After moving and cleaning the brewery I did another check of dead space.

In a HERMS system you need to tweak your water volumes for most brewing software. Most software will accommodate your dead space under your dip tubes (add your pumps and hoses if you use them). Most do not understand the loss from the false bottom and HERMS coil. The main thing you will need to account for is the increase of mash water and decrease in sparge water.

• The water needed to fill the MLT to the top of the false bottom while recirculating through the HERMS coil with the wort pump. This tells me how much water is in my MLT to get to “zero” before I add grains and mash water. I must take my basic water volume calculations and add this volume to my mash water. I have my volume markers on the MLT start at this “zero” point so I can ignore the added mash water unless I am adjusting my mash with salts or acids. I also use this full volume when calculating estimated mash pH.
• The water needed to fill the HERMS coil and hose to MLT. This needs to be added to the sparge volume as this much water will be required before any sparge water enters the MLT.

HLT dead space = 1.5qt  + Water pump & hoses = 2qt
MLT dead space = 2qt + Wort pump & hoses = 2.5qt
Boil Kettle dead space = 1.25qt + Wort pump & hoses = 1.75qt
Water pump & hoses = .5qt
Wort pump & hoses = .5qt
HERMS coil & hose to MLT = 1.5qt
MLT under false bottom = 8qt

Water added to mash volume = MLT under false bottom (8qt) + Wort pump & hoses (.5qt) + HERMS coil (1.5qt) = 10qt / 2.5 gallons

Water subtracted from sparge volume = MLT under false bottom (8qt) + Wort pump & hoses (.5qt) = 8.5qt / 2 gallons.

The net of all this is I subtract 2 gallons from my sparge water. Some systems allow you accommodate this difference now. For example beersmith now has an option for “recoverable dead space” which would be the volume under the false bottom less the MLT dead space. It would then reduce the sparge as necessary. Since my MLTs volume markings only start after this recoverable dead space it is easier for me to just subtract 2 gallons from my sparge water and move on with my brew day.