Strange hot recirc problem

[Plumbstar]

I have a customer with a 75 gallon Bradford. Everything is installed correctly with a timer & electronic thermostat controlling the pump. Brass nipple & tee before copper recirc. line connects to the heater. I was there on second pinhole leak in the 1/2" C piping adjacent to the pump (Grundfos UP15-29SU). Pump was very hot as well. You can see channeling in the pump unions, pipe & fittings. Everything is chamfered. There was a solid block of "something" where the brass nipple connected to the tank. I was able to hammer it out & everything seems to work fine now. I've run into this before. Any idea as to what's going on. Stray currents? Cavitation of some kind from the blockage? No soft water on this install. I'm in Newport Beach, CA

 

[Jadnashua]

Manganese deposits, I think, can be nearly black. Not uncommon in some water supplies. Water flow and temperature differentials can do some weird things. FWIW, that pump may be as much as twice as big as may be needed, keeping the velocity up, and the source of leaks. The water doesn't need to be moving fast at all. Once it does get hot when the timer turns on, (there, speed will help shorten the initial delay but uses more energy and can shorten the life of the pipes). The pump in my system is a 1/28-HP, and does the job. 12-years now, and no pinhole leaks (yet!?).