Does the heatpump cut out completly when the furnace comes on? It is definetly not recommended to run furnace and heatpump at the same time. That is the purpose of a fossil fuel kit or outdoor temp stat on Vision Pro tstat. The Hpump MUST be turned off before the furnace comes on. You can set any differential you want but I would turn the gas off to the furnace until this situation is rectified. Other wise you may find yourself shopping for a new heat pump.