Is it normal for two techs to give completely opposite furnace diagnoses like this?

Nine times out of ten, those sub-$50 promotional tune-ups are strictly designed to get commission-based sales guys past your front door. Nobody can roll a stocked service van, pay labor, and carry business insurance for twenty-nine bucks without an angle to push major equipment. What happened with your furnace is that it was simply **overfiring**. Most residential natural gas valves are rated to deliver around 3.5 inches of water column. When the manifold pressure is cranked higher than that, the burner dumps significantly more BTUs than the heat exchanger and ductwork can dissipate. The internal cabinet temprature spikes fast, wich bakes the internal insulation and trips the high limit switch within a few minutes to keep the unit from melting down. Lowering the manifold pressure back to spec directly fixes the overheating issue. High gas pressure doesn't mask a cracked heat exchanger either—if anything, overfiring puts way more thermal stress on the metal and makes flame rollout or combustion issues far more obvious. And claiming the inducer motor was over-amping is an old trick becuase shady techs know homeowners don't carry an amp clamp to verify the numbers. Your regular guy tested it right in front of you, which tells you all you need to know. Stick with your regular tech and ditch the coupon company. If you're still feeling paranoid about the heat exchanger, just buy a digital low-level carbon monoxide detector for the hallway near your bedrooms. If the metal ever acutally develops a dangerous crack, that monitor will tell you long before an aggressive salesperson does.

Man, stories like this make me so nervous as a first-time homeowner! I am brand new to learning how all this HVAC gear works, but it's honestly so fascinating seeing how much one setting changes everthing.

Speaking of gas pressure though, coud that also be what causes a crazy loud buzzing hum right when our burners kick on, or is that usally just an old transformer vibrating against the sheet metal? Now I'm super tempted to buy one of those digital manometers online just to see what our pressure is reading, or is that way too dangerous for a total beginner to mess with?!

If you want real peace of mind without guessing, pick up a Defender CA6150. Most standard hardware store alarms won't even sound off untill CO levels hit 70 ppm for an hour, wich is useless if you're trying to catch an early hairline crack or slight rollout.

The Defender reads down to 5 ppm and shows actual real-time numbers on the display. It's what most of the pros run in thier own homes, and it will keep you from stressing over that coupon tech trying to milk you for a whole new system.

Honestly, I'd even wonder if the promo guy bumped that regulator screw himself. Wouldn't be the first time a shady tech tweaked somethig on purpose just so it'd trip out right while you're standing there watching. You shoud definitly drop an online review warning folks about thier sales games. And hang onto your regular guy's number—finding an honest tech who acutally troubleshoots instead of pushing an eight-grand box swap is getting pretty rare.