Convert 3Gallon BIAB to 5Gallon BIAB using top off water?

I typically do 3 gallon BIAB batches as I only have a 5g Kettle. 

I set up for a 4gallon boil down to 3.5 with about 3.25 going into the fermenter after cooling, trub loss.

This gives me 3 gallons at bottling time... However i would like to scale this to 5 gallons but can still only boil about 4 gallons.

What would the equipment setup be for accomplishing this?  I created a new Equipment Profile that matches above but includes adding 2Gallons top-up water to fermenter.  When I scale the recipe to the new Profile it adjusts all the ingredients to match OG, IBU etc... but I am confused that the Est Post Boil Vol is 3.5 with a specific Gravity that matches the 3 gallon recipe. So then if i add my top up water, is that not diluting my gravity?  Is there something i am missing or should i go about doing this a different way?

TL;DR - Want to convert 3gallon AG BIAB to 5gallons but need to do so using Top-Up water (Boil restrictions)

Right now the program does not distinguish between the end of boil gravity and the diluted gravity into the fermenter.  The gravity target will include your two gallons of top off water.  It does the same thing with sugar additions post boil, so I have to zero out the sugar addition to get the target post boil gravity.

Unfortunately, doing the same with the top off water will only cause the program to increase the estimated pre-boil gravity target and the amount of water needed pre-boil.  This will flag your equipment as too small (it indicates this by turning the dot next to the 'mash tun volume needed' from blue to red on the mash tab.

The actual calculation is not hard to do though.  If you are topping off with 2 gallons and ending up with 5.25 gallons into the fermenter, then you can estimate the gravity target before the top off water by the ratio of sugar points.  So if your target OG is 1.060 for your 5.25 gallons into the fermenter, you can get the sugar points by multiplying the gravity points [(1.060 - 1) * 1000 = 60] times the volume [5.25 gallons] and then dividing this result by the estimated post boil volume.  This will give you 60 gravity points * 5.25 gallons = 315 sugar points.  315 sugar points / 3.25 gallons = 97 gravity points.  So your estimated post boil gravity would be 1.097.

So that means converting to the new Profile is not all i need to do?  I WILL need to adjust the grain bill to reach a Post Boil Gravity of 1.097 (as in your example) so that after i top off with the 2Gallons it will drop to 1.060?

You have two choices.

(1) stay with your current profile and omit telling the program of the 2 gal top off.  You would then need to increase your grain bill to a OG of 1097 for the recipe in order for the dilution into the fermenter to come out correctly at your actual target gravity

(2) create a new equipment profile with the 2 gal top off.  BeerSmith should give you the OG reading of the wort ONCE IT HAS BEEN DILUTED into the fermenter.  You can figure out the actual end of boil gravity by using the formula I spelled out in the last message.

If you go with (2), you will need to scale your recipe to the new equipment profile using the 'scale recipe' feature (or by hand if you prefer) from the 3.25 gal recipe to the 5.25 gal recipe.

You could always get a larger boil kettle. Here's one for $85 with welded fittings. The store that sells this is near me and I've physically looked it over. It's a nice, heavy duty ss and you can order online.

https://www.hopcraftsupply.com/products/8-5-gallon-stainless-economy-kettle-w-2-welds

Thanks Oginme! I think I know where to go from there.

Kevin, I do plan on upgrading to a larger kettle and burner next spring. Have a little one on the way that's gonna put brewing on a hiatus for a bit. Thanks for the deal info.