A little over 3 years ago we had a model LHERDV30 installed in our fireplace. Within 2 weeks we noticed a 3/4 black circle in the lower left hand corner of the glass. Service people came out several times and rearranged the logs and still the circle remains. Seals have been checked and the installation around the fireplace chimney has been looked at. Does anyone out there have any idea what might be causing this circle and how can I get it to go away permanently? Thank you for your help.