We changed a white rodgers thermostat out to a honewll round and now in order for the heater to come on you must first turn the fan to the "on" position. The heater will come on but won't turn the fan on so the safety sensors turn the heater off when there is no air flow. I was told that we got the wrong thermostat. That we had to get one that had a common on it. This is a 5 ton A/C unit with electric heat. I did not personally change the stat so I couldn't say if it was a 4 or 5 wire setup. If I turn the fan to the "on" position the heat works fine. The A/C works as correctly. Can someone advise?

Most heating units are wired up so that the heating unit closes a relay to run the fan after a timed delay or after reaching a certain temperature. If the fan starts as soon as the heat, you would get a blast of cold air. Apparently your unit was wired to have the White Rogers thermostat control the fan at the stat. It is possible you might only need to reset a jumper in the heating unit. Figuring out how too if you lack the installation manual could be difficult. Check around, often the manuals are left inside the unit. Let me know what you find. I may be able to dig up a little more.