Help Thimble Question

[Steamer]

Looking for some suggestions on how to determine how my thimble was built. I built my house in 1985 and had a 4 foot brick hearth installed to an outside double tile lined chimney. One flue is 8" round that services my Nashua wood stove the other is 8" square for my boiler. I remember the builder put a 12"x 4" solid header up high behind the stove so no wood studs would be needed behind the stove. I currently run flue temps of 300-350 and have no problem with creosote. Clean chimney once a year and get less than a coffee can. I wanted to install a Woodstock Fireview and when I contacted the company I was told the flue temps get up tp 600 degrees on startup but drop off significantly once the Cat is engaged. My question is how can I determine if the clay thimble has the necessary one foot of concrete or bricks surrounding it short of tearing it apart. Also the current height is perfect for the rear exit Fireview- do you think it would be difficult to tear out the clay flue and replace it with a steel insul flue. My hesitation is if the current flue is surrounded with solid concrete then I wont be able to install the insul flue. I would appreciate any suggestions. I intend to put the Nashua in the basement and power vent the boiler. Thanks in advance

 

[begreen]

It's not foolproof, but have you taken a mallet or hammer and tapped on the wall around the thimble. Often you can hear where the density of the wall changes that way. Or you could take a 2" finishing nail and probe at say 3" intervals.

Busting out the clay tile can be done, but it will be a chore. How tall is the stack? Perhaps you could drop down to a 5.5" insulated liner and leave the tiles intact?