I have a 2 loop system that uses 1 pump on each loop and 1 pump that circulates between the 2 loops. The power to the house was temporarily shut off for 3hrs and now only the lower loop is circulating hot water. The hot water will only go as far as the pump on the 2nd loop and it stops. I have changed out the 2nd pump thinking that was the cause but it didnt make any change. I also tried to purge the upper loop to get any air out that might be locking it up but still no change. The system ran for 3 years without any problems.