flame lift is caused mainly by excess flue pull usaly on opened flued appliances
in yours you may need a new burner if problem with flame retention ports
or you have very high winds going across the the square flue
or possibly case seal fooked unlikly as bg would picked that one up
in yours you may need a new burner if problem with flame retention ports
or you have very high winds going across the the square flue
or possibly case seal fooked unlikly as bg would picked that one up