Condar FlueGard Flue Gas Thermometer Probe Condar's FlueGard (3-39) thermometer is designed for insertion into a double-wall stovepipe. Measuring the temperature up to 1700 degrees Fahrenheit. Dimension: 8" LX 5" WX 2" H To install a FlueGard Thermometer, drill a14inch hole in the outer wall, and a316inch hole through the inner wall.