Thoughts on crazy high plenum temps — call the landlord immediately or test airflow first?

Shut that unit down tonight, honestly. 185°F is dangerously hot for a supply plenum. If you look at the rating sticker inside the furnace door, the maximum temperature rise is usally listed around 35°F to 65°F. Even on my old, oversized 100k BTU Goodman, supply air rarely crept past 130°F on the coldest winter days. The return duct visibly caving inward without even having a filter in the rack is a massive red flag. That kind of static pressure means the system is choking hard. If the previous filter got sucked into the blower, pieces of it might still be jammed inside the squirrel cage wheel blades, drastically cutting down your airflow. Starving the heat exchanger of air is exactly why your discharge temp is cooking at almost 190°F. Call the landlord's emergency maintenace right away instead of waiting until next week. Running it this hot risks cracking the heat exchanger or cooking the limit switches. Don't bother buying a filter right now—the technician needs to see how badly that return is choking and clean out the blower housing.

Don't freeze yourself out tonight by killing the furnace switch just yet. If you poked that probe right into the bottom of the plenum right above the cabinet, there's a strong chance your sensor is picking up line-of-sight radiant heat directly off the heat exchanger baffles. I did the exact same thing on an old Carrier 80% unit years ago with a digital kitchen thermometer and panicked when it read 192°F. Once I moved the probe around a 90-degree elbow about two feet downstream where the air actualy mixes, it showed a normal 138°F. Also, remember modern and even mid-age furnaces have a high-limit switch that trips if the internal cabinet actually hits 170°F to 200°F. If the burners aren't short-cycling every three minutes, the furnace's own safety isn't triggering. Check that every single supply register in the apartment is wide open so it's not choking on backpressure, knock the thermostat down to 65°F so it only runs in short bursts, and call the landlord in the morning during normal business hours instead of sweating thru an emergency shutdown.

Is the burner actually staying lit continuously, or are you observing it cycle off on the primary high-limit switch every few minutes while the circulating blower remains energized? Because if you are genuinely maintaining a steady-state discharge temprature of 188°F without tripping the limit—which on most residential upflow units breaks the circuit anywhere between 160°F and 190°F depending on switch insertion depth—you might not just have an absurd static pressure deficit across that choked return ductwork, but a stuck or bypassed safety control that is failing to drop out the gas valve.

Seen that oil-canning return trick plenty of times over forty years in the trade. If that sheet metal is bowing in like a crushed tin can, the blower wheel is spinning just fine—it's just starving for air from the house.

Dollars to donuts somebody tossed a rug over an old central return grate, or an old canvas flex connector rotted out and sucked flat on itself. Since you're renting, don't play hero trying to patch it up. Leave it to the landlord to get a proper tin-knocker out there before the ductwork pops right off the drive cleats.

Whoa, almost 190° is insane! What kind of digital probe did you stick in there? I just started learning about home HVAC stuff this month and got totally hooked—I acutally bought a handheld infrared thermal camera to play around with my own vents. whould pointing a thermal gun at the outside of the plenum even show that heat, or does bare sheet metal throw the reading totally off? Hope maintanence gets there fast, that suction sounds wild!

Make sure you've got fresh batteries in your carbon monoxide detectors tonight if you're keeping it running. Baking a furnace that hot's an easy way to crack a heat exchanger, and you won't smell CO if it starts dumping into the vents.

Also, don't bother buying that filter. You're renting—the second you mess with anything, the landlord'll try blaming the whole repair bill on you. Make emergency maintenance deal with it tonight.