I only heat enough water to dissolve the sugar well myself. But some heat all the water used...to prevent the development of the crud that sometimes starts growing in hummingbird feeders. Sorry, I don't remember the proper word for that "crud". It's harmful for hummingbirds. I try to remember to change out the nectar and clean the feeder often to prevent any such problems...using a vinegar solution to clean the feeder helps.
