Seems I once again stumbled upon household fix. Furnace has been blowing no heat. Repeated treks between office and basement, trying both to find a place selling replacement parts and getting advice from the Google, eventually led me to check outside to see if the intake and exhaust were clear.

A slight whistling sound from the tubes and suspicious, single animal tracks in the snow right to them made me think to see if either tube was blocked in the basement (thankfully they're in the clear). I'm pretty sure I felt and heard faint scratchy noises in the intake tube when I first took hold of it, but not a second time, no matter where I grabbed or how hard I tapped or shook. Maybe an animal or nest or other debris is in there. I continued grabbing and tapping and shaking along the length of the tube, hoping to maybe find a distinct difference in temperature. Shortly after that first sound, the exhaust tube started getting warm, and the heat started flowing to the other vents. I checked the airflow through the previously opened air filter bit, and it was warming! Even the vents in the basement were warming to the touch. I checked one of the vents upstairs (there are none in the basement), and warm air was starting to seep out. Not sure if a critter or some debris in there, but it's no longer blocking airflow! It might not be optimal airflow, but there's airflow. I checked the area outside again, but saw no additional tracks that would be incriminating to an animal escaping my shaking and tapping. The whistling had stopped, though, and instead of just air flowing out the exhaust, the expected plume of warm vapor was billowing forth. I'm not sure how to further check in tubes short of disassembly...I don't think my fish-tape can make the bends. I'll give it some time and go through the same thing again. Whatever it is that's blocking the airflow must have crawled in there yesterday or last night 'cause everything seemed fine until this morning. The wife is also thinking of calling someone to come clean the ducts and exhaust, so that should do the trick, right? Once cleared, I'll also put a mesh over both ports. I had queried why there weren't any when they were installed with the remodel a few years ago; the HVAC guy said that critters don't go in there because of the natural gas smell; maybe a month of sub-20F temperatures is too much for at least one little critter...