Comment by rsynnott
7 months ago
> like separate cold and hot water faucet
This was, oddly, for regulatory reasons; the concern was that a blocked mixer unit could cause hot water (considered potentially unsafe) to be forced into the mains supply (presumed safe). This hasn't been a concern with mixer designs for a long time, but it took til the 90s to get the rule changed.
The hot water was considered unsafe initially because early hot water cylinders over there were open tanks subject to incursion by vermin
This is a weirdly persistent myth, but no, that's not the case (think about it; what would happen if you had an open hot water tank? It would cool down very quickly, and you'd have horrendous humidity problems). _Cold_ water tanks used to be like that (sometimes still are), which I suspect is where the confusion comes from.
Hot water getting into the mains would be a concern anywhere; in particular, unless all equipment is in perfect working order, there's a legionnaires disease risk, but there are many other risks.
Thanks for the details! (Completely randomly just a few hours ago I got a many-many years old Tom Scott video recommended to me on YT about this topic.)