PICO style FB and "slack space"

I've read a bit about some folks that have the same problem I'm having, but don't feel I have enough info to make a good decision. I have a PICO style FB in my kettle MT that sits about 2 inches above the bottom. I have been batch sparging. I have a dip tube that gets all but about .25 gallons out of the tun.

My problem is that when I am mashing the "slack space" below the FB creates a situation where I have to either add more water or have some of the grain not in contact with the water. There is about 1 3/4 gallons of water below the FB. I thought about lowering the FB, but the drain sits very close to the bottom and doesn't leave enough room to fit the FB below. I've read that some people add the wort back to the MT after mashing to fill that slack space and have had luck with that. Others have said to adjust the sparge water to compensate for the 1 3/4 gallons below the FB.

I've tried adding to the grain bill, adding DME to the boil to make up for a low efficiency that I am assuming comes from lack of contact with the water, or too much water in order to cover the grain, but am looking for other ideas. I'd like to avoid buying a new pot if at all possible. I'm wondering if anyone is in this situation and has any suggestions to compensate. Thanks.

I have a similar issue with my mash tun (lots of volume below the false bottom but above the dip tube).

Some people prefer to add the volume to their strike water. I batch sparge so I prefer to follow denny's advice and aim for equal runnings from the mash and the sparge. For a more complete description, see my post here.

I tend to run in the mid to high seventies, so I haven't had any major efficiency problems. That said, I do RIMS, so the water below the false bottom is constantly recirculating through the mash bed. Do you recirculate?

Thanks. I've divided my total strike and sparge water in 1/2 before. I do mostly 5 gallon batches and have a large pot-15 gallons. Depending on the grain bill there are still times when if I take say 9 gallons total water for mash and sparge I'm only getting 4.5 gallons for each. This leaves about 2-2.5 gallons above the "slack space" and there can still be grain that is not covered with water. I use HERMS so I do recirculate during the mash.

As I understand it, Denny's recommendation for maximum efficiency isn't to use equal strike and sparge water, but to aim for equal runnings from the mash and the sparge. Doing this, it's hard for me to picture a scenario where you wouldn't have enough water to cover the grain. Here's the text from the other thread describing exactly how I calculate my volumes:

tophmck said:

I do a single batch sparge so I follow denny's advice of aiming for equal runnings from the mash and the sparge. So let's say I'm shooting for a pre-boil volume of 7 gallons, which means I'm shooting for 3.5 gallons of first runnings. Say my grain bill is 14.5 lbs, so I add 14.5*0.125=1.8 to account for grain absorption. Finally, I add the dead space of my kettle (i.e., how much water is left behind after I drain), which for me is about half a gallon. So I end up with 3.5+1.8+0.5=5.8 gallons of strike water. Then the sparge water calculation is easy -- just half the pre-boil volume, or 3.5 gallons.

Click to expand...

EDIT: Also, how low is your efficiency? While tweaking your water/grist ratio can help a bit, I wouldn't expect it to impact efficiency by more than a few percent.

Ok, I must have misread something about equal runnings vs equal water volumes, but I'm still not sure I get it. Using your calculations in the previous post. The 5.8 gallons of strike water would be plenty to cover in the mash (I think). You would have 3.5 gallons for sparge. If I've got 2 gallons of slack space beneath my FB, that's only 1.5 gallons on top of it. Forgive my if I'm missing something here.


Edit: Is the suggestion that I leave behind the 2 gallons under the FB (slack space) from the first runnings? This would fill the space with the remnants of the first runnings when I add the sparge water. ie. mash with 5.8 gallons. Empty 3.8ish gallons into boil kettle. Leave behind the 2 gallons and then add the 3.5 gallons to sparge?

OK, let's stick with the numbers from my example to make things clear. It may sound counter intuitive, but if the 5.8 gallons of strike water is enough to cover the grain, then the 3.5 gallons of sparge water will also be enough to cover the grain.

It's not that you're intentionally leaving any water behind, though. The water that's left behind is what's been absorbed by the grain (1.8 gallons) and whatever is below your dip tube (0.5 gallons). Since that water is already in the kettle, adding 3.5 gallons of sparge water just brings you right back up to having a total of 5.8 gallons of water in the kettle.

Basically, that's the whole point of the calculation -- you're just equalizing the water/grist ratio for the mash and the batch sparge. (Of course, you'll need to redo these calculations with your own grain bill and dead space.)