I haven't had a look at the Maxi-BIAB calculator for a while sorry Tarmac. You could try PM'ing stux who wrote the above version or Lambert who knows how to drive it.
I had a quick look and I think that might be an early version as some things in it don't seem to be adding up. For example, not sure why the Packaged Volume in cell B63 on the Maxi-BIAB page is not agreeing with the BrewLength on the Volumes sheet (16.7 L versus 23 L). These should agree.
There are several more important practical worries here though. The main one is that
we are asking for too much beer from such a small pot - 15.7 L.
Here's the problems that cause this...
Practical Liquor to Grain Mashing Ratios.
In full-volume BIABing, we might use around 7 to 8 litres of water to mash a kilo of grain on a normal gravity brew.
In traditional brewing, 3.5 litres per kilo might be used (1.25 quarts per pound). On the Maxi-BIAB sheet it says to mash in your 5 kgs of grain with 10.31 L of water. This is about double the traditional mash thickness and will cause you lots of problems. You might think you will end up with just a very concentrated wort but unfortunately at such a low liquor to grist ratio, things just don't work. So instead of your expected initial mash gravity of 1.123, you could end up with only about 1.060 believe it or not.
So, firstly, you need to get more water in contact with the grain during the initial mash. In your pot, you won't be able to do this.
A practical safe limit to your initial mash in your kettle might be say 3 kg of grain. So 3 kgs of grain plus 10.5 L of water.
Next problem...
Where will you fit the Sparge Water?
One you have mashed the above and drain the bag, you'd end up with about 12.5 L of wort in your kettle. There's not much room left to add any sparge runnings in. There's only really a few ways to get around this and they aren't very practical. We could reduce the amount of water you used above but, as mentioned, this can get unpredictable. You could also increase your boil time to try and boil off some water and add your sparged liquor in during the boil. Once again, pretty unpredictable.
The only really practical solution is to do a very small sparge. This limitation of mash and sparge water that can be used leads to the following...
Far Lower Efficiency
The lower the amount of water we have in contact with the grain during the mash and sparge means lower efficiency. More sugar stays behind in the grain. So, we get into a catch 22 situation. We need more grain and then we need more water!
Up to a point, this is okay but...
Any Dilutions After the Boil Begins Must Be Limited
You'll see on the Maxi-BIAB sheet that you'd be diluting 8.3 L of wort with 10 L of water. This is too much even if by some miracle you did achieve the high initial gravities you need. A general rule is that you should not dilute into fermentor volumes by more than 30%. We are going about 115%
.
What I Reckon...
I think make things as easy as possible on your first brews. Either halve your grain bill and do two brews one after the other (you will still have to sparge and dilute though) or buy a second pot and stagger the mash and boil (once again, you'll still need to sparge and dilute).
Maxi-BIAB can be a useful technique if used within limits. We have warnings for this built into the next site calculator (the BIABacus ) which should let you know when you are asking too much from your equipment. In this case, we are.
Your grain will keep well cool and dry so there is no need to brew immediately Tarmac. Much more enjoyable if we can get you started with a good brew day plan first and I'm sure we can do that. It's just going to mean a longer but more relaxed brew day
.
PP
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,
.
. The Calculator isn't set up for maxi-BIAB so interpreting the figures can be a little hard. What I'm assuming is that you held back that 4 L we talked about. What I'm wondering though is did that 4 L ever go back in? If not, that would explain all the volumes being too low and the gravities being too high.
.
and ruin your hard work. 
