I already said that, essentially, and am suggesting that if the barn is well ventilated, that this is not an issue for combustion gas. And or, --that this trap be modified or eliminated. The trap, at the least, is usually the freeze-up problem. Since it is a trap, it prevents condensate from fully exiting.
If the furnace is close to an outside wall, or other drain/ sump area such as a "dry well" you may be able to simply eliminate the trap and create a "for sure" gravity drain down and out. Make sure it is insulated all the way to the drain point and make sure it cannot "trap."
