my furnace did something strange this morning: it kept firing but only the fan was going, not the heater. that meant the furnace was actually cooling down the house instead of heating it. i noticed it when i was using the bathroom and felt cold air blowing out from the floor vent. i checked the thermostat and the temperature had dropped down to 56 degrees. i fixed it by setting the temperature down so it'd shut off the furnace and set the temperature up again to kick start the heater. that seemed to have fixed it and a minute later hot air was blowing out from the vents again. i've never experienced this glitch before, and i don't know whether it's related to the furnace or the nest thermostat. hopefully this doesn't happen again.

things seem to be getting back to normal as many more parked cars were missing this morning. i went out to reclear a path on the sidewalk snowbank and spent a little time reducing the size of said snowbank. i also shoveled off the snow from my portion of the backyard deck.

the dial on my circular polarizing filter for my lens has been getting hard to turn lately. so i decided to dab a little bit of bicycle oil (triflow) into the seam. now it's super easy to turn! later i did the same with one of the solar-powered tibetan prayer wheels i have in my kitchen. the gears inside seem to be a little sticky so it'd never spin on its own, so i dribbled a bit of oil into the axle and now it spins like crazy.

my parents came to pick me up in the afternoon. we stopped by burger king to grab some late lunch to go. i spent the rest of the day in belmont. my mother was busy knitting and watching old mission impossible episodes on netflix. i swapped out the microSDHC card on my father's phone with a new one i'd bought a few weeks ago. we also did some preliminary research on building a metal trellis for the backyard wisteria using pipes. after some chinese sauerkraut with rice for dinner, i got a ride back to cambridge.

in the late evening i began to notice the same problem i had this morning with my furnace, that at one point instead of blowing out hot air it'd only blow out cool air. i could temporarily fix it by resetting the temperature, but that's only a brief fix before cool air starts blowing from the vents again. i pulled out the furnace manual to look for some answers. apparently there's a diagnostic light that can indicate various conditions. i went to the basement to check it out. it was slowly flashing amber, which meant a normal call for heat. after 5-6 minutes though, it began to flash red 4 times. according to the manual, that meant the limit switch had been triggered. also the air blower will continue to run, just like in my case. it could be one of several problems, one of them being a dirty filter. so i pulled out the filter and sure enough it was covered in dust. since i didn't have a spare filter handy, i brought it back inside the house and vacuum-cleaned the filter for a quick fix. i re-installed the filter and fired up the furnace again. all seemed to be okay, so i went back upstairs.

i left too soon, because minutes later i felt cold air coming from the vents, despite the fact that the thermostat was still in heating mode. i went back downstairs. the furnace was flashing red, meaning there's a problem. instead of 4 flashes, it was flashing 8 this time. this meant the flame was lost during the heat cycle. causes include low gas pressure, faulty gas valve, or dirty/faulty flame sensor. of the three, cleaning the flame sensor is the only thing i can do on my own. but i was really hoping it was just a filter issue, which seemed like the easier thing to fix. also a lost flame error wouldn't explain why the fan continued to blow (that's an indication of 4 flashing red issue, heating limit triggered).

another weird thing that the furnace seemed to be doing that i never noticed before is it periodically "explodes" like there's a vacuum inside the furnace and suddenly it sucks in the front cover of the furnace causing a rumbling noise. this may be nothing, or could be another symptom of a busted furnace.

since it was getting late, i decided to call it a day and go back upstairs. a potentially broken furnace in the middle of winter can be a buzzkill, and i was in no mood to do anything else after that. tomorrow, i'll visit the hardware store to get a new filter. if that doesn't fix my problem, i'll try cleaning off the flame sensor. if that doesn't work, i may have to call the HVAC guy.