Once again a hero

The registers are spewing 59 degree air and everybody is happy, happy, happy.

The red wire shorted to the green wire (ground) inside the conduit. I expect that was exciting.

A slightly closer look. The glassy looking copper is typical of arc damage.

The wire I pulled out was thinner than the 8 THHN AWG I replaced it with. The breaker said 40 Amps so I went with 8 gauge.

In a perfect world I would have replaced the conduit with larger diameter because I think the NEC specifies no less that 50% air-space and I don't think I have that much. Some of that requirement is for heat dissipation. Some of it is for ease of pulling additional conductors if needed at a later date.

The cardinal sin of wiring is to run wire that is too thin to support the load the breaker will pass. The breaker is supposed to trip to save the wiring, and by extension the structure the wiring runs through. If I have to choose between sins, I will pick heavier wire and less airspace every day of the week.

Your mileage will vary.