I've used nashua 324A tape to seal up the custom metal duct I had to create to connect up my furnace to the existing heating duct. I used it on the inside and outside and was wondering if I'm at risk of a possible fire. The tape is rated to 325 degrees but I just wanted get some confirmation here.

Thanks.

If the tape is used on a supply or return air it is OK. If used on the flue pipe or other flue gas area it is not OK.

What tape should I use to seal up a custom sheet metal flue or for the flue pipe and flue gas area?

I did a test this morning with the Nashua 324A tape by putting two 4-inch pieces on the bottom of my skillet and made breakfast. It had a direct flame for five minutes without burning so I thought I was okay. The adhesive on the back of the tape will definitely burn though.

What tape should I use to seal up a custom sheet metal flue or for the flue pipe and flue gas area?

Do not use tape on any flue. Make the metal fit tight as per code.

I would avoid tape and just use a high temp silicone.

high temp silicone

Not UL approved for the application.

Thanks for your answer! Yesterday, I had a chimney co. to clean our flues. The man placed Nashua tape around the vents at furnace and water heater. Immediately, the horrible plastic burning smell started venting all over the house. I demanded that he removed the tape. He obviously does't know what he is doing! He could have caused us to loose out house to a fire!! Never apply tape to metal ducts nera the furnace or water boiler!! HVAC is right!

That person had no idea of what they were doing. No I wonder if the rest of his work was OK or not?

That person had no idea of what they were doing. No I wonder if the rest of his work was OK or not?
Yesterday, while the chimney co. was vaccuming, cleaning and taping my flues, I search for "Nashua tape on metal duct at furnace" and came up with HVAC100 response in this thread. Thank you so much for enlighting me. Today the chimney co. came back to replace my flue cap (on top of the furnace and boiler). They stuck the new cap on top of the flue on the roof. They used caulk to seal the cap to the flue. After 5 minutes started to rain, so I think the caulk will not be able to dry. There is a tornado like storm coming to Smithtown tonight, and I hope my flue cap doesn't fly away and land on somebody's window or windshild. The plastic burning fumes from the Nashua tape subsided, but did not disappeared totally. Two hours ago, after inspecting the furnace for the 15th time, I found out you can remove the front panel. There I found a big metal duct covered by Nashua 324 tape. I was so mad I was ready to burst on flames myself! I spent the next hour pealing the tape with my own fingers which were cut many times. Then I scrubbed the stinky burned glue on the duct with a metal sponge, then I cleaned the duct with a damp cloth. I changed all our filters, including the huge perfect fit filter on the side of the furnace. That filter is everything but perfect fit, it took me 45 minutes to fit each of its 15 folded parts. I also ran to our local hardware store and bought the new humidifier filter that is on the side of the furnace, (there are 21 steps to change that filter), I also got an indoor filter 14/30/1. After all that I was in tears. Finally, the plastic radioactice like smell disappeared, however I will sleep not knowing if my cap will end up in the Long Island sound. I will not know what else they manage to bamboozle up on the roof where they replaced the flue cap. If wasn't for HVAC100 I would be still breathing caustic fumes! Thanks again.

Your are lucky you took the time to check it all out most people do not. Hopefull all else is OK.