Cold water coming from hot water taps (turning hot when one tap is opened!)

[gwynant]

The boiler is filling up the unvented cylinder with hot water as it should but the hot taps and showers have increased in pressure and only release cold water (maybe a dash of hot water blended in).

Intriguingly, when a particular hot tap that is near to the tank is opened (I haven't tested yet with the others), the nearest shower goes hot and operates as normally.

Any ideas what this could be, and approx. price to repair? I'm a bit baffled! Thanks

 

[cross thread]

could be a mixer valve or shower faulty letting cold push up the hot.