Perfect Storm I have a 5 ton Climate Master W2W open loop system. I get a lockout about once a season. It is a Code 4 (Water Coil Low Temperature Cut-Out Limit) Not a source side flow issue (15GPM @ 48 degrees). It's not a dirty coil. I know what is causing it but I am not sure how to remedy it. It is the load EWT goes down to around 55 -60 degrees and the Source EWT is around 48. Based on the performance chart from Climate master the Load EWT is out of performance design. The load low temperature happens when the last heat call from 5 of 6 zones depletes most of the buffer tank hot water and it is satisfied before the heat pump can turn back on. Leaving the buffer tank return in the 70's. Then the basement slab zone comes on and dumps 50 degree water into the buffer tank. I have reduced the basement slab manifold water flow to about .2 gpm per loop which equals about 1 GPM for the 5 loops. The FP1 temp bulb plummets to around 27 degrees. (I verified this with another temp gauge) If it could run for another 3 minutes it would probably rise back up again. I thought about maybe mixing. I thought about keeping the buffer tank up to temp all the time not just when it calls. I thought about reducing the load side flow from 15 GPM to 10 for the first 8 -10 minutes of run time. Any thoughts?