hot and cold tap

[thundercrack]

hi all, I have recently purchased off of e-bay, a mixer tap. now, correct me if i'm wrong, but I don't think that when I turn the hot and cold on together to mix and get the right temperature, that the cold tap should push the hot water back up the pipe, so when you shut both of them off, and turn on the hot on its own again, all you get for ages is stone cold water. is this supposed to happen? I realise that the cold is coming in at mains pressure and the hot is only gravitating from the loft tank, but it seems strange to me.

[buildersmate]

I think you've answered your own question - there is much an imbalance in pressures between your hot and cold that the mains pressure is forcing water back up the hot pipe. I guess the only way you can get 'warm' to come out is by having the valve set over to one side such that just a tiny volume of cold water is getting through.
A way to stop this is to fit a pressure regulator/limiter into the cold feed pipe.

[musicman1956]

If your mixer was an approved kitchen mixer, the two waters would be totally separate until they both left the spout. The hot may surround the cold or be side by side with the cold. (The spout has two flow channels in it, one hot and one cold) so when you put your hand under the flow you would feel two separate water temperatures. If you have a standard bath filler or basin monoblock which is single flow then what you have dscribed wll happen. You have the wrong type for your system. Try fitting a non-return valve on the hot pipe. Or maybe change the taps back, if cold water goes up your hot pipe it will back fill the cistern and eventually overflow and can lead to mains contamination. via cross flow. (Water regulatons (water fittings) act 1999.)