This question is related. The difference here is that there is a delay between adding the water and the element coming on.
On Yom Tov there is insufficient water in the urn.
A small amount of cold water can be added to the urn without tripping the thermostat. A large amount of water causes the main heating element to come on but not immediately, only after say 30 seconds or one minute.
Is there any source which gives permission to add the large amount of water?