I have a Purex Minimax 250 plus natural gas spa and pool heater. It is around 14 years old. It started fine after the winter and I used it for several weeks with no issues. Recently I saw black smoke coming form the vents. I looked at the burners and the flames were extending toward the front of the unit where all the electronics are. I purchased and replaced the burners then noticed some electronic housing had melted from too much heat so I replaced them all. Less than 15 minutes it had melted again. At this point could there be an issue with the heat exchanger? I cleaned the unit but did not open the chimney area to check for blockages. Where should I look next or should I just replace the unit? Thanks. Mark