I have a 5 year old Armstrong Hiboy direct drive oil furnace that also provides A/C from an outdoor Trane unit. A few nights ago with the A/C on, there was barely any air coming from the registers. Opening the furnace access panel, the blower was running but very slow. I shut the unit off and then turned it back on and the blower did not turn but made a humming sound. I turned off the unit again, gave the blower a spin, turned it on and it was now running at a normal high speed. After running about an hour, I cycled the power again and the blower was back to running slow.
I havenít pulled out a meter yet to test, but reviewing the wiring (black, blue and white to the motor) and schematic, there is a relay switch (with low voltage violet wires) and a fan limit switch. Just wondering if these components could be the issue or if itís the motor itself? Bad starter cap or winding?