Ok. My mistake. Learned from this one. Long story short... My 3 week old Wilo pump locked up, I guess caused an overheat and the aquastat was doing what it's paid to do. Here's what I don't understand,... Why did the temp probe read open? If the temp is high, would this be considered normal? I think the A419 displays ~5' higher temp than actual. The reason that the fan would not previously come on unless mech stat was turned past 220 was that the water was very hot. The local supplier I got the pump from did warranty the pump. I'll probably not purchase a Wilo pump again, but now I'm wandering if this shop loop should have a stainless or bronze pump due to being an open loop ?? I've talked to several local OWB owners close proximity to me and they all run cast pumps. Maybe it's just my luck. I owe PM for an A419 and sensor when it arrives, but at least I'll have spare. Thanks to Rory @ P&M for the assist on this one. I will say, the 2840 held heat for an amazing 3 days with no blower. Water temp never got below 120'F. Stay warm....