A tech came to perform annual maintenance on our two zone units in early October, before the cold season. He mentioned that the heat exchanger looked a little rusty in one of the units, and proceeded to tell me a long story about 2 another jobs.
Less than 3 weeks later, a whole unit failed. Another tech said it could be 3 problems, but he didn't have the test kit he needed, nor enough time to check. He showed me how to bypass the safety and reactivate the furnace and made no plans to return.
I asked him many questions about purchasing a new unit. I was told that the owner would call me with pricing, which he never did.
I wound up with a very unsafe furnace that passed a safety check and then I was taught to bypass the safety. In the end, the heat exchanger was completely rusted out and cracked in five places... completely visible to the naked eye and very dangerous.
I left another message for the owner to discuss the safety component of my experience and again he never returned my call. Not impressed and a little frightened by this experience. Fires are dangerous.