I do small batch (2.5 gallon) brewing and recently upgraded to a HERMS setup to improve my mash temp stability. I use a 5 gallon round beverage cooler for my mash tun, with a 9" domed false bottom. I circulate with a chugger pump through a 1/2" immersion coil that I also use for chilling the wort.
My problem is that the mash becomes stuck within a minute or two of recirculating. I've tried cutting back the flow from the pump as much as possible but it still sticks. I've tried adding rice hulls (generally around 1/4 to 1/2 lb for 5-6 lbs of grain which seems like a lot) and it helps some but I still have to watch it like a hawk and it takes a lot of patience to get it flowing well.
I've tried different mash thickness as well, from 1.25 qt/lb up to around 1.75 qt/lb. It doesn't make a difference. The issue seems to be the grain bed compacts like concrete no matter how much water is in there.
Here's what my false bottom looks like:http://www.biyhomebrewsupply.com/collections/equipment/products/9falsebottom
Do I need a different type of FB? If so, what do you recommend? Other ideas?