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Going on the real cheap for a nickel sized hole, there are furnace repair/sealing silicone caulks available at Home Depot good for 1000 degrees. Cost around $15 for 10 oz caulking tube. I have made some lasting repairs where welds at y junctions failed. Other areas have begun to leak but original repair is still solid 4 years later. https://www.homedepot.com/p/3M-10-1-fl-oz-Red-Fire-Barrier-CP-25WB-Plus-Sealant-CP25WB-10/100166701 "3M™ Fire Barrier Sealant CP 25WB+ shall be a one component, ready-to-use, gun-grade, latex-based, intumescent firestop sealant capable of expanding a minimum of three times its dried volume when exposed to temperatures above 1000°F (538°C)." Beyond 1000 degrees, ya got other problems.