I have a 13 year old Heatilator direct vent fireplace original to the house. Over the last couple of seasons I had problems with the flames going out randomly, sometimes right after I lit the fireplace or after it had been running for a few hours. This past season not only did I have the flames going out, I could not get the unite to ignite at all towards the end of the season. The pilot flame is steady and blue in color and has never blown out. I have never removed the glass cover on front, nor has the unit ever been serviced. What could be causing this? Also, what is the service life for a direct vent unit? Should I have it repaired or replaced?