Thanks. I found the problem. flue was not blocked, but for some reason it was nocked out, so combustion products was coming back to the boiler instead of the fresh air.
I am regestered, but not enough experece. it is condencing boiler. could somebody tell why boiler works fine when i let a bit of air by opening a cover?
could somebody help please? vailant boiler locks out when the cover is on. if the cover is taken off then the boiler works fine. what could it be? Thanks