I have a Rheem Criterion II furnace and a Honeywell thermostat has an "on" switch for the fan. The company that services my furnace said if I turned tha switch to on and a separate "system" switch to off, the fan would turn on. It doesn't. Is my furnace able to turn on just the fan? If so, how do I do it?

Do you have an Air Conditioning system connected to your heating system?

They told you correctly.. might not be wired correctly or you might have a problem with control board or relay.