Fireplaces listed to the ANSI Z21.88 Vented Room Heater standard are intended as supplemental heaters. You would need something listed as a Warm Air Furnace in order for credit by a mortgage company, FHA, etc. to recognize it as the primary heat source. I'm not aware of any dual listed but I know there are some out there that should be. The first that comes to mind is a Rinnai unit that vents with PVC yet is a fireplace.
HTH,