I just ordered a Harman Accentra insert. I noticed that you can run a 3" pipe just past the flue to draw in outside air for combustion. This is adjacent to the 4" exhaust pipe. Just how important is this to efficiency? Obviously, if you use inside air for combustion and blow the exhaust outside of the house, you have to draw air into the house to replace it. That's going to be cold air coming in. I'm a little nervous because the dealer didn't seem to install the inserts in this manner. They were going to look at the manual and get back to me. I think that I will tell them that this is the only way that I will have it installed. Am I just being crazy, or is it important to use outside air for combustion?