The thermostat was replaced about a year ago today. The temps will go up to operational and stay right there solid where the thermostat is supposed to open (and I tested it prior to installation and it opened at the right temp). This is prior to the powered fan coming on, btw. But as soon as it gets below 35f, like I said, I can't seem to keep near operational temp. if the thermostat was stuck open or open pre-maturely, I'd be seeing trouble getting to temp all the time, not just when it's below freezing.