Converting two-vent fireplace insert into one-vent: how much combustion air access does it need?

Question

We recently took out a woodburning stove and are replacing it with a propane insert.

The insert is set for a 2-pipe direct vent configuration (one for combustion air, one for exhaust). Our contractor suggested converting to a single pipe--exhaust, and then just use interior combustion air. I was hesitant at first, but he made a good case--namely we're saving a whole lot of money in not having to extend the double wall pipe the 30+ feet up the chimney. But in addition, he said it's a common conversion in our area (relatively mild climate) and that it will be sucking a whole lot less air up the chimney than the old wood stove did anyways.

Since this is will be more decorative that utilitarian (it will be used on the occasional holiday or party and as back-up heat on the those power-outage occasions) I'm not too worried about the drop in efficiency. In fact, in the grand scheme of things, I'll be better off putting the money I saved on the vent pipe into a much more efficient propane HW heater.

Now, my question: Should I include a vent in the mantle surround to make it 'easier' for the insert to get combustion air? The installer said there's no need, as it's not drawing a lot of air, and there are enough holes on the underside of the fireplace and gaps in the surround that it should be fine. I thought I'd get a second opinion just to be sure. :)

The surround will be tiled on the front, but the outside is just a wood 'shadow box' type of construction, so I could easily pop in a 2" circle vent or such on the side. Any harm in that? Any benefit?

Answer 1

I think you have considered all the right aspects. However, even in our mild climate (Pacific Northwest), we opted to run both 2 inch flex lines, but we had only 12 feet to go. It is our primary heat source and during our coldest nights we really appreciate the lack of draw through the cracks and crannies.

There is no harm in creating a draw through the surround, but a 2 inch circle seems like it is an invitation for trouble: what 3 year-old could not resist dropping a candy bar or an unfinished hot dog through there?

Maybe it could be a slit the width of the side instead.