to turn off the propane flow....


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## indaswamp (Jan 29, 2018)

Holding steady @ 282* on top, 271* on bottom.

Dave, That is the plan when I install the Tejas burner with external venturi. 

This was a test run for figuring out the regulator and BTU requirements. I'll post pics. when I start installing the system. I'm also installing a safety shut off valve.


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## indaswamp (Jan 29, 2018)

Oh, and Dave, I think the blow outs were more prevalent with the high pressure regulator than the low pressure one. It was breezy and gusty this afternoon- not one blow out even with a low flame.


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## indaswamp (Feb 12, 2018)

I preparation for smoking 150lbs. of venison sausage in the next few days, I decided to try and cut the BTU's in half on the burner. I wanted something temporary as a test, this is what I came up with...

4D nails cut down and inserted in the burner holes...












Took about 5 minutes using plyers to snip them. Inserted in the outer ring of jets..












Been down with the flu for the last 3 weeks...finally feeling better. When I catch up and get some time I'm going to install my Tejas burner with external venturi. This ought to suffice until then.


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## daveomak (Feb 13, 2018)

GREAT JOB !!!  Thinking outside the box again HUH !!!!


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## indaswamp (Feb 13, 2018)

daveomak said:


> GREAT JOB !!!  Thinking outside the box again HUH !!!!


<thumbs up>
Yep!


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## indaswamp (Feb 16, 2018)

This is working real good Dave. Smoking 100# of sausage today, wind is gusting to 20mph and the flame is still lit-no blow-outs. I can fine tune the temp. +/- 3*.....

Awesome!!!!!


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## daveomak (Feb 16, 2018)

How cool is that....   REAL COOL !!!   Congrats on the improvement to your temperature control....


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## indaswamp (Feb 16, 2018)

I'm Happy, Happy, Happy!:D


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## indaswamp (Feb 17, 2018)

Smoke is going sideways today....LOL! Another stiff wind day, but the burner is chuggin along great!


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## daveomak (Feb 17, 2018)

Are you just "*TESTING*" the burner or are you smoking something...


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## indaswamp (Feb 17, 2018)

Dave, I'm smoking all the sausage we made Tues/weds.
https://www.smokingmeatforums.com/t...cs-added-pic-heavy.272745/page-3#post-1800335

This is round 2; 60# in the smokehouse. Just added more wood and bumped the temp. to 130*.


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## daveomak (Feb 17, 2018)

Nice....   Very nice....


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## indaswamp (Feb 17, 2018)

Dave, I barely had to touch the needle valve today. I'd watch the top therm. inch up...141.5, 141.7, 142...
And then within 5 minutes, the INT would go up 1~2 degrees. The smokehouse kept a constant +30* on the INT all through the smoke. Just chugged along slowly walking up the temps. It was an awesome smoke day today! Even in the wind!!! When she got to 168*, I cracked the fire door open to stabilize the temp. in the smokehouse until target INT was achieved.


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## indaswamp (Feb 17, 2018)

Dave, do you think a diffuser plate/ baffle would help even out the flow of smoke and heat? I might have read about one on Widnelly and Donwne (spelling?). The one I saw was a large aluminum plate with holes drilled every square inch...think the holes were 3/8". Something like 1200 holes. The plate was situated about 6" above the smoke box on the burner. This evened the heat out uniformly across the entire smokehouse. I could very easily fabricate one as I have access to a MIG machine and scrap aluminum.

Whatcha think?

*edit to add link:
https://www.meatsandsausages.com/smokehouse-plans/smokehouse-baffles

*see bottom right pic. @ link above.

Also-I'm considering going to (2) 4" top vents for more draft when drying sausages if the smokehouse is full so I do not have to crack the chamber door. Of course I will fab up some aluminum vent covers too.


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## daveomak (Feb 17, 2018)

Did you see the layer of "drain rock" on a wire screen...  One member has used this method, that I know of, and says it's an awesome diffuser and heat stabilizer...







	

		
			
		

		
	
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## indaswamp (Feb 17, 2018)

Yea I saw that Dave. I would want it for safety as well. While smoking the 100# batch of sausages, I noticed quite a bit of drippings falling down to the bottom of the smokehouse. This would act as a good flame arrestor as well.


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## indaswamp (Feb 17, 2018)

Yea, I imagine the rock is a great thermal heat sink to moderate temp spikes....

The hardi board does the same thing and once it heats up helps to moderate the temp. I thought about stacking fire brick along the inside bottom wall for a heat sink, might look at rock on a baffle now....or maybe coarse crushed fire brick???


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## daveomak (Feb 18, 2018)

If you use it, have a drip pan so the rock doesn't get covered in grease....  The grease attracts mice and rats and.......


