I am replacing my old zero clearance wood burning fireplace with a natural vent gas fireplace. I plan to utilize the existing 8" diameter chimney and run a 5" aluminum liner through it. I did the same thing for my gas boiler downstairs. I have been given a big push against this from the company I am buying my fireplace from (he heard from a guy). He is concerned with the two B-Vent appliances using up all of the air in my 2000 sq ft house for combustion. I have a B-Vent gas boiler feeding a HRV for my primary heat and will use the fireplace for ambiance. I have looked in the codes and can not see anything stating that what I am going to do is illegal, I have also talked with other installers and they don't seem to share the concern. I live in Ontario Canada and would like a little reassurance.