So you bought a vent free gas log or a vented gas log to retrofit your fireplace from wood burning into “clean” gas burning, and you wonder can I plug a gas fireplace chimney?

You read all about how your “clean” burning gas log can operate at 99% efficiency, because it doesn’t need to be vented through a chimney to the outside. Now, you are wondering how to plug a gas fireplace chimney, so you can keep all that heat in the house?

Honestly, I don’t know how installers or manufactures of gas log fireplaces can in good conscience sell gas log retrofits anymore. Vented gas logs are a setup for drafty rooms and high gas bills with no heat to show for it. Vent free gas logs are promoted as “clean burning”, even though they have been proven by a University of IL study that shows they put high levels of NO2 fume contaminants into your home.

Just google “vent free gas log dangers” and you will see hundreds of articles on that topic. Some US states have banned them entirely, for good reason! You will notice all the “Pro” gas log articles are by places that benefit from you buying one, and the “Con” articles are health agencies or building science agencies.

Personally, If I bought a house with a gas log of any kind, the first thing I would do is pull that gas log out and plug a gas fireplace chimney afterward.

Let’s zoom out and look at the different types of gas burning fireplaces, and how you can use them.

clamp to hold damper open

Clamp to hold damper open

Unvented gas logs (also called vent free gas logs) should not be retrofitted into a firebox that is a wood burning fireplace. You cannot simply drop one in, run a gas line to it, and plug a gas fireplace chimney. You can’t plug a gas fireplace chimney with a Flueblocker, a Chimney Balloon, or a metal damper of any kind. The reason is it will burn a non-metal chimney plug, and a metal damper can cause heat and fume spillage into the house in a manner that is not safe. Odds are your mantel area is not designed to have the intense heat of a vent free gas log spilling up the face of it.
Vented gas logs are mostly designed to be retrofitted into and old wood fireplace. But the downside is, by code you will need to clip open or remove the damper from the fireplace. This leaves a sizable hole open all the time. It is like me telling you to keep a window open 6″ all year long. Why would you want to do that? You wouldn’t!