Where are you getting these numbers exactly? I've read the original post 20 times now and can't make heads or tails of what you're trying to say. Your aquastat cuts out at 180 I think, right? So boiler temp is theoretically 180. Inlet to the exchangers is 178 at this point and pulls down to 152 when the air handler starts and stays there indefinitely until the boiler cycle begins again, at which point it starts warming up the temps at the exchanger supply to 175ish again? I'm ever so confused. If your supply temp at the boiler is 180 and your supply water at the exchanger is only 152, your underground is not carrying any heat. What is the return temp at the boiler, with both exchangers in the house calling? As long as that temp stays above 140 degrees, there is nothing to worry about