Planet calculator vs feldon calculator for half moon sizing

[kdfiter]

Hello all
I'm building another reverse flow out of a 60 gallon air compressor tank and I punched in the numbers on planet calculator and feldons and I'm getting a pretty big differnce in sizing for the half moon cutout..I was planning a sqaure fb @ 18×18×18 on feldon it says I need 46.665sq snd my half moon would be 11" wide by 7 3/4 high ..planet calculator say for 47sq my half moon would be 16 wide by 4" high ... I'm leaning more to the planet calculator just so I have more room under the grate .my last smoker build I used strictly feldon calculator and I think my half moon was a little to big ..any advise would be great

 

[daveomak]

We have been here before....

https://www.smokingmeatforums.com/threads/fire-box-opening.256023/

 

[pete mazz]

My calc is easier;)

 

[daveomak]

It probably won't give you as good a smoker as the tried and true tutorial the members here helped to put together...

 

[kdfiter]

Thanks all ..I forgot I posted on this smoker build awile back..I built a smaller one instead and now im just getting back to this one

 

[kdfiter]

So I have my fire box almost done (except for the top) and I'm wondering if I went to big my box is 20L×18W×20H ...feldon caculator says all I need I'd 4600 cu and my box is coming out too 7200cu in which Is 155.8 ..is my box too big? I was planning on putting insulation just on the bottom with a plate over it but that will be 1in high ..just wondering if I'm going to have issues with using too much wood

 

[kmmamm]

If everything everything else is on size, 55% is pushing the limits of my comfort zone. Is the CC oversized as well? I hav found It is better to be a bit oversized on the fb, but be sure you oversize the air intakes as well. Also might want to go a bit over on exhaust as well. I like to use two inlets set into the fb below the fire grate and then one about mid way up the fb (I prefer to install the inlets in the door).

 

[kdfiter]

So I read in another post that you actually calculate your firebox area by how high you put your fire grate ..not sure how true that is ..but now I'm leaning on shrinking the inside of the width of the fb by adding secondary inside walls ..so pretty much a box in a box ..kind of dreading doing that though with the way the price of steel is

 

[pete mazz]

firebrick

 

[kmmamm]

Don't forget the bottom of the fb will fill with ash during the cook. Which effectively reduces the available volume..the logic some builders use for figuring only the volume above the grate.
Steel prices aside, I would not shrink he box just yet. Do some cooks with the larger box. If it turns out to be a turd, there are a few tricks you can use to shrink the volume after it is built without performing radical surgery.

 

[kdfiter]

That's not a bad idea trying it out first ..so what are the some of the problems that happen with a oversize fb

 

[kmmamm]

Could see any or all of following if fire box is too large:
Poor/inconsistent drafting
Excessive moisture
Creosote build-ups
Excessive cc temps
Wide cc temp swings
Incomplete fuel burn
There are so many other variables in a build related to airflow and draft, it is hard to predict what might happen. Chances are it will be ok...but the only way to know is to fire it up and put it to work. If you experience pain, at least you have a likely starting point to work from.