Thimble Connection

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New Member
Aug 17, 2022
New York
When I bought my house approximately 1 year ago the 40 year old oil furnace was venting into a masonry (concrete block w/ clay flue) chimney. Also venting to the same flue was a Fisher? wood stove. After having a professional chimney sweep come in they recommended removing the woodstove from the setup. So we removed the woodstove and ran just the furnace vent into the chimney.

Flash forward a year and I have replaced the oil-burning furnace with a direct vent lp furnace. This frees up the masonry chimney to be used to vent the woodstove I disconnect the prior year. The stove pipe for the woodstove is all 8" but the inside diameter of the clay thimble is only 7 1/2". Is this common? Do I need to have something fabricated to accept an 8" stove pipe on one end and reduce down to 7 1/2 " on the other end to fit into the thimble? Or is there a common adapter out there for this?

The one side of the chimney that has the thimble is also faced w/brick. The thimble is recessed into the brick and that opening is 9". This again leads me to believe I am missing a piece of stove pipe that would fit into the thimble and that accepts the 8" stove pipe. Unfortunately I was not paying attention and either the chimney sweep, the installers for the lp furnace, or more than likely I tossed this magical piece of metal.

Any help on what this adapter would be called or what I need to make this work is greatly appreciated. Thanks!


Staff member
Nov 18, 2005
South Puget Sound, WA
Yes, that is common. Before reconnecting, have the chimney inspected inside and out, and cleaned. If the block chimney is interior it should have 2" clearance from combustibles all the way up through the roof. If exterior, then that drops down to 1".

The adapter is called an 8" to 7" reducer. If you can't find that you may need to use a 7" to 8" increaser, reversed and that may need recrimping. The stove pipe crimp always points toward the stove.