So my old 50 year old Holly wall furnace is pumping out carbon monoxide so I thought it was time to replace it. I went and bought a Williams wall furnace at Home Depot thinking it would be a quick retrofit. We took out the old one and quickly realized that the venting needed to be replaced. It is really old big oval vent, with another smaller oval vent inside it. It also has a register venting to the room which I was told needs to come out. The biggest issue is the ceilings are vaulted so it's a bit difficult getting to the actual vent. My question is what is the best way to replace the old venting? I have basic tools, tin snips etc. I'm concerned about actually replacing the oval pipe going up through the roof with round, though. Am I in over my head here? I'm pretty handy around the house. I regularly do basic plumbing and electrical work. I'm an ME. I design and build small electro-mechanical machines for a living. This doesn't seem like it should be that hard to me.