US Fireplace Products Seal Tight Damper - 12" x 16" Designed with Two Gaskets 50% Larger Opening Than Competition Energy Efficient / Tight Seal Limited Lifetime Warranty Dont use on chimneys serving vented gas logs. Vented gas logs require the fireplace damper to be removed completely or fixed to a permanently open position; this is to assure that soot, fumes and carbon monoxide do not enter your home. Use only on vent-free gas logs if approved by the appliance manufacturer and the local authority having jurisdiction. However, we dont recommend it because the moisture generated from the gas logs could damage the chimney structure. Dont use on chimneys serving other fuels, wood stoves or other appliances such as inserts, furnaces, etc. 8 X 8 Fits inside flue from 6-3/4 X 6-3/4 up to 9 X 9 8 X 17 Fits inside flue from 6-1/4 X 14-1/2 up to 8-3/4 X 16-3/4 9 X 13 Fits inside flue from 6-1/2 X 10-1/2 up to 9 X 12-3/4 12 X 16 Fits inside flue from 9-1/4 X 13 up to 11-3/4 X 15-1/2 13 X 13 Fits inside flue from 10-1/8 X 10-1/8 up to 12-3/4 X 12-3/4 13 X 18 Fits inside flue from 9-5/8 X 15 up to 12-3/4 X 17-1/2 18 X 18 Fits inside flue from 15 X 15 up to 17-3/4 X 17-3/4