Thanks so much! This was interesting: when I put the SFS back together and tried the heater once more just to see, the same thing happened but I got an E05 error code for the first time. Every other time I just had the red light underneath the panel.
It starts out at air temp, then goes up to 100s, 200s, 300s, then 400s and then off. It doesn't bounce around too much really, and only takes about 30 seconds to turn off because of high 400s
Thank you so much for the information. I ran the heater without the thermal regulator in place, and it went up to 480 degrees super quickly, so it looks like my next step is to replace the stack flue sensor and then try again!