On a call for heat the draft motor starts but the air proving switch does not close. I replaced it with the same result. Everything works normally if I close the switch manually with my own air pressure. Someone told me that somethimes the motor, due to age (this one is 13 years) will not spin fast enough to produce enough pressure to close the switch. Any help would be appreciated.

Check for any restrictions that would not let enough pressure to activate the control.

The fitting that the tubing connects to on the induced draft blower housing may be fouled, this is common, run a small wire or drill bit through it to insure its not restricked, Good luck, Mike

If your vent pipe is restricted it won't close. If it's a 90% furnace(vented with PVC) you may have had a bird build a nest in the vent pipe. Is your furnace vented with PVC or metal? What brand is it?