Payne gas furnace - blower fan running continuously
Payne model 376B size 048095
Blower fan runs continuously no matter what the t-stat is set to temp-wise
Thermostat is apparently OK. I have 2 furnaces in here; the other furnace is a smaller Payne (376B 036075). Both have same Honeywell T-8200. I swap the two t-stat units and they both work when connected to the 036075 but neither works with the 048095. The t-stat connected shows no LEDs on while blower running, while the good furnace always shows amber LED when running.
The fusible links running to the limit switch: the closest 2 of the 3 are black and crispy; the 3rd one looks OK in its transparent sleeve. On the working furnace, those fusible links are all 3 OK in their transparent sleeves.
a multimeter test of the limit switch shows continuity while the furnace is unplugged. (I'm nervous of probing around with live voltages, even at just 120v)
I replaced the filters above the blower. They were absolutely filthy as if they had not been cleaned in several years!
while blower fan is running, the squirrel cage fan is not running.