Public Service Announcement:
We all know it is much quicker and more convenient in your busy schedule to throw some water in a pot and boil it for your instant coffee, instant cereal, top-ramen, spaghetti, etc. But there are reasons you should never use water from the hot side of the kitchen faucet, even though it saves you about 5-7 minutes.
Here comes the science:
The insides of a hot water heater contain metals that can, and do corrode. Some of the pipes in your home that are not made of PVC may have lead soldering. Hot water will dissolve metals, especially lead, much quicker than cold water will. Not to mention that over the years of daily use of drawing gallons throughout the day in cycles causes the water from the local utility, with all of it's impurities to collect and precipitate in the bottom of the hot water heater. This is a prime breeding ground for bacteria. Perhaps they cannot survive in an environment where the water is around 140°, but as soon as the water cools down enough due to a power outage or extended leave (if you turn off your water heater), all the necessary nutrients are there in an 80 gallon soup.
I personally saw a demonstration by a company that was in the business of manufacturing and selling water distillers for the consumer market. They had 2 five gallon glass jugs filled with water. One of the jugs was filled with with the water that been taken from an 80 gallon water heater that had been in use for a few years, and boiled down from 80 to 5 gallons. The other was filled with distilled water. While the distilled water was crystal clear, the other had a 3 inch layer of precipitate containing heavy metals, impurities, and bacteria. Truly a revolting sight. Also, ask any gas or electric utility worker or plumber who does work and repairs on water heaters and they will confirm this and probably offer a few horror stories of their own.
If you plan to cook with tap water, run the cold water for a minute or so and use that cold water for your cooking. You may want to save the 'non-potable' water that you flushed from the pipes as you can use it for cleaning and such. And by all means, NEVER draw warm water for infant formula. Since infants are developing at a rapid rate, they are much more vulnerable to the concentrations of impurities and lead and could potentially get lead poisoning from warm tap water.
Lead toxicity and water treatment information taken from the following pamphlet
Cooperative Extension Service
University of the Virgin Islands
#2 John Brewers Bay
St. Thomas, VI 00802-9990