There's no recirculation possibility, so of course there won't be any sort of thermosiphon in this setup. I've only seen sidearms used for that, as I'm not sure the plate type would allow for effective flow without a pump though maybe I'm wrong. I'm curious how much that plate HX actually preheats when the tank is filling. Have you felt or put a thermometer on the exit of the HX when it's operating? You could put a recirc line between your drain port and the pressure relief valve, though it is unfortunately on the top which can cause a thermal block in a thermosiphon setup. Then you could heat your DHW nearly 100% from the boiler apart from high usage times. If the thermosiphon doesn't work, could have a small recirc pump hooked through the contacts on your bottom thermostat so it's only running when there's still cold water at the bottom of the tank.

hi, i am getting hot water to the heat exchange but the water in the tank is cold can you please explain me what is the problem