Well, seeing that you're on Propane allready, the furnace would have originally shipped with the Natural Gas venturi, all you have to do is get the Natural gas venturi from the manufacturer (although, you should have the original one that shipped with the unit because they had to get a propane venturi after the fact to convert the unit to Propane in the first place and all original spare parts SHOULD have been left with you since you bought and paid for them in the original purchase price and got teh upcharge for the propane conversion).