The problem with a bottom draining FB. well not a problem just something to be aware of.
My FB has an inch ss rim fitted, therefore there is a void below it. and the grains sit on top.My system requires 7 litres of water to fill that space up to the level of the FB.
If my recipe calls for the mash to be done in say 10 litres of water for 90mins. and I only add 10 litres of water, that means there is 7 litres in the void and the grains are having to mash in only 3 litres of strike water, a bit thick !
I have to add the 7 litres onto the strike water amount so the grains are sat in the prescribed 10 litres of strike water and there is the 7 litres underneath....17 litres in all to come off the final total of water.
After the mash I run off approx 8 litres of the mash water to ensure I get clean runnings and that all that water from the void is removed and returned to the top of the mash, and allowed to trickle through the grains.
I then continue with either fly sparge or batch sparge...remembering that I've already used 17 litres of water not just the 10 litres of the recommended strike water.
This caught me out a couple of times when I first started and I kept getting my final volumes wrong.
Becaue my tun is bottom draining it is not really " dead space " as its drawn off and returned.
Doing it this way ensures the sugars washed out of the grains are mixed thorough out the mash water.
You can fit a heater in this " void " although any grains getting through may get " burnt " and you would really need ( as mentioned ) some kind of re circulation, in effect some kind of basic hermes system.
Just try and remember you have a " dead space " under the fb,....but its not really..lol.