What is this knob for on my fireplace?

That would be the flue control. The flue is the metal duct work, ceramic piping or brick chimney stack that runs from a firebox to the top of a chimney to the outdoors. The flue directs exhaust gases and debris from a fireplace upward to vent outdoors into the atmosphere. The flue is effective because it makes use of the stack effect that happens when there's a significant temperature differential between the indoor and outdoor ends of the flue. This has the effect of drawing the exhaust gases and ash, which are lighter weight than the cold outdoor air, up through the flue and venting them outside. Some heat is lost as well in this process, and modifications can sometimes be made in or above the firebox to minimise this ancillary heat loss.

The flue has a secondary purpose of allowing fresh air to enter it, which aids in feeding or oxygenating a fire. At the fireplace end that knob allows exhaust gases to escape, and fresh air to enter, when the fireplace is in operation. The lever can be closed to prevent cold air from entering a home when the fireplace is not in use.

Before I use the fireplace I would do one of two things. Get the model# and go to hearthmaster website and down load the owners manual or contact a heating pro to explain to you how it works. The somewhat generic info on Hearthmasters website shows the knob to be the gas shutoff. If you turn it off the pilot (if it has one) will be extinguished. That means if you turn it back on and donot or cannot relight the pilot you will have a gas leak. The wall switch could be for a fan unit or an ignitor if the unit is pilotless. What all this means is that you need more information before turning or switching any thing. This is a case where knowledge is heat and no knowledge is a potential explosion.

Having built and been specing equipment for all kinds of fireplaces, I would have to say that this key is used to turn a quarter-turn gas valve. If you like to reference it, you can find the same key, with the valve set up, manufactered by Arrowhead Brass.

I am not satisfied this was answered correctly. This (removeable) key has a gas shutoff valve at the other end. In my installation, the valve itself was installed below the floor, probably by an untrained do it yourselfer from the looks of it. The knob was located behind a curtain which my grandson found immediately and shut the gas fire place off for me. I would recommend the key be removed and placed on the fire place mantle. I don't think codes allow gas system valves to be installed above basement ceilings, with or without a key through the floor.

It is the gas supply adjustment valve. The knob raises or lowers the flame in the fireplace. I have one!

There is no harm in turning it off while the fireplace is operating, as long as your know how to relight the pilot light (if any).

If the gas goes off, you're good to go. If the gas does not go off, you're probably looking at the prior gas heater's shutoff.

Turn this off during the summer season to save gas and reduce CO poisoning risk, if you have a standing pilot light.

This is a really old thread, but I don't see the correct answers here. I also have a valve exactly like that and a switch.

The valve controls the propane flow into the gas fireplace log set. It only controls the flow into your home. Once open it goes to your fireplace. The gas fireplace logs will then automatically open a valve to use the gas when you light it.

My switch turns on a blower below my fireplace that heats the room. The walls of the fireplace need to be hot before the blower will blow warm air.