Background on my setup: I have a stove set on an expanded hearth in front of an old fireplace. It's rear-vented, going back about 20" to a T with a cleanout at the bottom that then goes up the old masonry chimney, which is now lined with a 6" liner (photo attached).
When I do a reload after the stove has cooled off, and I'm getting the flue temps back up, the liner will suddenly drop about half an inch at the T - it's very visible, sudden and makes a clear 'thunk'. I think this is just temporary expansion, since it's happened a number of times but the pipe still overall looks like it's in the same place (it's not slowly continuing to drop down). I've never noticed it snapping back up into place, so I assume that happens more slowly.
Is this normal? Should I be worried about stress on the insulating block off board surrounding the liner? Does this mean I'm heating the flue too quickly? The probe thermometer never gets above 800, so I don't think I'm overheating it, but it does heat up pretty fast.
![[Hearth.com] Weird Chimney Liner Behavior](https://www.hearth.com/talk/data/attachments/291/291266-e10b286c0f8a34bcadb8e2f4ba821510.jpg?hash=PVRRna94nK "[Hearth.com] Weird Chimney Liner Behavior")
When I do a reload after the stove has cooled off, and I'm getting the flue temps back up, the liner will suddenly drop about half an inch at the T - it's very visible, sudden and makes a clear 'thunk'. I think this is just temporary expansion, since it's happened a number of times but the pipe still overall looks like it's in the same place (it's not slowly continuing to drop down). I've never noticed it snapping back up into place, so I assume that happens more slowly.
Is this normal? Should I be worried about stress on the insulating block off board surrounding the liner? Does this mean I'm heating the flue too quickly? The probe thermometer never gets above 800, so I don't think I'm overheating it, but it does heat up pretty fast.
