Note: on the
crossover pipe, the arrow go both ways; flow is from the right bank to
left when T-stat closed, and reverses to go left to right when T-stat
red section is controlled by the thermostat,
when this one is close, the coolant continue to flow through the purple
hose. (and coolant is still flowing through this one even if the
thermostat is open).
Both heaters are not controlled by the thermostat