Does the mfg really spell this out in this way? I don't recall anything like that from my mfg papers.
My (NG) furnace is 19YO. Every time I've thought about having it inspected, I've looked in there, cleaned out a little dust, and that was that. Heat exchanger is warranted for 20 years. I have loosened the burners to stick an inspection mirror in there as far as I can see, and it looks fine. I vacuumed and brushed them a couple times. I have a relative who works on industrial boilers, he says that is all he does with his NG furnace.
I do have a CO detector nearby, and another in the house (both are due for replacement though, I understand they have limited lifespans).
I have done some maint on it - a few years ago, the blower motor started 'sticking' on start up very intermittently (apparently a 'dead spot' in the windings), maybe once a year or less. Two years ago, the blower started squeaking, I pulled it and oiled it. Last year, it 'stuck ' again, so I bought a new blower motor for $110, and replaced it. Pretty easy job, working great now. But I pulled a dumb one, and bent the fan getting it out, but a new fan was only $50. Still less than a service call, even with my 'ooops'.
I had to oil the draft inducer motor once. Takes a partial dis-assembly, but not hard either. Total main cost for 19 years (not including filters) - ~ $160 and a few hours of my time ( < $9/year! ).
I'm starting to wonder if I could keep this furnace running 'forever'. A replacement draft inducer, control board, and maybe the gas valve are the only main components that might fail, and those are ~ $200 ea. Then you just have minor items like the sensors, switches and stuff - peanuts, and I could easily DIY those.
That leaves the Heat Exchanger. I looked at it, and it seems like you could just pull it out by unscrewing a few things, and pop in a new one. I don't know if the mfgs sell replacements though. Not only would it save the cost of a new furnace, but I'd eliminate the install labor charge. That would be huge. Other than those things, it's just a sheet metal case that should last longer than me. Anyone keep a furnace running forever?
-ERD50