[sitharus]

That's due to old-fashioned plumbing. Wayyy back in the day when water networks were fairly new they often couldn't provide the water pressure we have now, especially in hilly areas. It was fine for cold water, but not enough to pressurise a hot water tank.

The solution was a header tank in the attic. This stored water from the supply and fed it down to the hot water cylinder, providing some pressure.

The problem is the header tanks weren't a sealed system so animals could make their way in. In particular when using rodent control poison as this makes them seek water. Generally it'd be noticed pretty quickly, but not immediately.

So the hot water was generally safe for washing but shouldn't be used for drinking or cooking.

With modern closed systems (from the 70s onwards really) this isn't an issue.

[coolspot]

Where I am from we are having centrally-heated water, so we were told that hot tap water has anti-scaling chemicals unlike cold tap water which has only excessive chlorine.

[throw0101a]

> With modern closed systems (from the 70s onwards really) this isn't an issue.

The water department for the City of Denver still recommends not to use hot water:

> Hot water systems like tanks and boilers contain metallic parts that corrode as time goes by, contaminating the water. Hot water also dissolves contaminants in pipes faster than cold water.

* https://www.denverwater.org/tap/psa-dont-drink-or-cook-with-...

Also Vancouver (BC):

> Confused, I decided to send the question over to Metro Vancouver, which provides water to most of the 2 million residents in the Vancouver region. Bill Morrell, Metro’s media relations guy, quickly got me this answer from Bob Jones, their water quality expert. The bottom line: Use cold water for boiling.

* https://vancouversun.com/news/staff-blogs/why-you-shouldnt-u...

EPA:

> 2. USE ONLY COLD WATER FOR COOKING AND DRINKING. Do not cook with, or drink water from the hot water tap. Hot water can dissolve more lead more quickly than cold water. If you need hot water, draw water from the cold tap and then heat it.

* https://www.epa.gov/sites/default/files/2015-09/documents/20...

[dabluecaboose]

Worth mentioning that hot water is also better at dissolving things than cold, which is why aquarists should avoid putting anything from the hot water tap into their aquariums. It can introduce copper which is bad for my shrimp.