the calculator says I screwed up

[barlowds]

I am in the middle of my first build and I should have used more of the engineering side of my brain and less metal shop teacher side of my brain.  I didn't know the calculator existed untill I had the fire box made already.  I am bulding a RF 30" dia 60" long. I was inspired by a lang and since they used a 20x20x20 box so did I.  Turns out my diam and cook area is larger that a lang.

I have 8000 ci in my box the calculator calls for 14,130.  What kinds of problems will I be running into if I have a fire box that is 60% of what I realy need?

Thanks

Doug

 

[barlowds]

here are a few pics of the mock up

photo 2.JPG

• barlowds
• Apr 1, 2014

photo 1.JPG

• barlowds
• Apr 1, 2014

 

[lendecatural]

The good news is you have shop skills to make changes. In your climate, you will need the flow/heat. What is the height of the segment you cut out of the cook chamber? The Feldon's and the circle calculator says you need 6.5" for a segment height so that part may be good. The choices you have now are if you want to make new, modify the firebox, or go with a forced air fan like the BBQ Guru....

 

[barlowds]

I have 67" and the calc calls for 64, I will look into the guru.  I am not in love with the modifications that I have come up so far..

 

[lendecatural]

I just did some more homework on this one and it is a bit worse than I thought. Dave Omak very rightly corrected me on another post and indicated it is best practice to use 150% of the calculator suggestion for CC FB opening and area under the RF plate. The real problem you are seeing is the calculator error that goes with the firebox you specify for the opening. If you correct the firebox to reflect 100% as it relates to the cook chamber, you end up with 112 as the minimum recommended opening; therefore, if you add 50% to make it breathe better, the real number should be around 112 x 1.5 = 168 square inches or a segment height of 8.5 inches on the circle calculator. Here's a link with your numbers for Feldon's

http://feldoncentral.com/bbqcalcula....69&fi=2,4,0,42.00,5.40&fc=11.94,16.89,112.00

I agree, the options at this point aren't something to be in love with but that will be a big chamber to keep up to temperature in your climate....The alternative may be that you get a forced air system that may not keep up either after you have invested even more work but I don't know what the BBQ Guru guys recommend for chamber/firebox flow restrictions, I do know they offer several CFM fans that may be able to overcome the problem provided you have enough area for a good coal bed. Keep us posted on what you find out and ultimately decide!

 

[daveomak]

One option is to remove the small FB and save it for another build.... make the necessary mods to the CC and then you will be happy..... maybe not at this point in time but down the road.....
Hey.... stuff happens.... not a big deal.... I call them "VALUABLE" learning experiences....


Dave

 

[txsean]

I'll agree with that! And like LenDecaturAL said, I imagine you'll need the heat! I almost made the same mistake on mine, using my actual FB numbers instead of those that gave me 100% required. Thankfully mine was already greater than 100% so the change was minimal. 

A bit of work now but at least you aren't too far into it. The trade-off is down the road, a better running cooker and most of all piece of mind that its right!

 

[barlowds]

Thanks you guys I took a look at the numbers.  The good news is that a new sheet of steel is cheeper that a guru, and it is only a few minuets of programing on the plasma cam to build a new one, Who knows craigs list if probably full of guys looking for a premade 20x20x20 firebox.

I am not worried about the opening, nothing is welded up yet so that is just a matter of making the cut bigger, but thank you for catching that.

 

[daveomak]

Don't worry about making the FB too big..... You don't even have to change the other numbers.... they are all based on the size of the CC and everything will work perfect..... You can even make more room for ash collection under the fire grate.... Dave

 

[barlowds]

Dave
Are you saying as long as I have a big fire box all is well or saying don't worry about making the box bigger as long as the other numbers are good?

 

[daveomak]

don't worry about making the box bigger as long as the other numbers are good?


Yep.... that's not going to affect anything... all the other numbers are based on the size of the CC.....

Dave