Hello, I've read through some of the threads on this forum and it looks like there are a lot of knowledgable people who read these posts. I'm hoping that someone can take a look at my issue and give me their opinion. Here we go: I've got a 95% efficiency propane forced air furnace. This of course means that it's vented through PVC and has a dedicated PVC line to bring in combustion air. So far, so good. But there is also a cold air run that is connected to an exterior vent to pull cold outside air into the cold air return when the system is running. While I understand that there is some benefit to bringing in fresh air, I have to think this is hurting the efficiency of the system. This week temperatures here in Michigan are headed for single digits or lower, that's some serious cold air being introduced to the system. I'm considering closing off this return path. My question is: is this a bad idea? Why or why not? Thanks in advance for any advice that anyone can provide.