G'day Aaron - any reason why you can't boil some water in the kettle and add a bit of hot water during the boil? I do this with every batch I make because I am doing 25 L batches in a 30 L urn (for what it's worth, I always do a 90 minute boil - can't really say why, just that it's conventional and I am trying to get a consistent process before I consider changing something like this).
Quite right dick, yep, you sure can add more water, although it is better for efficiency if you've sparged with it first!
Seriously, I actually do both- I'll often add sparge and also just straight boiling water to my piddly kettle (a big double ewe 19 L stockpot). Because I usually do over- gravity boils and dilute prior to pitching, the actual post- boil SG is not really that important as it will be changed later anyway, in the beginning I figured I may as well have the pot as full as possible all the time to help with hops utilisation (see below).
Dunk sparging is one way to do this, it uses the additional water to extract more of the remaining sugars out of the mash, the actual numbers are not huge, equate maybe to 3 L of additional full- strength beer (eg. yesterday yielded 5 L * 1.034 of sparge from 4 kg of grain), but when the volumes are fairly small anyway, this becomes significant and can be even more helpful in offsetting trub and fermentation losses.
Aaron, if you are squeezed for kettle volume, perhaps you should do an over- gravity boil and incorporate a dunk sparge/ mashout step after mashing, I do this most of the time as I use just a 19 L stockpot as my kettle for 23 L batches (I am also putting together a guide for this very method). After getting the first runnings in your kettle/ pot and putting it on the stove, just drop the bag of grain into another bucket or pot and add about 1 L - 2 L of water (just off the boil) per 1 kg of grain, stir thoroughly, let it sit for 10 minutes, then lift & drain as before. This gives you more liquor to add to the boil to replace evaporation losses but also harvests more sugars left in the grain after the first lift. Always use hot water for this BTW, never use cold or it will end up adding nasty- tasting tannins, also it helps if the water is slightly acidified with a pinch of citric acid (not essential though).
Suggest also if you do the dunk sparge to reduce the mashing water rate to around 4 L per 1 kg of grain or there may be just too much liquor. That's a fairly thick mash for a BIAB though, but you should be able to squeeze 3 kg of grain into that pot, maybe even 4 kg and still get reasonable efficiency.
As mentioned, if you're not sparging but have a concentrated boil then adding boiling water is fine to make up for evaporation losses, obviously if you add too much though you'll dilute it below the desired specific gravity (i.e. Original Gravity at the start of fermentation). This will require additional boiling to evaporate but keep in mind that this will also increase the bitterness if any hops have been added at that point. If you add cool water, the boil will halt, in which case you should pause your boil timer until it comes back up to the boil, so only add boiling water if you want to avoid that. A domestic kettle is handy for this.
One other thing to remember in over- gravity boils is that hops utilisation begins to drop off with wort of over 1.050. It is easily adjusted for though, I estimate about 10% more hops per 10 gravity points (i.e. 1.060 is 10 gravity points over 1.050). The human palette is going to struggle in picking up this difference though, so I wouldn't be overly concerned if your boil is creeping over 1.050. Also, when adding adjunct sugars, add them right at the end, which will help with hops utilisation efficiency during the boil.
Happy to discuss!
Ps. Sorry for all the odd- looking numbers and units, just seeing if the unit converter works properly!
Give me a beer and I will move the world. Archimedes