When I was a child, my older brother said...

that he didn't want my mom to use hot water when boiling water for pasta or whatever. He claimed that it was "stale" and "nasty" from sitting in the hot water heater for an extended period of time. Now, if I'm in a hurry, I'll use hot water to facilitate getting boiling water quicker when cooking. On the other hand, if I got time, I'll start with cold tap water, supposedly in the belief that it is somewhat cleaner. But after all these years, I still remember his concerns, and now I'm wondering if they were unfounded. Is there really any difference in how clean cold water straight from the pipe is compared to water that may have set in the water heater for an extended period of time? One could argue that the hot water killed off bacteria that would otherwise have been present, but of course that's a moot point when you consider it's going to be boiled anyway. Besides, there probably aren't many bacteria that would be killed in 120 to 130 degree water that wouldn't be present in ordinary tap water anyway... Just a random thought... I've always kind of wondered about this. Anyone? Oh, and I'm assuming that the water heater in question is not backed up with sediment. Of course that would be rather nasty...
candeh
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Hi,
Environmental Engineer here, The cold tap water is safer. You are correct, bacteria grows in the hot water heater.
Craig

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I have never used the water from the hot tap for boiling a pot of water for pasta, nor do I use it for consumption. Hot water is more likely to have (salt) contaminants. Cold water is more likely to contain dissolved gases, but boiling will remove the gas.
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snipped-for-privacy@thelast.mile wrote:

I don't often use hot water for cooking and caution other people not too. However, a biological concern is not particularly valid unless your incoming water isn't clean or you don't use your hot water tank much.
However, the concern is valid. A major concern is that hot water dissolves more things and to a greater extent than cold water. If you have an older house that uses lead solder, you will get more lead in the water if you use hot water for cooking. But even if you have iron pipe or copper pipe with lead free solder, stuff (who knows what) is getting dissolved by the hot water as it passes through the pipe. Sure it is minuscule. None of this means much with just a few times, but if you consistently use hot water to make drinks and to cook for 30 years, you will have received a lot more contaminants and some of those will accumulate in you compared to always using cold water. It simple to avoid. Why step on a banana peel, when you can see it and step over it?
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