Melted Sight Glass on Sealed Combustion Furnace

Has anyone seen this before? I couldn’t find another thread for it.
5 year old Lennox sealed combustion furnace with a melted sight-glass at combustion chamber.
Called it out for Specialist Inspection & Service.
I would think that there would be a risk of CO leakage, especially if it completely melted away, not to mention that some overheating situation must have caused it. ???

By the way, I can’t express enough how much help this Board has been since I started “Inspecting” 6 years ago! Thanks, Guys!


I doubt that is melted glass but may be a sign of improper combustion.
I used to blow glass and using typical lead glass even takes lots of heat.

Plastic makes sense.

Yeah, I knew it was plastic. I guess the term sight-glass was misleading. So, according to that HVAC site, it sounds like maybe a plugged secondary heat exchanger? I know we don’t need to determine the exact problem- was just curious, I guess. I think I’ll check with the Buyers in a few days to be sure they addressed it, and maybe hear what the HVAC specialist had to say. If I learn anything new, I’ll post it.