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## indaswamp (Feb 20, 2018)

Alright... this afternoon I scoped out some of the modifications for the smokehouse. I located a 3" hole saw and will be drilling out the 2" holes to 3" holes. I only want 2 openings so I only have to be concerned with 2 vent covers instead of 4. I still have some aluminum plate left so I will be cutting donuts for inside the smoke house from that. I will also cut donuts for the outside with a tab. Turns out the roll of tape I found has a perfect 3" I.D. and 5" O.D. - awesome!






I will cut a 5" circle with a matching tab and this will be my vent cover. These will mount over the donuts above so the vent covers can slide on aluminum and not the wood siding of the smokehouse.

Worked on the corner brace to support the door as well...







Will stop by my buddy's shop tomorrow morning to have it welded up. I should have installed the hinges all the way at the top and bottom of the door, but I did not realize how heavy the door would be with the hardi board on it. This ought to fix it for good.

Gonna be in the top right corner. I have a ratchet strap holding tension on the opening to pull it plumb and square for installation tomorrow.


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## indaswamp (Feb 21, 2018)

Dropped the pieces off to have them welded, picked them up at lunch. Knocked off around 3pm and rushed home to install. Took me about an hour buying hardware, lining everything up, drilling the holes, and bolting in place...







Door closes perfectly now! No sag! Diagonals measure within 1/16" of each other...close enough for me! Now to make the vents larger. Hope to get started on that tonight and finish up tomorrow....then a test run to check out the larger vents.


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## daveomak (Feb 21, 2018)

Me likes the lining in the smoker body..  Yes me does..


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## indaswamp (Feb 21, 2018)

Dave, You takin about the smoke deposits or the aluminum flashing over the hardi board?


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## daveomak (Feb 21, 2018)

Yes, the aluminum flashing...   That's a very nice lining and a good job installing it...


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## indaswamp (Feb 21, 2018)

It was tedious and took a lot of time to install. Had to take my time and take good measurements to bend it. It is installed with aluminum nails so no zinc in the smokehouse.


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## daveomak (Feb 21, 2018)

AND......   No electrolytic corrosion...   You are two steps ahead of me...


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## indaswamp (Feb 21, 2018)

Yep. I got that idea looking at ideas on the internet. I think the guy is an old member here on SMF, not sure if he posts any more though.

I'll tell ya Dave, I'm really impressed with that hardi-board...really moderates the temps once it heats up.


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## daveomak (Feb 21, 2018)

It's hard to beat good thermal mass...


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## indaswamp (Feb 21, 2018)

Yep. And it does not take too long to heat up like a cinder or brick smokehouse would. I like it a lot and glad I decided to go that route instead of just all wood.


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## indaswamp (Feb 23, 2018)

Got the holes cut, but had to get creative and make a jig for the hole saw. I offset the 3" hole up and towards the front of the smokehouse. I did not want the center of the hole to be lower and needed more draft higher to clear stale smoke on the ceiling.

I had to draw a 2" circle and line that up from the inside with the jig on the outside, then screw the jig in place. and use the 3" hole saw cutting from the outside.






Once cut through the plywood, the jig held the hole saw in place to score the smokehouse wood plank.







Once it was scored about 1/8" deep, I no longer needed the jig and removed it for less friction on the hole saw.






Cut through the cypress plank and the hardiboard. I had to feather it once I got close to the aluminum flashing so as not to tear it.

Cleaned up the holes, vacuumed the dust, cut the flashing tube, wiped it down with acetone and put high temp silicone on it to install. Did you know a purina dog food can is exactly 2 15/16" in dia.? me neither until I started looking for the right size cylinder to hold the flashing tube in place for the silicone to stick and hold the shape...







Both sides are in place. I will let the silicone cure over night and finish it tomorrow. I have the aluminum disks and vent cover made, just got to clean them up and install and it's done.


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## daveomak (Feb 23, 2018)

You dig pretty deep in that box of tricks, I see...


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## indaswamp (Feb 23, 2018)

Thanks Dave. I come from a family of engineers; Dad was mechanical. Jut kinda how I grew up.


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## indaswamp (Feb 24, 2018)

Finished up the vents today...

Plates cleaned up and prepped to install.





Tabs bent down, ready for high temp silicone...






Thought I snapped a picture of the inside after I installed the plate, guess not. Here is the outside plate and vent cover installed...







I used a metal roof screw with a neoprene washer for the vent to rotate on. I will need to find a small #10 spring steel washer and a small #10 2.5" long bolt for a permanent attachment point. This will do for now.

Whole Chickens are on sale...I'll be cranking her up tomorrow to see how this has changed the draft in the smokehouse.


