How do combI boilers work

[JamesF]

Yes, water that's been in a header tank isn't generally considered potable, which is why the kitchen tap is connected to the mains. That said, boiling it for 90 minutes is probably going to kill pretty much anything that might do you harm, and modern header tanks are supposed to be sealed to stop most stuff getting in.

James

[Jim]

I've used hot water from the combi to make up a kit before (i.e. no boiling) and it was fine. We don't have any kind of header tank though - the combi is fed directly from the mains.