Have had a problem with one of these going off on F05 previous engineer has fitted a pump and it went off i got the job and noticed the system pressure was low and the vessel had lost its charge so repressurised vessel and refilled system,it had been converted to a sealed system.I also checked the burner seal because i was told if they perish it could cause it to overheat and trigger the stat and i checked the flow and return diff temp which seemed out it would fluctuate between 24 and 30 which makes me think blocked heat ex.Is there any way i could check the temp it went to lockout and could this be anything else 
