Replacing old honeywell CT3500A4453 THERMOSTAT with a new RTH6530D where does the bluw wire go on the new unit?

It all depends on where the blue wire is hooked to in the furnace. If you can pull the covers on the furnace and let me know where the blue wire is hooked in the furnace, I should be able to tell you where to hook it at the stat. Also, where was the blue wire hooked to the old stat.

The blue wire connects to a black wire coming from a relay? O power supply? Inside the furnace.The blue wire at the old unit was connected to "B".

Thanks for the reply; I actually just purchased another thermostat (3M brand) that has the "B" connection like my old one... Problem solved! Too bad tech support people from Honeywell had no clue as to how to take care of this!

Thanks for the update, glad you got it worked out.