I have a Grundfos 15-58 on the cold side of a flat plate heat exchanger that isn't holding its prime (its pumping air). The diagram attached shows the design I am using. What it doesn't show clearly is that the HX and pump is at ground level in the garage. The tank is in the basement. I have the pump on high speed and the head is within the pump's capacity. Why do you suppose the cold side won't pump?