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## indaswamp (Feb 24, 2018)

I decided against the tab on the vent. Thought it might be prone to bend. Went with just the circle vent. I cut a 1/2" dia. round bar stock handle 3/4" long and then drilled a 3/8" hole where I wanted it installed. Had it welded from the back for a clean look. Just used a tiger disk to sand the weld down flat.


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## daveomak (Feb 25, 2018)

Very clean installation...  you did good...


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## indaswamp (Feb 25, 2018)

Thanks Dave. I'm smoking some chickens today to see how the larger vents workout and to start re-seasoning the bare spots in the smokehouse.


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## indaswamp (Feb 25, 2018)

New vents worked great today smoking chickens...








Smoked with Apple and Hockory wood pellets and dust @275* for about 4 hours till leg temp reached 170*.


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## daveomak (Feb 25, 2018)

Looks like something worked for you today....   awesome...    Dave


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## indaswamp (Feb 28, 2018)

So Dave, I've been thinking about that baffle. I'll give you the parameters I'm playing with...

Inside wall to wall in the smokehouse is 28.5" wide X 30" deep. I'm leaning towards 3/16" aluminum plate for the baffle because it's what I have access to. Very easy to clean, I'll weld a handle on the front to make it easy to pull out. My question is on drilling the holes. How small? How spread out? I was thinking maybe (1) 3/8" hole every sq. inch. That would give me about 650 holes, right at 75cu.in. of space for the smoke and heat to travel through. The smokehouse interior is 68" tall and roughly 36 cu. ft. volume.

Is 75 cubic inches enough flow through for heat and smoke? Do you think 3/8" holes are big enough? I use a 15" cast iron pan as a diffuser/wood pan on the propane burner, should I drill small holes through the plate directly above the ring of this pan?

The plate will be roughly 10" above the burner and 6" above the CI pan....

Or, should I drill larger holes on 1.5" or 2" spacing????


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## daveomak (Mar 1, 2018)

Holes in the plate idea is something you will have to experiment with...   size, spacing etc...    All of the variables are based in physics...  air flow, temperature, viscosity, pressure...   Then there's the thermal mass issue..  Aluminum absorbs BTU's rapidly and gives them up rapidly...  and it burns... it's one monster of a mess once the aluminum catches fire.. been there, done that..  never thought it would happen... evidently the fat from salmon burns hot enough to torch 3/16" angle aluminum..  Rocks, slow on the BTU up take and slow on the discharge...  or what ever term fits...   I'm kind of a simple guy....   If the rocks work, why change them..


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## indaswamp (Mar 19, 2018)

Dave, 
I think I have the answer. That 'Ah ha' moment...
I have some left over hardi board from the build. I will cut a piece big enough to use as a baffle... it's 1/4" thick and will perform nicely. Great thermal mass too. I'll need to play with the hole sizes though.


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## Gwanger (Mar 24, 2018)

indaswamp said:


> I preparation for smoking 150lbs. of venison sausage in the next few days, I decided to try and cut the BTU's in half on the burner. I wanted something temporary as a test, this is what I came up with...
> 
> 4D nails cut down and inserted in the burner holes...
> View attachment 353586
> ...


Indiaswamp, nice looking sausage, Looking into mods that you made to your LP burner, when you closed half of the holes with 16 pny nails that you shortened, did you have to pound them into the burner or just a tight friction fit.Don't want to destroy my burner. I am a sausage maker myself and would love to be able to get my LP smoker to 100 deg. for the lowest temp.


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## indaswamp (Mar 24, 2018)

Friction fit. The weight of the nail is enough to keep it in place against low pressure propane. How many cubic ft. is your smokehouse?


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## Gwanger (Mar 24, 2018)

indaswamp said:


> Friction fit. The weight of the nail is enough to keep it in place against low pressure propane. How many cubic ft. is your smokehouse?


Never measured the smoke chamber but it is the double door Masterbuilt LP smoker and have had problems with too much heat even for smoking meat 285 deg. want to be able to smoke my sausage and fish not able to do at these temps.Interested in your mods that you made. Great looking sausage, Im jealous.


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## indaswamp (Mar 24, 2018)

Measure the width, depth, and height of your cooking chamber in inches. Divide by 12 to convert to decimal feet. Multiply those numbers together and that is the cubic square feet of your smokehouse. Depending on how much space you have, will determine roughly how many BTU's you need. I do not know what the burner for you MES looks like; nails may not work. Might need to use screws like Dave posted.

And thanks for the compliment. I love making sausage, been doing it for a long time. Finally build a smokehouse large enough so I can make big batches for friends and family. I'm having fun with it....


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## indaswamp (Mar 24, 2018)

In trying to look up the specs. for your unit, I found this:


Did you ever try this? Did you still get blow outs?


