I had a phone call saying he could not re-light the boiler and could I arrange a time to fix the boiler. On this day the customer could hear some birds tweeting which seemed to come from the boiler, after speaking to his family they concurred it would be impossible for birds to get into the boiler so they thought the sounds were from outside.

I called some days later and removed the front cover where I could see three birds inside the boiler, after stripping out the burner and flue hood I found four more to the back of the boiler.

The family of birds had got through the terminal of a balanced flue.