I have a Rheem criterion gas furnace which works great ac/heat but I want to use the fan only without the ac/heat and the fan will not run in the on position but does run with the ac or heat on, any suggestion's... I put in a new thermostat and it is installed properly ( honeywell) I called the Honeywell company to make sure and they said it's in the furnace

Check the green wire in your stat may be loose or broken