Sealant for exhaust pipe

[Russ]

The exhaust on my Lennox furnace is an oval connector and in order to connect to it I had to bend my round exhaust pipe to an oval shape.  The connection isn't very good and I see there is some leakage around it.  I have the metal tape for sealing up the sections of pipe but the tape wont work very well on the furnace itself.  Does anybody know if there is there some type of high temp caulking that would work in this situation?

[Dead Eye]

Depends on how hot. I say mig weld it.

You could try plastic refractory, roll it then work it in place like play doe.  Then slow fire it hard. 

[Russ]

I ended up getting some fireplace calk.  I think that is what it is called     I sealed it up last year and now this year it has all cracked up, so I might have to redo it.  But it did seem to work well.

[diesels]

Most all 100% silicone caulk is rated to 500F. If the flue gas stream is hotter, the silicone hardens into its constituent components ( brown chunky granules). You should have a folding spike type digital temp thermostat to test the flue gas temp. Everybody should have a folding spike type digital temp thermostat. This instrument tells you what is that you cannot see. You want less than 450F. If you vent 450F through a safe flue, you are being smart and run little chance of burning your house down. I have used silicone caulk for years in sealing flues, cleanout doors, and other flue related chores. The tape is OK but costs! - diesels