I'm working on putting a new natural gas fireplace into an existing firebox. I'm lucky because the gas line was extremely close, the firebox was very large, plus it has a 6" flexible vent which runs up the old chimney. The vertical rise is roughly 25' and it's nearly a perfect straight shot up to the top.
So here's my question. The manufacture indicates I need a 4"/7" venting system (generic double wall config), but I'm guessing I could simple run a 4" down the existing 6" and I'd be safe. I'm a little nervous that I could starve the unit of fresh air, but it's such a low BTU I don't see how this would be a problem. I know the exhaust flue will be the right side so I'm not creating a dangerous situation.
I'm not sure if I've given enough information or not, but I'd be curious to get thoughts.
Here are more details.
Max input = 18,000 btu
Max output = 11,500 btu
615 aq inch viewing area