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Vaillant GB 221 H pilot goes out after using hot water?

I have replaced the thermocouple and it lasted 3 days without going off. could this be over heating and tripping the overheat thermostat. if this is the case whats the cure.
mac, June 2008
sounds like an overheat problem to me which could be coming from scaled up heat exchanger for domestic hot water.

boiler cannot transfer its heat to water enough and gets too hot and trips stat. if stat is interrupting thermocouple

iain, June 2008
link Click here to see other fixes for Vaillant.