Hot/cold taps not mixing

[Toonman]

Hi I have mixer taps on my kitchen/bathroom taps and when I put the hot on a then try to cool it with hot then the hit goes off, what could be causing this?

 

[Ric2013]

Mains cold, gravity (old-fashioned vented cylinder) hot? If so, not really suited for mixer taps, unless you get a bi-flow tap without in-body mixing.

 

[MARTIN STANJER]

Mains cold, gravity (old-fashioned vented cylinder) hot? If so, not really suited for mixer taps, unless you get a bi-flow tap without in-body mixing.

I agree with Ric2003. If this is the set up, you could try throttling the cold water into the tap by partly closing the isolation valve on the cold inlet to the tap (if isolators are fitted!). What is happening is that when you turn on the cold as well as the hot, the greater pressure of the cold forces cold water backwards up the hot pipe. Then when you turn the cold off, the 'Hot' water will still flow out cold until this cold water has been flushed out of the hot supply line) This can be properly fixed by fitting a check valve (non-return valve) in the hot supply, but you would still have to adjust the tap carefully to get enough hot output. Some mixers are better than others, but all can suffer this. (Some taps do come with check valves, but normally these are kitchen taps. This is not only for the reason described, but also to comply with the water regulations regarding backflow into the mains on taps where hot and cold can mix in the cartridge or spout)