Water not staying hot

[cdh]

Hi

My brother has Ferroli Optimax 25c boiler, but is having a problem with the hot water.

Central Heating is working fine. However, if the central heating is on and you turn the hot water tap on, the water will come out hot but then become cold after a few seconds. If the central heating is off, the water comes out cold.

Any ideas what the problem could be?

Thanks

 

[Llan1observer]

Diverter valve sticky or shot

 

[bellairaphon]

check the ntc too or diverter