The Fireplace throat is one of the most important areas of the fireplace construction. The throat is essentially the termination of the firebox and the beginning of the venting system of the fireplace. This is where the traditional throat damper is placed during construction and is normally completely visable without any special tools are accesses.
An Interesting point about the throat is that it is NOW required to be 8" above the face opening of the fireplace. If your opening is 36 x 24, your damper/throat assembly must be at 32" high. Formerly, the requirement was 6" above.
If the throat is not constructed properly, placed in the right location or installed in an obstructive manner, the fireplace will have a tendancy to leak smoke out of the top of the opening. This is one of the things we look for on inspection and is one of many reasons a fireplace may smoke. Typically, it is not the only reason we find smokey fireplace conditions.
When fireplaces are restored due to damage or unsafe conditions, the throat is typically reconstructed without a throat damper assemble. The restorations typically have top sealing dampers that are more energy efficient and do not resist the path of the smoke from the firebox.
Check out PART 3, The Smoke Chamber coming next week.