Shabbos Urn

Question

As you may have gathered, I'm new around here. I have been browsing through previous Q & A's and would like to make comments on them if I may. Someone asked if you could add water to a Shabbos urn and the reply was that one could, provided the urn has been removed from the stove. My understanding is that one is only permitted to add water that is (a) already boiled up and (b) still retains some heat from its boiling. Is that correct?