Need help figuring out HERMs MLT temp. issues

[deprecated]

I'm having a terrible time maintaining mash temperatures where I would like them to be. Mashing with PID set to 153, the majority of my mash sits at 145 except for area immediately around HERMs return.

My setup (followed Kal's guide):

Approx. 32G kettles (19.5" diameter by 26" H)

- HLT is setup identical to Kal's guide but does use center/bottom drain.

- MLT:
False bottom is made from SS - 0.06" Thick / 0.09375" dia. holes / 0.15625" stagger.
I do have 3G of deadspace below false bottom and uses a center/bottom drain.
The return from HLT HERMs coil passes through an analog temp. gauge (outside HLT wall) which is attached to street elbow (inside HLT wall) then silicone return hose inside MLT.
Using 2 809 pumps.

Things I've noticed:
- PID temperature always matches analog temp. gauge on MLT return.
- Wort temperature coming out of silicone hose in MLT is within a degree of PID and analog gauge, as would be expected.
- Grain bed seems to compact often and slow down recirculation quite a bit.
- With 32 lbs grain (2-Row, Crystal, Munich) and a few handfuls of rice hulls this still occurs. Frequent stirring does help but obviously heat loss remove lid.

I'm guessing at this point....

Maybe I'm losing too much temperature through 3G of deadspace?
Perhaps I need to more evening distribute wort back into MLT (rather than just return hose as in Kal's guide)?
Need to start with slower recirculation rate to keep it higher over full mash time?

Any ideas???

 

[phreaky]

I would start with a combination of more rice hulls and slower recirc rate. If the mash tun is getting compacted enough that it is slowing the pump output, I would think it is no longer allowing the hotter wort coming in the top to flow through it and heat it up, but instead it is probably flowing around it. My system is a little smaller, but with a 15-20 pound grain bill, I use .5-.75 lbs of rice hulls, and can recirculate at half throttle on my March pump for the whole 60 minute mash. Good luck figuring it out.