I live in an old mill house with chimney located in center of house. Originally there was a fireplace on each side of the chimney. At some point the fire places were closed in with concrete. There is a 6" flue thimble in the center where I have hooked my wood burning stove. Recently my wife noticed that the concrete behind the wood stove is still warm for hours after the stove has been shut down and stove is cool. There is a void in the chimney below the flue thimble with no clean out. If this is a safety issue, what can I do to correct this. The wood stove is our only heat source and we are both are out of work. Not a lot of money. Have priced flue liner at a cost of $1500. The stove is a VC Acclaim. Can anyone help? Hope this is enough info. Thanks