If the water doesn't get used for awhile because nobody's home, and you have a regular water system, the tank keeps the stored water hot enough to prevent nasties growing in it. Even so, there's occasional reports of legionella contamination in systems where the storage tank isn't kept hot enough...
Nobody cares if a closed hydronic system gets nasties, it's a closed system. You aren't drinking it, or showering in it. Also, the oxigen gets forced out of it after a few cycles, there's nothing for the nasties to feed on, and there's usually/probably antifreeze & other additives in it.
If the water doesn't get used for a while because nobody's home, and you have an open radiant setup, you're storing a bunch of water at ideal growth temp.
You should check out what the guys at heatinghelp.com have to say about radiantec - might make you think twice before linking anything from there. (Search the forum for "open radiant", as they don't name radiantec by name, because radiantec trolls the forum & threatens to sue & so on.)
Or just note that their open direct system, as designed, is non-code-compliant. At least here in NY, it would be illegal.