Julius Ballanco, Editorial Director for PM Engineer magazine, is a registered professional engineer and recognized national plumbing expert. He offers the following explanation:
Your question regarding the quality of water between a kitchen faucet and a lavatory faucet has been one of those mysteries that has been analyzed quite thoroughly. Faucet manufacturers have spent millions of dollars researching the quality of the drinking water delivered from various faucets. We now have a national consensus standard, NSF 61, that specifies what criteria the faucet must meet in delivering a given quality of water.
Let me first respond by saying that some people are of the opinion that kitchen faucets provide a better water quality than bathroom faucets. Others claim that the best water is from their water dispenser on the refrigerator--even though that supply comes from the same pipe as the one serving the kitchen faucet.
Believe it or not, the research has proven rather conclusively that the water quality from a kitchen faucet and bathroom faucet is the same. There are variations between manufacturers. If you have the same manufacturer for both the kitchen faucet and bathroom faucet, however, you are getting almost identical water from both places.
So what is the difference? Most people like to drink water that is very cold. The water coming through a refrigerator is cooled inside the refrigerator, hence, it is probably the coldest water available in the home. In the bathroom, the water is often very cold because the person getting a drink first uses the water closet (toilet) and flushes. This starts the flow of cold water. Then they wash their hands, which continues to flow the cold water. By the time they take a drink, the water is nice and cold. This fools the brain into thinking it tastes better. (Just think, would you like to taste water that is 100 degrees Fahrenheit?)
As for the kitchen, in most homes it is located the farthest from the source of water. To keep the cost down, most water meters are located closest to the bathrooms, because they have the most piping. In comparison, kitchens have a small amount of piping and can be located farther from the meter. As a result, it takes some time to bleed the water in the piping to obtain the coldest water available. You tend to want a drink immediately, not in three or four minutes. Furthermore, studies of the human body have shown that we can sense the difference of three degrees F in the temperature of water.
Water sitting in a pipe for a long time will raise in temperature because of exposure inside the home. (If the thermostat is set at 70 degrees F, the water will rise to close to that temperature.) Water in the piping located below ground will be approximately 55 degrees F. During the winter months, the temperature can drop to 45 degrees F. That colder temperature is what does it.