Spent the morning having our ancient gas furnace inspected and learned a ton

Finding a tech who actually diagnoses instead of immediately pushing a new install is half the battle with these older beasts. When I had our 1988 Lennox checked last fall, the main extra thing I asked for was measuring the temperature rise across the heat exchanger. The data plate had a 40 to 70-degree range, and sticking a probe thermometer into the supply and return plenums showed we were creeping up over 76 degrees because the squirrel cage blades were caked in dust. Another simple test that saved us a massive headache was having him check the run capacitor on that blower motor. It was rated for 7.5 microfarads and tested down at 4.1, which explained why our airflow had felt sluggish all month. A five-minute swap for an inexpensive part kept that motor from burning out in the dead of January.

Since you already verified the heat exchanger is intact, definitely pick up a plug-in carbon monoxide detector with a digital readout if you don't have one already. Having that visual ppm display in the hallway near the bedrooms takes away any lingering anxiety about running older iron.

Glad the exchanger checked out, but my focus with older units is always the electrical side. A clean heat exchanger doesn't help much if the blower quits at 2 AM in January. Whenever I have someone look at my 90s Heil, I specifically ask them to test the run capacitor microfarads against the plate specs and pull amp draws on the motors under load.

Those simple meter checks caught my blower capacitor dipping down to 3.2 µF on a 5 µF rating two winters ago. The motor was running hot to the touch and probly woud've seized up during our first real freeze. The tech had a fresh Genteq capacitor on his truck and swapped it out in five minutes for twenty bucks. I'd also ask your tech wich components on your Rheem are standard shelf items versus obsolete parts. I ended up buying a spare universal flame sensor and an extra thermocouple to keep in a drawer near the basement stairs. Even the best independent mechanic can't do much on a Sunday night if the supply house is closed til Monday morning.

If it's an older Rheem, check if that blower motor has oil ports. alot of the younger techs don't even know what they are since almost everthing's sealed bearings now. Mine's got those little spring caps on each end, and just three drops of non-detergent 20W oil every fall keeps the thing whisper quiet. Also, if you're running a standing pilot, keep a spare thermocouple taped right inside the furnace door. It's an eight-dollar part and saves you a weekend freeze-up when the original finally gives out.

Totally agree on making sure the burners and safeties get that full rundown, but one small thing guys easily skip on those natural draft green Rheems is checking for spillage right at the draft hood. All it takes is holding a match or smoke pencil along the rim of the diverter a minute or two after the burners kick on. A cracked exchanger isn't the only way you get fumes in the basement—if that chimney flue has loose mortar or an old bird nest restricting the updraft, it'll spill right back into the room even if the combustion numbers look spotless.

Did he leave the analyzer probe in the flue when the main blower kicked on? Scoping a cold heat exchanger is fine, but have you ruled out hairline cracks that only open up once the metal expands under load?

If the O2 levels spike or the CO numbers jump the second the blower starts pushing air across the outside of the drum, that's usally how those older clamshells acutally show failure. A visual camera pass on cold steel can look pristine and still miss a stress crack that leaks under pressure.