Making fireplaces work in a green home

A few weeks ago our two fireplaces showed up and they’re now installed.  Fireplaces can be a large source of air pollution in a home (particulates, carbon monoxide, and other noxious gases), so the guidelines for tight homes are pretty strict.  In this case we’ve gone a bit further than the requirements by installing power-vented gas fireplaces.

The power-vented units have several features that will significantly reduce the risks of unhealthy air: a tight-fitting glass enclosure, a pilot light that only ignites when the fireplace is started, and power-vent fans to pull combustion gases out of the fireplace and supply outside air to the firebox.

Our chimney is really just a box to protect the exhaust flues.  Building a big brick chimney attached to the house is a large heat loss (thermal bridge) and a source of air leakage, so we’ve instead designed a box outside the insulation of the house.  The exhaust flues exit from each fireplace, pass through the exterior wall of the house through an insulated metal insert, rise within the chimney,  and terminate in the power-vent units.  The power-vent units are designed to draw a vacuum in the fireplace so that combustion gases are drawn out of the firebox and can’t leak into the house. The PV unit also provides an air intake so that outside air enters the fireplace instead of warm air from inside the house exiting through the fireplace.

Once the flues and power-vent units were installed, the team sealed up the chimney, made it watertight, and flashed it in preparation for its brick coating.  Now our mason is at work installing the brick veneer.  We’ll install a chimney vent at the top (to match one hidden in the bottom) to ensure the interior of the chimney doesn’t collect moisture, and then once we’ve got gas and electricity in the house we’ll be able to fire them up!

the framed “chimney”
chimney box with the flues installed
family room fireplace
after a layer of waterproof membrane and fireproof drywall, the brick veneer goes on
the two power-vent units concealed on the chimney