Water goes hot then cold

[blake floyds]

Can any one help

I have a vokrea lina 28 combi boiler

the heating is fine but run any hot water tap and it goes hot for 30 secs then cold for 30 secs

boiler lites for about 20 seconds then goes out then relites

Pump is ok
Diverter valve has been changed
gas pressures have been checked
there is resisence across the thermostat
heat exchanger has been cleaned out


any one got any ideas

 

[DP]

Do a search. Plenty on this topic on this forum

 

[bengasman]

If you have sludge in the second h/e, you can clean it out, but it will be back.