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## Gwanger (Mar 24, 2018)

indaswamp said:


> In trying to look up the specs. for your unit, I found this:
> 
> 
> Did you ever try this? Did you still get blow outs?



Indaswamp, that looks like my smoker,will try soon, need to get lp, need it for my beer making and LP smoker still in 30'3 here. Haven't used smoker in over a year bc I was frustrated with not being able to control temps, you have helped more than I can say. thnx, anxious to try now!!!


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## indaswamp (Mar 24, 2018)

Take some measurements and I can help you better...


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## Gwanger (Mar 24, 2018)

indaswamp said:


> Take some measurements and I can help you better...


I just measured, 24 in.h,16 in.w x 14 in.d


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## indaswamp (Mar 24, 2018)

You have 3.11cubic feet in your smoke chamber. Looking @ the specs. I see your MES has a 15,400 BTU low pressure burner. I doubt you will need that many BTU's to run @ temps. under 170*. You may need to block off some jets if you still get blowouts running the burner on low flame. I do not know how the burner is made, might need to block holes with screws.


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## Gwanger (Mar 24, 2018)

indaswamp said:


> You have 3.11cubic feet in your smoke chamber. Looking @ the specs. I see your MES has a 15,400 BTU low pressure burner. I doubt you will need that many BTU's to run @ temps. under 170*. You may need to block off some jets if you still get blowouts running the burner on low flame. I do not know how the burner is made, might need to block holes with screws.


Indaswamp, thnx for all legwork you have done and finding the video on running the smoker and finding the temp adjustment to get lower temps. The temps here tday were in the 30's still,waiting for spring to be sprung.Thnx again,Do you ever smoke any Gators? Will try to lower temps like shown on video b4 I start plugging holes in burner.Since I am a retired carpenter I have plenty of screws and nails to plug the jets.


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## indaswamp (Mar 24, 2018)

Never smoked gator. Would be very tough I would reckon.


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## indaswamp (Mar 24, 2018)

Hey Gwanger...
Turn your burner on and count how many individual propane jets it has. Then take 15,400 and divide it by that number. Now you know how many BTU's you are reducing with each one you cover with a screw. This is how you dial the heat in. I recommend spreading them out, don't bunch them up all on one side of the burner or you will get uneven heat and hot spots in your smokehouse.


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## daveomak (Mar 24, 2018)

The jets need to be close enough together to light one another...   Not like lighting 70 birthdays candles..


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## Gwanger (Mar 24, 2018)

indaswamp said:


> Hey Gwanger...
> Turn your burner on and count how many individual propane jets it has. Then take 15,400 and divide it by that number. Now you know how many BTU's you are reducing with each one you cover with a screw. This is how you dial the heat in. I recommend spreading them out, don't bunch them up all on one side of the burner or you will get uneven heat and hot spots in your smokehouse.


Indaswamp thank you again I will do the math b4 closing off jets in burner.I will check the temps again tomorrow, maybe Mon. my daughter and husband just bought first house and need help moving stuff on Sunday. Thnx again.George, I thought gator would taste like chicken. LOL


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## Gwanger (Mar 24, 2018)

daveomak said:


> The jets need to be close enough together to light one another...   Not like lighting 70 birthdays candles..


Dave thnx for info,I will look b4 I leap.


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## daveomak (Mar 25, 2018)

AND... the orifice in the venturi of the burner also regulates the total BTU output of the burner......


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## Gwanger (Mar 25, 2018)

daveomak said:


> AND... the orifice in the venturi of the burner also regulates the total BTU output of the burner......
> 
> View attachment 358470


Thnx for added info Dave


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## indaswamp (Mar 25, 2018)

The area of the inside cross section of the pipe between the venturi and the burner plays an important part as well. It needs to be the correct size and length for proper fuel mixture and to pull in enough air for proper combustion. If it is too large, the venturi will not work properly.


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## Gwanger (Mar 26, 2018)

indaswamp said:


> The area of the inside cross section of the pipe between the venturi and the burner plays an important part as well. It needs to be the correct size and length for proper fuel mixture and to pull in enough air for proper combustion. If it is too large, the venturi will not work properly.


thnx for added info you and Dave have been a great help, thank you both. I will dig out my gas smoker tomorrow and fire it up, I calibrated thermometer this am so I can get an accurate temp. reading


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## Gwanger (Mar 26, 2018)

Gwanger said:


> thnx for added info you and Dave have been a great help, thank you both. I will dig out my gas smoker tomorrow and fire it up, I calibrated thermometer this am so I can get an accurate temp. reading


I also have cold smoking attachment with my gas smoker, works good.


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