If you look you will see on the SPi models that the expansion tank has a small hose that goes to the throttle body and then the throttle body has small pipe at back to the pipe from the heater matrix.
This is to keep the throttle body warm in winter, you can in fact bypass the throttle body and connect the expansion tank directly to the pipe from heater matrix and it may provide a slightly cooler throttle body in summer when coolant is pretty much hot all the time, some 80-90°.