Sorry,
I think I now understand. The secondary reservoirs position in the image was confusing as well as the lack of the other reservoirs, even though you had mentioned them. It looks as though in addition to the heater and the regular system
pump it requires a pump that feeds the heater that would be on 24/7. There is water flow, to recirculate the water through the hose (coils) but the heater has no restrictions in regards to pressure. The main thing is that both the heater and the pump would need to be on 24/7 or at least all night and on a timer to turn them off when it becomes day.