It can be either. There's fancy thermosiphons, etc, but you can also just preheat water going into a water heater by pretty dumb means. You can run circulatin

It can be either. There's fancy thermosiphons, etc, but you can also just preheat water going into a water heater by pretty dumb means. You can run circulating pumps. You can have a separate working loop. Etc, etc, etc. The Carter panels were pretty simple boxes with pipes and water circulated through them, I think.