Hi, I have a Trane XV90 furnace. I keep the fan running on low speed all the time. This year when the furnace is running for a while it will never kick into high speed. I can tell when the burner picks up speed, after 5 minutes of running, but the fan never does. It worked good with the AC in the summer but nothing now. Any ideas?

Are you for sure its calling for 2nd stage heat? If you are in high fire and the blower is not ramping up you would start to trip limits and the system would shut down.

I'm just going by the sounds the furnace makes. It sounds like the burner picks up speed after 5 minutes but the fan noise stays the same. Last year the fan definitely picked up speed when the burner made this noise. I hear what you're saying though.