We’re installing a high-efficiency wood-burning fireplace insert. Should the intake air be drawn from a crawl space surrounded by a concrete perimeter wall or should the wall be cut to draw fresh air?
The Canada Mortgage and Housing Corporation (CMHC) doesn’t recommend the installation of fresh-air inlets because they don’t always reduce the chance of a combustion back draft and they can be a fire hazard. Under certain conditions, combustion gases and flames can exhaust through the fresh-air inlet instead of up through the chimney. Since the inlets are not designed for this, they don’t provide protection of the surrounding structure and a fire can result.
If you choose a unit that’s specifically designed by the manufacturer for an outside air source and it’s properly and safely installed, you should be okay. It’s possible that a cottager could use an insert with a built-in fresh-air inlet and never have a problem; however, the CMHC’s position is that fresh-air intake inlets don’t offer a major advantage over inserts that draw combustion air from inside, so why bother using one when there’s the possibility of risk?