I can't believe I'm sharing this with everyone but here it goes... Growing up, my parents and I lived in an old farm house. So bad that when you looked in the knot holes of the clapboard siding, you could see the back of the tounge and groove pine that was in the living room. Single pane windows w/ no storms. Just reminding everyone that we are central NY. Anyway, the house had a really nice fieldstone fireplace and a wide, block chimney with another flu for the old fuel oil furnace we never used. Dad had bricked in the fireplace and inserted a thimble, and built the stone hearth out to put a woodstove on it. We used a square Shenandoah woodstove that vented out the side, and once a month, Dad would send me outside at night to "watch for anything hot landing on the roof." He would have that stove so hot, trying to burn anything out of that chimney, there would be a faint, blue flame shooting out the top, the sides of the stove would be a little red (and eventually very warped), and the section of pipe going into the thimble would be bright orange. I dont ever remember him replacing it. Now if I was you, it probably wouldn't hurt to replace it to make you feel a little more at ease. I'm sure it's fine but for what the pipe costs, I certainly would.